UNIT 2

1. Explain BJT?

*BJT: -The BJT is constructed with the three draped semiconductors region’s separate by two Pn junctions as shown below

-Basic epitaxial planner structures.

-Three terminal with region’s are called emitter, base and collector.

-The physical representation of the two types of BJT’s,

One type consists between two regions separated by a P region (npn) and other type consists of two p regions separated by an n region (pnp).

-The Pn junction joining the base region and the emitter region is called the base emitter junction.

-The Pn junction joining the base region and the collector region is called the base collector junction.

-The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.

*Base Transistor Operation: -

        In order for the transistor to operate properly as an amplifier the two pn junction must be correctly biased with the external D.C vtg.

-The next figure shows the proper bias arrangement for both the npn and pnp transistors for active operation as an amplifier.

-In both the cases the base emitter

(BE) junction is forward biased & the base collector junction (BC) junction is reverse biased

• As from above figure consider n-p-n transistor. The forward bias from base to emitter narrow’s the BE depletion region and the reverse bias from base to collector widens the BC depletion region shown in figure.
• The heavily doped N-TYPE emitter region is full with conduction band(frep)  electron’s that easily diffuse through the forward biased BE junction into the p-type base region where they become minority carrier’s same as forward biased diode region
• The base region is lightly doped & very thin so that it has a very limited number of holes.
• Those only a small percentage of all the e-flowing the BE junction can combine with the available holes in the base.
• The relatively few recombined flow out of the base lead as valance electrons, forming as small base current.
• Most of the e flowing from the emitter into the thin lightly dooped base region do not recombine but diffuse into the BC depletion region.
• The BC depletion region diffuse e is being pulled across the reverse biased BC junction by the attraction of the collector supply vtg.
• The electrons now move through the collector region, out through the collector lead into the +ve terminal of the collector vtg source. This forms the collector electrons current.
• The collector current is much larger than the base current.
• This is the reason transistor exhibit current gain.

2.      What is transistor current?

Transistor Current:-

Transistor’s Characteristic’s and parameters:-

-The transistors is connected to d.c bias vtg for both the npn&pnp types VBB forward biases the base emitter junction &Vcc reverse biases the base collector junction.

3.      Explain CE configuration?

Common Emitter (CE) configuration

The important points about the CE

Configurations are as follows: the emitter acts as a common terminal between I/p and output. The /p Vtg. Is called between base and emitter

Hence VBE is the I/p Vtg. And IB is the input current.

The O/p is taken between the collector and emitter. There for VCE is the O/p Vtg. And IC is the O/p current.

In order to operate the transistor in its active region the base emitter (BE) junction is forward biased and collector base junction is reverse biased.

• Current relations in CE Configuration:

IE = IC + IB

Where IC = d.c.IE + ICBO

Rearrange this eqn to get

IC – ICBO = d.c.IE

- = IE = IC + IB

] = IB -

IC ] = IB +

] +

• Current gain : as is the ration of o/p current TC and I/p current IB it called common emitter current amplification factor or simply current gain. Thus transistor acts as current amplifier.

The value of is match higher than

=

= = =

IC = IB + ……..  I

But =

1 + = + 1

1 + =

1 + =

Put in eqn I

IC = IB + (1+ ICBO above eqn can be expressed as

IC = IB + ICEO

Where ICEO is the reverse saturation current for the CE configuration which is given by

ICEO = (1+ ) ICBO

• Reverse leakage current :

The Reverse leakage current of a transistor operating

In the LE configuration is denoted by

ICEO = (1 + ) ICBO

As the value of

1. Match greater than 1  ICEO >>>> ICBO
2. As IC =

Put IB = 0

IC = (1 +

The Reverse leakage current (ICEO)

Increases with increase in the temperature this current flows in the same direction as that of IC there for the collector current IC will increase with increase in temperature even when IB is constant

This is called as thermal instability so in CE configuration thermal stabilizing CKT must be included.

• Relation between IC and IB (Current gain )

We know that

IC

Though ICEO is large it is much smaller as compared to. Therefor the eqn for IC gets modified as

C =

• Relation betn& :

We know that

but

= =

=

Similarly we can obtain the expression for d.c in terms of as follows

d.c =    but

=

Multiply & divide numerator & denominator for by IE

d.c =

d.c =

=

4.Explain FET? What are the types of FET?

The FET consists of a semiconductor channel with electrodes at either end referred to as the drain and the source.

A control electrode called the gate is placed in very close proximity to the channel so that its electric charge is able to affect the channel.

In this way, the gate of the FET controls the flow of carriers (electrons or holes) flowing from the source to drain. It does this by controlling the size and shape of the conductive channel.

The semiconductor channel where the current flow occurs may be either P-type or N-type. This gives rise to two types or categories of FET known as P-Channel and N-Channel FETs.

In addition to this, there are two further categories. Increasing the voltage on the gate can either deplete or enhance the number of charge carriers available in the channel. As a result, there are enhancement mode FET and depletion mode FETs.

As it is only the electric field that controls the current flowing in the channel, the device is said to be voltage operated and it has a high input impedance, usually many megohms. This can be a distinct advantage over the bipolar transistor that is current operated and has a much lower input impedance.

5.What are the types of MOSFET?

Type of MOSFET

1. Depletion type MOSFET
2. Enhancement type MOSFET
3. 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

• 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

1. Operation with VGS = 0
2. 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.

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.

6. Explain the V-I characteristics of MOSFET?

V-I Characteristics

Characteristics of n-channel enhancement type MOSFET :

Transfer characteristics drain  characteristics

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                                                                     

7. Explain MOSFET as a switch ?

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.

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 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.

8. What is Barkhausen Condition ?

Condition that are required to be satisfied to operate the circuit as an oscillator are called as Barkhausen condition.

In oscillator circuit there is no input signal hence the feedback signal it say should be sufficient to maintain the oscillations.

Barkhausen condition states that 

• The loop gain (Aβ) is equal to unity i.e. =1
• The phase shift around the loop is zero or an inteqer multiple of 2, i.e. <βA = 2n

Where n=0,1,2,……..

9. What are the types of oscillators?

Types of Oscillator-  An oscillator circuit is usually made up of transistor. A transistor can work as an oscillator to produce undamped continuous oscillations of any desired frequency it oscillatory and feedback circuits are properly connected to it. The oscillators differ only in the way of feedback to supply losses to the oscillatory circuit. The following are the important oscillators-

• Wein bridge Oscillator
• RC phase shift oscillator
• Hartley Oscillator
• Capitals Oscillator

10. Explain the functional diagram of op-amp?

The block diagram of op-amp consists of :

• Types of inputs

      Non-inverting input

      Inverting input

• Input stage

      It has a dual input.

      It is a differential amplifier which provides balanced output.

      It provides voltage gain.

      Establishes input resistance of op-amp.

• Intermediate stage

      It also has a dual input.

      It is a differential amplifier which provides unbalanced output.

      The output of the input stage becomes input for the intermediate stage.

      Due to direct coupling, the dc voltage is well above 0V.

• Level shifting stage

      The output of the intermediate stage becomes input for the level shifting stage.

      It is an emitter follower with constant current source.

      It is used to shift dc level at the output to 0V wrto ground.

• Output stage

      This is the final stage.

      It’s a push pull amplifier.

It raises swing in output voltage as well as increases current supply capability of op-amp.

11. Explain the characteristics of op-amp?

It has the following characteristics :

1. Infinite input resistance [Ri =∞]
2. Zero output resistance [RO = 0]
3. Infinite voltage gain [AV = ∞]
4. Infinite bandwidth [BW = ∞]
5. Infinite Common Mode Rejection Ratio
6. Infinite slew rate
7. Zero offset [ ie,V1 = V2 , VO =0]
8. The above characteristics do not change with the change in temperature 

12. Explain op-amp as inverting and non-inverting amplifier?

Inverting amplifier

• As seen in the above figure, input is applied to only one terminal i.e. inverting input terminal.
• The other input terminal is supplied 0V i.e. grounded.
• Hence,

V1 = 0V and V2 = Vin

• Therefore output voltage Voutis given by,

Vout = A( - Vin )

Where, A is the voltage gain of op-amp.

• The negative sign implies that the output voltage is 1800 out of phase w.r.to the input voltage.
• Hence, the inverting amplifier amplifies the input signal by voltage gain A and inverts it at the output .

Non-Inverting Amplifier

• As seen in the above figure, input is applied to only one terminal i.e. non-inverting input terminal.
• The other input terminal is supplied 0V i.e. grounded.
• Hence,

V1 = Vin and V2 = 0V

• Therefore output voltage Voutis given by,

Vout =AVin

Where, A is the voltage gain of op-amp.

• Here, the output voltage is more than the input voltage by voltage gain A and are also in phase with each other.