Unit - 2
Transistor Circuits
Bipolar junction transistor or BJT is simply known as Transistor. A transistor consists of two pn junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types.
The prefix 'trans' means the signal transfer property of the device while 'istor ' classifies it as a solid element in the same general family with resistors. Accordingly, there are two types of transistors
- n-p-n transistor
- p-n-p transistor
An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type as shown in Figure 1 (i).
However, a p-n-p transistor is formed by two p-sections separated by a thin section of n-type as shown in Figure 1(ii).
Figure1: (i) n-p-n transistor (ii) p-n-p transistor
In each type of transistor, the following points may be noted:
(i) Transistor may be regarded as a combination of two diodes connected back to back.
(ii) There are three terminals, one taken from each type of semiconductor.
(iii) The middle section is a very thin layer. The thin layer plays an important role in the function of a transistor.
A transistor has two pn junctions. One junction is forward biased and the other is reverse biased. The forward biased junction has a low resistance path whereas a reverse biased junction has a high resistance path. The weak signal is introduced in the low resistance circuit and output is taken from the high resistance circuit. Therefore, a transistor transfers a signal from a low resistance to high resistance.
A transistor (pnp or npn) has three sections of doped semiconductors. The section on one side is the emitter and the section on the opposite side is the collector. The middle section is called the base and forms two junctions between the emitter and collector.
Figure 2: (i) p-n-p transistor (ii) n-p-n transistor
Emitter: The section on one side that supplies charge carriers (electrons or holes) is called the emitter. The emitter is always forward biased w.r.t. Base so that it can supply a large number of majority carriers. In Figure 2 (i), the emitter (p-type) of pnp transistor is forward biased and supplies hole charges to its junction with the base. Similarly, in Figure 2 (ii), the emitter (n-type) of npn transistor has a forward bias and supplies free electrons to its junction with the base.
Collector: The section on the other side that collects the charges is called the collector. The collector is always reverse biased. Its function is to remove charges from its junction with the base. In Figure 2 (i), the collector (p-type) of pnp transistor has a reverse bias and receives hole charges that flow in the output circuit. Similarly, In Figure 2 (ii), the collector (n-type) of npn transistor has reverse bias and receives electrons.
Base: The middle section which forms two pn junctions between the emitter and collector is called the base. The base emitter junction is forward biased allowing low resistance for the emitter circuit. The base-collector junction is reverse biased and provides high resistance in the collector circuit.
Before studying action of transistor, some important facts about transistor should be keep in mind.
- The transistor has two pn junctions i.e. it is like two diodes. The junction between emitter and base may be called emitter-base diode or simply the emitter diode. The junction between the base and collector may be called collector-base diode or simply collector diode.
- The emitter diode is always forward biased whereas collector diode is always reverse biased.
- Three regions of transistor are emitter, base and collector. The base is much thin as compared to emitter while "collector is wider than both. To keep it simple it is assumed emitter and collector is considered to be of same size.
- The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base. The base is lightly doped and very thin it passes most of the emitter injected charge carriers to the collector. The collector is moderately doped.
- The resistance of emitter diode (forward biased) is very small as compared to collector diode (reverse biased). Therefore, forward bias applied to the emitter diode is generally very small whereas reverse bias on the collector diode is much higher.
Working of Transistor
(i) Working of npn transistor: As discussed in the npn transistor emitter-base junction is forward biased to emitter-base junction and collector-base junction is reverse biased. The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current Ie.
Ie =Ib + Ic
Figure 3: n-p-n transistor
As these electrons flow through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with holes to constitute base current Ib. The remainder i.e. more than 95% cross over into the collector region to constitute collector current Ic In this way, almost the entire emitter current flows in the collector circuit. It is clear that emitter current is the sum of collector and base currents i.e.
(ii)Working of pnp transistor: In the pnp transistor emitter-base junction is forward biased to emitter-base junction and collector-base junction is reverse biased. The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current Ie. As these holes cross into n-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, therefore, only a few holes (less than 5%) combine with the electrons.
Figure 4: p-n-p transistor
The remainder (more than 95%) cross into the collector region to constitute collector current Ic. In this way, almost the entire emitter current flows in the collector circuit. It may be noted that current conduction within pnp transistor is by holes. However, in the external connecting wires the current is still by electrons.
Thus the input circuit (i.e. emitter-base junction) has low resistance because of forward bias whereas output circuit (i.e. collector base junction) has high resistance due to reverse bias. As we have seen, the input emitter current almost entirely flows in the collector circuit. Therefore a transistor transfers the input signal current from a low resistance circuit to a high-resistance circuit.
Transistor’s Configuration:-
1) Common Base configuration(C.B)
2) Common emitter Configuration(C.E)
3) Common Collector Configuration(C.C)
COMMON BASE CONFIGURATION
- The notation and symbols of pnp and npn transistors are given below:
Figure 5: PNP CB and NPN CB (Ref. 2)
- Here the base is common to both the input and output sides of the configuration.
- The flow of holes will govern the direction of current.
- Hence, Ic = Ib + Ie
Where Ic, Ib, Ie are the collector, base and emitter currents respectively.
- The graphical symbol of the PNP common base configuration is
Figure 6: PNP common base(Ref. 2)
- The arrow in the above symbol shows the direction of emitter current in the device.
- Now, to study the behavior of the device we require two characteristics:
Input Characteristic Curve
Figure 7: Input Characteristic Curve (Ref. 2)
- It is the relation between the input current IE to the input voltage VBE for various levels of output voltage VCB.
- It is also known as driving point characteristics.
Output Characteristic Curve
Figure 8: Output Characteristic Curve (Ref. 2)
- It is the relation between the output current IC to the output voltage VCB for various levels of input current IE.
- It is also known as collector set of characteristics.
- It has three basic regions:
- Active Region
- Here, base-emitter junction is forward biased and collector-base junction is reverse biased.
- As input current IE increases above zero, output current IC increases to a magnitude equal to IE as determined by the basic transistor current relationship.
- So the first approximation determined by the curve is
IC ≈ IE
2. Cut-off Region
- It is defined as the region where the collector current IC is equal to 0A.
- Here, the base-emitter junction and the collector-base junction both are in reverse bias.
3. Saturation Region
- It is the region that lies towards the left of VCB = 0V.
- Here, the base-emitter junction and the collector-base junction both are in forward bias.
COMMON EMITTER CONFIGURATION
- The notation and symbols of npn and pnp transistors are given below:
Figure 9: NPN CE and PNP CE (Ref. 2)
- In the above figure all the currents are shown in their actual conventional directions.
- The current relation developed earlier is still applicable,
IE = IB + IC
Where IE , IB , IC are the collector, base and emitter currents respectively.
- The graphical symbol of the PNP common emitter configuration is
Figure 10: PNP common emitter (Ref. 2)
- Now, to study the behavior of the device we require two characteristics:
Input Characteristic Curve
Figure 11: Input Characteristic Curve (Ref. 2)
- It is the graph between the input current IB to the input voltage VBE for a range of values of output voltage VCE.
- Note that the magnitude IB of is in micro amperes and that of IC is in milli amperes.
Output Characteristic Curve
Figure 12: Output Characteristic Curve (Ref. 2)
- It is the graph between the output current IC to the output voltage VCE for a range of values of input current IB.
- It has three basic regions:
Active Region
- Here, the base-emitter junction is forward biased and collector base junction is reverse biased.
- These are the same conditions that existed in the active region of the common base configuration.
- This can be employed for voltage, current or power amplification.
Cut-off Region
- Here IC is not equal to zero when IB is zero.
- For linear amplification purposes, it is defined as IC = ICEO .
- The region below IB = 0µA is to be avoided for undistorted output signal.
- When the transistor is used as a switch, the condition should be ideally IC = 0mA for a chosen VCE voltage.
Saturation Region
- It is the region that lies towards the left of VCE = 0V.
COMMON COLLECTOR CONFIGURATION
- The notation and symbols of npn and pnp transistors are given below:
Figure 13: NPN CC and PNP CC (Ref. 2)
- In the above figure all the currents are shown in their actual conventional directions.
- It is used for impedence matching purposes as it has high input impedence and low output impedence.
- It can be designed using common emitter characteristics.
- The output characteristics of common collector is same as that of common emitter configuration for all practical purposes.
- The output characteristics are a plot between IE versus VCE for all values of IB.
- The input current of common collector is same as that of common emitter configuration.
- Here the region of operation will ensure that maximum ratings are not being exceeded and output ratings have minimum distortion.
Figure 14: Output Characteristic Curve (Ref. 2)
- The characteristics specifying the minimum VCE that can be applied without entering the non-linear region is saturation region.
- The maximum power dissipation is given by,
P = VCE . IC
Key Takeaways
- A transistor consists of two pn junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types.
- There are two types of transistors n-p-n transistor and p-n-p transistor.
- The emitter diode is always forward biased whereas collector diode is always reverse biased.
- Three regions of transistor are emitter, base and collector. The base is much thin as compared to emitter while "collector is wider than both. To keep it simple it is assumed emitter and collector is considered to be of same size.
- The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base. The base is lightly doped and very thin it passes most of the emitter injected charge carriers to the collector. The collector is moderately doped.
- Transistor’s Configuration:-Common Base configuration(C.B), Common emitter Configuration(C.E), Common Collector Configuration(C.C)
BJT as a switch
Suppose we had a lamp that we wanted to turn on and off with a switch. Such a circuit would be extremely simple, as in the figure below.
a- mechanical switch
b- NPN transistor switch
c- PNP transistor switch
Figure 15. Transistor as switch
Suppose we insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp. Remember that the controlled current through a transistor must go between collector and emitter.
Figure 16. Transistor: a) cut-off lamp b) saturated, lamp on.
The choice between NPN and PNP is really arbitrary. All that matters is that the proper current directions are maintained for the sake of correct junction biasing.
In the above figures, the base of either BJT is not connected to a suitable voltage, and no current is flowing through the base. Consequently, the transistor cannot turn on. Therefore, connect a switch between the base and collector wires of the transistor as in figure.
If the switch is open as in figure (a), the base wire of the transistor will be left “floating” and there will be no current through it. In this state, the transistor is said to be cut-off.
If the switch is closed as in figure (b), current will be able to flow from the base to the emitter of the transistor through the switch. This base current will enable a much larger current flow from the collector to the emitter, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated.
BJT as an Amplifier
Transistor raises the strength of a weak signal and hence acts an amplifier. The transistor amplifier circuit is shown in the figure below.
Figure 17. Transistor as an Amplifier
The transistor has three terminals namely emitter, base and collector. The emitter and base of the transistor are connected in forward biased and the collector base region is in reverse bias. The forward bias means the P-region of the transistor is connected to the positive terminal of the supply and the negative region is connected to the N-terminal and in reverse bias just opposite of it has occurred.
Vee is applied to the input circuit along with the input signal to achieve the amplification. The DC voltage VEE keeps the emitter-base junction under the forward biased condition regardless of the polarity of the input signal and is known as bias voltage.
When a weak signal is applied to the input, a small change in signal voltage causes a change in emitter current this change is almost the same in collector current because of the transmitter action.
In the collector circuit, a load resistor RC of high value is connected. When collector current flows through such a high resistance, it produces a large voltage drop across it. Thus, a weak signal (0.1V) applied to the input circuit appears in the amplified form (10V) in the collector circuit.
Input Resistance
When the input circuit is forward biased, the input resistance will be low. The input resistance is the opposition offered by the base-emitter junction to the signal flow.
Hence, it is the ratio of small change in base-emitter voltage (ΔVBE) to the resulting change in base current (ΔIB) at constant collector-emitter voltage.
Input resistance, Ri=ΔVBE/ΔIb
Where Ri = input resistance, VBE = base-emitter voltage, and IB = base current.
Output Resistance
The output resistance of a transistor amplifier is very high. The collector current changes very slightly with the change in collector-emitter voltage.
The ratio of change in collector-emitter voltage (ΔVCE) to the resulting change in collector current (ΔIC) at constant base current.
Output resistance = Ro=ΔVCE/ΔIC
Where Ro = Output resistance, VCE = Collector-emitter voltage, and IC = Collector-emitter voltage.
Current gain
It is the ratio of change in collector current (ΔIC) to the change in base current (ΔIB).
Current gain, β=ΔIC/ ΔIB
Voltage Gain
It is the ratio of change in output voltage (ΔVCE) to the change in input voltage (ΔVBE).
Voltage gain, AV=ΔVCE/ΔVBE
Power Gain
It is the ratio of output signal power to the input signal power.
= current gain x voltage gain
Biasing refers to the application of dc voltages to establish a fixed level of current and voltage. The proper flow of zero signal collector current and maintenance of proper collector emitter voltage for the passage of signal is called Transistor Biasing. The circuit which provides transistor biasing is called Biasing circuit.
The need for biasing circuit is that if a signal of low voltage is given as input it has to amplify and meet these two conditions:
- The input voltage should exceed cut-in voltage for transistor to be ON.
- BJT should be in active region to be operated as amplifier.
Transistor Regions Operation:
- Linear-region operation:
Base–emitter junction forward-biased
Base–collector junction reverse-biased
2. Cutoff-region operation:
Base–emitter junction reverse-biased
Base–collector junction reverse-biased
3. Saturation-region operation:
Base–emitter junction forward-biased
Base–collector junction forward-biased
Types of Bias
- Fixed Bias
In fixed bias since it is C-E configuration
1st step: Locate capacitors and replace them with an open circuit
2nd step: Locate 2 main loops
BE loop (input loop)
CE loop(output loop)
Base – emitter loop
Collector emitter loop
Problem:
Determine the following for the fixed bias configuration:
i) IBQ and ICQ
Ii) VCE Q
Iii) VB and VC
Iv) VBC
Sol:
IBQ = Vcc – Bbe/Rb = 12 – 0.7/ 240K = 47.08 µ A
ICQ = β I BQ = (50) ( 47.08 µ) = 2.35 mA
VCE Q = VCC – Ic Rc
= 12 – (2.35 mA)(2.2 KΩ)
= 6.83 V
VB = VBE = 0.7 V
VC = VCE = 6.83 V
Using double -subscript notation yields
VBC = VB – VC = 0.7 V – 6.83 V = -6.13 V
The negative sign reveals that the junction is reverse biased as it should be for linear amplification.
Transistor Saturation
a) Actual b) Approximate
IC (sat)
Fixed bias configuration
Load Line Analysis
(a) – circuit
(b) Characteristics
Emitter bias Configuration
BJT bias circuit with emitter resistor.
DC equivalent
Base – Emitter loop
Collector -Emitter configuration
Problem:
For the emitter-bias network find the following:
a) IB
b) IC
c) VCE
d) VC
e) VE
f) VB
g) VBC
I B = Vcc – VBE/ RB + (β +1) RE = 20 V – 0.7V / 430k Ω + (51)(1kΩ) = 40.1 μ A
Ic = β Ib
= (50) ( 40.1 μ A )
= 2.01 mA
VCE = VCC – Ic( RC + RE)
= 20 V – (2.01mA)(2kΩ + 1kΩ)
= 13.97 V
Vc = Vcc – Ic Rc
= 20 V – (2.01mA)(2kΩ)
= 15.98 V
VE = IE RE = 2.01 m A x 1 k Ω
= 2.01 V
VB = VBE + VE
= 0.7 + 2.01 V = 2.71 V
VBC = VB – VC = 2.71 – 15.98 = -13.27 V
The addition of the emitter resistor to the dc bias of the BJT provides improved stability, that is, the dc bias currents and voltages remain closer to where they were set by the circuit when outside conditions, such as temperature and transistor beta, change.
Saturation level
Load Line Analysis
Voltage Divider Bias
Approximate analysis
Transistor Analysis
Load Line Analysis
Determine the levels of ICQ and VCEQ for voltage divider configuration using exact and approximate techniques and compare solutions.
β . RE ≥ 10 R2
(50)(1.2 k Ω) ≥ 10(22 k Ω)
60kΩ≥ 220kΩ
Rth = R1 || R2 82 kΩ || 22 kΩ = 17.35 kΩ
Eth = R2 Vcc/ R1 + R2 = 22kΩ (18V)/ 82kΩ + 22kΩ = 3.81 V
IB = Eth – VBE/ Rth + (β +1) RE = 3.81 V – 0.7V / 17.35 kΩ + (51 )(1.2kΩ) = 3.11/ 78.55 kΩ = 39.6 μ A
ICQ = β IB = (50)(39.6μA) = 1.98 mA
VCEQ = Vcc – IC(RC + RE)
= 18V – (1.98mA)(5.6kΩ + 1.2 kΩ)= 4.54V
Common Emitter amplifier
The common emitter amplifier is a three basic single-stage bipolar junction transistor and is used as a voltage amplifier. The input of this amplifier is taken from the base terminal, the output is collected from the collector terminal and the emitter terminal is common for both the terminals. The basic symbol of the common emitter amplifier is shown below.
The circuit diagram shows the working of the common emitter amplifier circuit which consists of voltage divider biasing used to supply the base bias voltage as per the necessity. The voltage divider biasing has a potential divider with two resistors connected in a way that the midpoint is used for supplying base bias voltage.
Here R1 resistor is used for the forward bias, the R2 resistor is used for the development of bias, the RL resistor is used at the output it is called as the load resistance. The RE resistor is used for thermal stability. The C1 capacitor is used to separate the AC signals from the DC biasing voltage and the capacitor is known as coupling capacitor.
If R2 resistor increases, then there is an increase in the forward bias. The alternating current is applied to the base of the transistor of the common emitter amplifier circuit then there is a flow of small base current. Hence there is a large amount of current flow through the collector with the help of the RC resistance.
The voltage near the resistance RC will change because the value is very high and the values are from the 4 to 10kohm. Hence there is a huge amount of current present in the collector circuit which amplified from the weak signal, therefore common emitter transistor work as an amplifier circuit.
The current gain of the common emitter amplifier is defined as the ratio of change in collector current to the change in base current.
β = ∆ Ic/ ∆ Ib
The voltage gain is defined as the product of the current gain and the ratio of the output resistance of the collector to the input resistance of the base circuits.
Av = β Rc/Rb
Characteristics
- The voltage gain of a common emitter amplifier is medium
- The power gain is high in the common emitter amplifier
- There is a phase relationship of 180 degrees in input and output
- In the common emitter amplifier, the input and output resistors are medium.
Applications of Common Emitter Amplifier
- The common emitter amplifiers are used in the low-frequency voltage amplifiers.
- These amplifiers are used typically in the RF circuits.
- In general, the amplifiers are used in the Low noise amplifiers
Advantages of Common Emitter Amplifier
- The common emitter amplifier has a low input impedance and it is an inverting amplifier
- The output impedance of this amplifier is high
- This amplifier has the highest power gain when combined with medium voltage and current gain
- The current gain of the common emitter amplifier is high
Disadvantages of Common Emitter Amplifier
- In the high frequencies, the common emitter amplifier does not respond
- The voltage gain of this amplifier is unstable
- The output resistance is very high in these amplifiers
- In these amplifiers, there is a high thermal instability
- High output resistance
Common Base Amplifier
Common Base Amplifier has the following:
- The input is given at the Emitter of the BJT.
- The output is taken from the Collector of the BJT.
- The base terminal, which is common to both input and output, is often connected to ground.
Common Base Amplifier Circuit
The common base amplifier circuit consists of voltage divider bias configuration.
Base of the BJT is the common terminal and is at AC ground due to the capacitor. The input signal is given to the emitter through capacitor coupling. Output is taken at the collector and the load is capacitively coupled to the collector.
In order to determine the characteristics, we need to construct an AC equivalent model of the common base amplifier.
Voltage Gain
The voltage gain of CB amplifier from emitter (input) to collector (output) is given by
AV = Vout / Vin = Vc / Ve = Ic Rc / Ie (r’e || RE) ≈ Ie Rc / Ie (r’e || RE)
Assuming RE >> r’e, then AV ≈ Rc / r’e
Here, Rc = RC || RL
r’e = AC Emitter Resistance
The voltage gain of a common base amplifier is very high without the phase inversion.
Current Gain
The current gain of CB amplifier is output current divided by the input current. From AC equivalent mode, Ic is the output current and Ie is the input current.
Since Ic ≈ Ie, the current gain Ai ≈ 1.
Input Resistance
The input resistance is the equivalent resistance looking it at the emitter. It is given by
Rin = Vin / Iin = Ve / Ie = Ie (r’e || RE) / Ie = r’e || RE
Usually, RE is much greater than r’e.
If RE >> r’e, then Rin ≈ r’e.
This means that the input resistance of a common base amplifier is usually very low.
Output Resistance
The output resistance is the Thevenin equivalent at the output of the common base amplifier looking back into the amplifier. The AC collector resistance r’c is in parallel with RC and it is usually much larger than RC.
Hence, Rout ≈ RC
Common Collector Amplifier
Resistors R1 and R2 form a simple voltage divider network used to bias the NPN transistor into conduction. Since this voltage divider lightly loads the transistor, the base voltage, VB can be easily calculated by using the simple voltage divider formula as shown.
Voltage Divider Equation
Therefore:
With the collector terminal of the transistor connected directly to VCC and no collector resistance, (RC = 0) any collector current will generate a voltage drop across the emitter resistor RE.
However, in the common collector amplifier circuit, the same voltage drop, VE also represents the output voltage, VOUT. RE depends greatly on IB and the transistors current gain Beta, β.
As the base-emitter pn-junction is forward biased, base current flows through the junction to the emitter encouraging transistor action causing a much larger collector current, IC to flow.
Thus, the emitter current is a combination of base current and collector current as: IE = IB + IC.
However, as the base current is extremely small compared to the collector current, the emitter current is therefore approximately equal to the collector current. Thus IE ≈ IC
As the amplifiers output signal is taken from across the emitter load this type of transistor configuration is also known as an Emitter Follower circuit as the emitter output “follows” or tracks any voltage changes to the base input signal. Thus, VIN and VOUT are in-phase producing zero phase difference between the input and output signals.
Voltage gain:
Vout = Vin x RE/ r’e + RE
Thus Av = Vout/ Vin = Ie x RE/ Ie(r’e + RE)
Since RE is much greater than r’e (r’e + RE) ≈ RE
And the two emitter currents cancel out
Av = Vout/Vin = RE/RE ≈ 1
A common collector amplifier is constructed using an NPN bipolar transistor and a voltage divider biasing network. If R1 = 5k6Ω, R2 = 6k8Ω and the supply voltage is 12 volts. Calculate the values of: VB, VC and VE, the emitter current IE, the internal emitter resistance r’e and the amplifiers voltage gain AV when a load resistance of 4k7Ω is used. Also draw the final circuit and corresponding characteristics curve with load line.
Base biasing voltage, VB
I = VCC/ R1 + R2 = 12/ 5600 + 6800 = 968 µ A
VB = I x R2 = 968 x 10 -6 x 6800 = 6.5 V
Collector voltage, VC. As there is no collector load resistance, the transistors collector terminal is connected directly to the DC supply rail, so VC = VCC = 12 volts.
Emitter biasing voltage, VE
VE = VB – VBE = 6.5 – 0.7 = 5.8 V
Thus
VCE(off) = VCC – VE = 12 – 5.8 = 6.2 V
Emitter Current, IE
IE = VE/RE = 5.8 / 4700 = 1.23 mA
AC Emitter Resistance, r’e
r’e = 25mV/IE = 25mV/1.23mA = 20.3 Ω
Voltage gain, AV
Av = RE/ re + RE = 4700/ 20.3 + 4700 = 99.6%
Comparison of all configuration
Sr.no | Parameter | CB | CE | CC |
1 | Common terminal between I/p & o/p | Base | Emitter | Collector |
2 | Input Current | IE | IB | IB |
3 | Output Current | IC | IC | IE |
4 | Current Gain | α d.c =  |  |  |
5 | Input Voltage. | VEB | VBE | VBC |
6 | Output Voltage. | VCB | VCE | VCC |
Comparison of all Modes
Parameters | Common-Emitter (CE) Amplifier | Common-Base (CB) Amplifier | Common-Collector (CC) Amplifier |
Input resitance | Moderate βre | Low re | High βRE |
Output resistance | High RC | High RC | Low re |
Voltage gain | High  | High  | About 1 |
Current gain | High β | Low, about 1 | High (1+β) |
Power gain | High | Moderate | Low |
Phase shift | 180o | 0o | oo |
Ci = cwi + cbe + cbc( 1-av) Co = cwo + cce + cbc( 1-AV)
= cwi + cbe + cmi = cwo + cce + cmo
Where cmi = input miller capacitance where cmo = o/p miller capacitance
At increasing frequencies, the reactance xc will decrease in magnitude resulting in a shorting effect across the output and a decrease in gain.
For high frequency response, various parasite capacitances (cbe, cbc, cce) of the transistors are included along with the wiring capacitors ( cwi, cwo) for analysis.
For high frequency response, cs, cc, ce are assumed to be in short circuit state.
Input capacitance ci includes wiring capacitance cwi, the transistor capacitance.
Cbe and miller capacitance cmi.
The o/p capacitance co includes wiring cce and miller capacitance cmo
Cmi = cbc (1-av)
Cmo = cbc (1−11−1/av)
For the input network, the – 3db frequency is defined by
Rthi = rsrs // r1r1 // r2r2 // rππ
Ci = cwi + cbc + cmi
Cmi = cbc (1-av)
At very high frequency, the effect of ci is to reduce the total impedance of the parallel combination of r1r1, r2r2 , rπrπ & ci. The route is a reduced level of voltage across ci, a reduction in In and a gain for the system.
Rtho = rc // rlrl // re
c0c0 = cwot + cce + cmo
Cme = cbe ( 1 - 1av)1av)
∵∵ 1 >> yav cmo = cbc
At very high frequency, XCO will decrease and consequently reduced the total impedance of o/p parallel branches. The net result is vo will also decline towards ‘o’ as the reactance xc becomes (zero) or smaller.
The purpose of using transistor as a switch is that the current at the base controls the current present at the collector directly. If the current at the base exceeds the minimum cut-off voltage then transistor acts a close switch otherwise it will remain in open switch condition.
Figure 18. Transistor as Switch
When bias is applied to the base of the transistor both the types in bipolar junction transistor are used as switches. The areas at which the operation of the switch is preferred is either it should be completely in the region called saturation or the cut-off operating region.
Key Takeaways:
A transistor can be extensively used for switching operation either for opening or closing of a circuit.
MOSFETs are three terminal devices with a Gate, Drain and Source and both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available. The main difference this time is that MOSFETs are available in two basic forms:
- Depletion Type – the transistor requires the Gate-Source voltage, ( VGS ) to switch the device “OFF”. The depletion mode MOSFET is equivalent to a “Normally Closed” switch.
- Enhancement Type – the transistor requires a Gate-Source voltage, ( VGS ) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.
The symbols and basic construction for both configurations of MOSFETs are shown below.
The four MOSFET symbols above show an additional terminal called the substrate and is not normally used as either an input or an output connection but instead it is used for grounding the substrate. It connects to the main semiconductive channel through a diode junction to the body or metal tab of the MOSFET.
The line in the MOSFET symbol between the drain (D) and source (S) connections represents the transistors semiconductive channel. If this channel line is a solid unbroken line then it represents a “Depletion” (normally-ON) type MOSFET as drain current can flow with zero gate biasing potential.
If the channel line is shown as a dotted or broken line, then it represents an “Enhancement” (normally-OFF) type MOSFET as zero drain current flows with zero gate potential. The direction of the arrow pointing to this channel line indicates whether the conductive channel is a P-type or an N-type semiconductor device.
The construction of the Metal Oxide Semiconductor FET is different to that of the Junction FET.
Both the Depletion and Enhancement type MOSFETs use an electrical field produced by a gate voltage to alter the flow of charge carriers, electrons for n-channel or holes for P-channel, through the semi-conductive drain-source channel.
The gate electrode is placed on top of a very thin insulating layer and there are a pair of small n-type regions just under the drain and source electrodes.
With the insulated gate MOSFET device no such limitations apply so it is possible to bias the gate of a MOSFET in either polarity, positive (+ve) or negative (-ve).
This makes the MOSFET device especially valuable as electronic switches or to make logic gates because with no bias they are normally non-conducting and this high gate input resistance means that very little or no control current is needed as MOSFETs are voltage controlled devices.
Both the p-channel and the n-channel MOSFETs are available in two basic forms, the Enhancement type and the Depletion type.
MOSFET is another type of field effect transistor. The MOSFET has become one of the most important devices used in design and construction computers
MOSFET [metal oxide semiconductor field effect transistor]
Type of MOSFET
- Depletion type MOSFET
- Enhancement type MOSFET
- Power MOSFET
Enhancement type MOSFET: classified in to two type n. Channel or p. Channel E MOSFET
A) N-channel E MOSFET
B) P-channel E MOSFET
N-channel E MOSFET
Figure 19: N-channel E MOSFET
- A slab of P-type semiconductor is used as substrate. The substrate is sometimes connected to the source otherwise it is brought out as the fourth terminal.
- The drain and source terminal are connected to the n-type doped regions through the metallic contacts.
- The insulating sio2 layer is still present which isolates gate terminal from the substrate.
Effect of the insulting sio2 layer:
Due to the presence of the sio2 layer between gate terminal and n-type channel the i/p impedance of MOSFET is very high this is a desirable fracture of a MOSFET. Due to high i/p impedance the gate current IG= 0 for the d.c operating conditions.
Operation: the operation can be explained with two different operating
- Operation with VGS = 0
- Operation when VGS is +ve
1) Operating with VGS = 0 Volt
If VGS = 0 and a positive voltage is applied between its drain and source, then due to the absence of the n-type channel a zero drain current will result.
2) Operation when Vgs Positive:
The positive potential at the gate terminal will repel the holes present in the p-type substrate.
Figure 20: Operation on N-channel E MOSFET
Formation of induced channel in n-channel enhancement MOSFET
This creates a depletion region near the sio2 insulating layer. But the minority carriers ie the electrons in the p-type substrate will be attracted towards the gate terminal and gather near the surface of sio2 as shown above
As we increase the positive VGS the number of e- gathering near the sio2 layer increases to such an extent that it creates an induced n-channel which connects the n-type doped regions.
The drain current then starts flowing through this induced channel. The value of VGS at which this conduction begins is called as the threshold Vtg. & is indicated.
V-I Characteristics
Characteristics of n-channel enhancement type MOSFET:
Transfer characteristics drain characteristics
Figure 21: Characteristics of n-channel enhancement type MOSFET
P-channel enhancement type MOSFETS:
Figure 22: P-channel enhancement type MOSFETS
The construction of p-channel EMOSFET is exactly opposite to that of a n-channel EMOSFET.
Characteristics:
Drain Characteristics of transfer Characteristics of p-channel E MOSFET
Figure 23:Characteristics of p-channel enhancement type MOSFET
Regions Of Operation, MOSFET As Switch & Amplifier
Operation: the operation can be explained with two different operating
Operation with VGS = 0
Operation when VGS is +ve
Operating with VGS = 0 Volt
If VGS = 0 and a positive voltage is applied between its drain and source, then due to the absence of the n-type channel a zero drain current will result.
Operation when Vgs Positive:
The positive potential at the gate terminal will repel the holes present in the p-type substrate.
Figure 24: Formation of induced channel in n-channel enhancement MOSFET
Formation of induced channel in n-channel enhancement MOSFET
- This creates a depletion region near the sio2 insulating layer. But the minority carriers ie the electrons in the p-type substrate will be attracted towards the gate terminal and gather near the surface of sio2 as shown above
- As we increase the positive VGS the number of e- gathering near the sio2 layer increasesto such an extent that it creates an induced n-channel which connects the n-type doped regions.
- The drain current then starts flowing through this induced channel. The value of VGS at which this conduction begins is called as the threshold Vtg. & is indicated
Effect of increase in the drain to source Vtg.:
Figure 25: Effect of increase in the drain to source Vtg:
Effect of changes in VDS at fixed VGS on the channel width:
- The positive gate to source voltage VDS is kept constant and the drain to source voltage VGS is increased gradually.
- Due to this the gate terminal becomes less and less positive with respect to the drain so less number of electron are attracted towards gate terminal & the induced channel becomes narrow.
- Eventually the channel width will be reduced to a point of pinch off and the saturation condition will be established which is same as that in a JFET
- That means any further increase in VDS at the fixed value of VGS will not affect the saturation level of ID unless breakdown condition are encountered.
Symbols:
1) N-channel EMOSFET
2) P-channel EMOSFEJ
BJT is a bipolar device [minority & majority carriers both contributes to the current flow]
E MOSFET is a unipolar device ie current flows only due to the majority carriers
BJT thermal runaway can damage BJT thermal runway does not take place.
BJT low i/p impedance, E MOSFET → High i/p impedance
- Voltage ampɤ :
- Current and vtg gain
Current gain AI is defined as the ratio of o/p current
Vtg gain is given by
Av = Vo/V
In this expression if Ro is very small as compared to RL then Av≈ A
Thus we can say that the overall gain of the ampere Av will approach its open circuit value A if Ro << RL
Problem: An amplifier has a signal i/p Vtg. Vi of 0.25v and draws 1mA from the source. The amɤ delivers & v to a load at 10 mA. Determine the current Vtg and power gains. Also find the i/p resistance of this ampɤ what must be the open ckt. Vtg of the source Vs to provide an ampɤ i/p VtgVi of 0.25 v when the internal resistance of the source is 50 Ω?
Given Vi = 0.25v, Ii = 1 Ma, Vo = 8 v, Io = 10 mA, Rs = 50 Ω.
Solution:
Vtg gain Av = 
Current gain 
Power gain AP = Av × AI = 32 × 10 = 320
Input resistance Ri = 
Open circuit Vtg of Vs
Vs = 0.3v
MOSFET as switch:
- MOSFET can act as a switch jest like a transistor a typical switching Ckt using N-channel EMOSFET shown below
- A positive going i/p pulse (high i/p) turns on the EMOSFET. Maxm drain current flows and the o/p Vtg drops from +VDD to RD ID (on). So the EMOSFET acts as closed switch.
- Similarly when the i/p is low (zero) no drain current and hence the o/p is equal to +VDD. So the EMOSFET acts as open s/w
EMOSFET as an AMP:
The purpose of amplifier is to amplify the weak signal faithfully. For amplification MOSFET should operate in a saturation region.
Figure 26: EMOSFET as an AMP
N-channel E-MOSFET Ampɤ
- It can operate with positive gate and drain Vtg where as in p-channel with –ve gate and drain Vtg
- For amplification MOSFET should operate in saturation region where the drain current ID remains constant
- The threshold Vtg (VT) is the minimum gate Vtg (VGS) required to induce the channel between source to drain
- Fig shows single stage common source ampɤ using Vtg bias method
- Vin is A.c. Signal
- Resistors R1 ,R2 and R5 provides the proper and stabilized operating point
- C in i/p coupling capacitor where is coat is the o/p coupling capacitor which blocks the d.c signal
- Cs is the bypass capacitor so that signal available at source terminal. Never pass through Rs otherwise o/p Vtg reduces.
Key Takeaways
- MOSFET is another type of field effect transistor. The MOSFET has become one of the most important devices used in design and construction computers
- MOSFET [metal oxide semiconductor field effect transistor]
- Type of MOSFET are Depletion type MOSFET, Enhancement type MOSFET and Power MOSFET
- Enhancement type MOSFET: classified in to two type n. Channel or p. Channel E MOSFET
- The operation can be explained with two different operating; Operation with VGS = 0 and Operation when VGS is +ve
- Vtg gain is given by
- MOSFET can act as a switch jest like a transistor a typical switching.
Fixed Bias Configuration
Fig 27 Fixed Biasing Circuit JFET
Fixed dc bias is obtained using a battery VQG. This battery ensures that the gate is always negative with respect to source and no current flows through resistor RG and gate terminal that is IG =0. The battery provides a voltage VGS to bias the N-channel JFET, but no resulting current is drawn from the battery VGG. Resistor RG is included to allow any ac signal applied through capacitor C to develop across RG. While any ac signal will develop across RG, the dc voltage drop across RG is equal to IG RG i.e., 0 volt.
The gate-source voltage VGS is then
VGS = – VG – Vs = – VGG – 0 = – VGG
The drain -source current ID is then fixed by the gate-source voltage as determined by equation.
This current then causes a voltage drop across the drain resistor RD and is given as
VRD = ID RD and
Output voltage
Vout = VDD – ID RD
Self-Bias Configuration
Fig 28 Self-Biasing for JFET
This is the most common method for biasing a JFET. Self-bias circuit for N-channel JFET is shown in figure.
Since no gate current flows through the reverse-biased gate-source, the gate current IG = 0 and, therefore,
vG = iG RG = 0
With a drain current ID the voltage at the S is
Vs= ID Rs
The gate-source voltage is then
VGs = VG – Vs = 0 – ID Rs = – ID Rs
So, voltage drop across resistance Rs provides the biasing voltage VGg and no external source is required for biasing and this is the reason that it is called self-biasing.
The operating point (that is zero signal ID and VDS) can easily be determined from equation and equation given below:
VDS = VDD – ID (RD + RS)
Thus, dc conditions of JFET amplifier are fully specified. Self-biasing of a JFET stabilizes its quiescent operating point against any change in its parameters like transconductance. Let the given JFET be replaced by another JFET having the double conductance then drain current will also try to be double but since any increase in voltage drop across Rs, therefore, gate-source voltage, VGS becomes more negative and thus increase in drain current is reduced.
References:
1. Integrated Devices & Circuits by Millman & Halkias, TMH Publications.
2. Electronics Devices and Circuit Theory by R. Boylestad & L. Nashelsky, Pearson Publication
3. Electronic Communication System by G. Kennedy, TMH Publications.
4. Basic Electronics by Sanjeev Kumar & Vandana Sachdeva, Paragaon International Publication
