Unit 3

Question 1

bipolar junction transistor (BJT)

Answer 1

three terminal device either p n p, or n p n junction

depends on the active participation of both the majority carriers, and the minority carriers

transfer + resistor

as current is transferred from a low to a high-resistance circuit.

Question 2

emitter base and collector

Answer 2

The Emitter (E) supplies charge carriers (either electrons or holes) and it is a heavily doped region.
area is moderate in size.

➢The Base (B) middle portion of the transistor that forms two PN
a lightly doped
region. It is the smallest in width

➢The Collector (C) collects charge carriers (either electrons or holes) is a moderately doped region.
the widest of all the three regions as maximum heat is dissipated in this
region.

Question 3

common base(CB) operation

Answer 3

• Since the n -type material is very thin and lightly
  doped, small number of these (holes) will
  recombine with the electrons in base region

= base current (microamperes).

large number of these majority carriers from
the emitter region will diffuse across the reverse biased junction into the p -type material to the collector terminal.

Question 4

Transistor works as an
Amplifier

Answer 4

operation
Active region(commonly
used)

Emitter base junction

Forward

Collector base junction

Reverse

Question 5

digital OFF switch

Answer 5

operation
cut-off region

Emitter base junction

Reverse

Collector base junction

Reverse

Question 6

digital ON switch

Answer 6

operation

Saturation region

Emitter base junction

forward

Collector base junction

forward

Question 7

Attenuator

Answer 7

operation

Inverse Active region (rarely used)

Emitter base junction

Reverse

Collector base junction

Reverse

Question 8

Npn and pnp nmenonic

Answer 8

npn => “not pointing in”
pnp => “pointing in permanently”

Question 9

pnp and npn operate in …

Answer 9

active region

Question 10

biasing the emitter base junction (EB Jn) and CB are

Answer 10

forward and reverse

Question 11

how to decrease base width

Answer 11

greater Vcb => collector base depletion regions increases thus base width decreased

Question 12

Base Narrowing Early Effect

Answer 12

• lesser chance for recombination within the “smaller” base region.
• The charge density is increased across the base, and the current of minority carriers across the collector-base junction increases.

➢The Early effect is an increase in the collector or “output“ current with
increasing collector emitter V

Question 13

what is the range of: a

Answer 13

0.9 to 0.998

Question 14

for CE config what is realation btw VCE and IB

Answer 14

VCE is increased, causes a decrease in the current IB

Question 15

for CB config what is relation bte VCE and IE

Answer 15

VCB is increased, it causes an increase in the emitter current

Question 16

CE active region

Answer 16

B-E junction is forward biased. Collector voltage is
greater than the base voltage which reverse biases the C-B junction.

VCE > VBE
In this region, IC depends on IE
and is almost independent of VCE.

Question 17

CE cutoff region

Answer 17

Both junctions are reverse biased. No base current. Only a
small reverse leakage current between C and E. For practical purposes, current
is assumed to be zero.

Question 18

CE saturation region

Answer 18

The E-B and C-B junctions are forward biased.
Ic varies with VCE and is independent of IB
and β

Question 19

CB cutoff region

Answer 19

The region where the collector current IC is approximately
equal to 0 (IC < ICBO ). The device is basically OFF i.e., there’s no input voltage,
but only reverse – bias output voltage and negligible reverse saturation current

Question 20

CB active region

Answer 20

The operating range of the amplifier. It is noticed that IE is
approximately equal to IC (IE ≈ IC ). BJT is a current controlled device in the active
region and is independent of the output voltage VCB

Question 21

CB saturation region

Answer 21

The region to the left of VCB = 0V. Note the exponential increase
in collector current IC as VCB approaches 0V. In this region, IC depends on VCB but is
independent of I
E

Question 22

Limits of Operation

Answer 22

The limit of operation ensures that the transistor works within its
maximum rating with minimum signal distortion.

Question 23

what is VEsat

Answer 23

Specifies the minimum VCE that can be applied without transistor going to
saturation region.

Question 24

why should the cutoff region below IC = ICEO (= βICBO)

Answer 24

This region must also
be avoided to have minimum distortion .

Question 25

what is biasing

Answer 25

application of DC voltages to establish a fixed level of current and voltage in a transistor. resulting DC current and voltage on the characteristic curve establish an Operating Point. It gives the DC values of IB , IC and VCE in the absence of AC input AKA Quiescent point or Q point where Quiescent means ‘silent’ i.e., in the absence of ac signal. Biasing circuit can be used to set the operating point within the active region

Question 26

how does temp affect affect operating point

Answer 26

As temperature increases, ICEO increases, resulting in changing the Q point. The variation of operating point due to variation in temperature is indicated by stability factor (S). Higher the stability of the circuit, better is the circuit.

Question 27

transistor bias

Answer 27

Biasing is the process of providing DC voltage which helps in the functioning of the circuit. A transistor is biased in order to make the base- emitter junction forward biased and base-collector junction reverse biased, so that it maintains in active region, to work as an amplifier.

Question 28

1. Linear-region (or Active region) operation

Answer 28

Base–emitter junction fwd-biased Base–collector junction reverse-biased

Question 29

2. Cutoff-region operation:

Answer 29

Base–emitter junction reverse-biased Base–collector junction reverse-biased

Question 30

3. Saturation-region operation:

Answer 30

Base–emitter junction forward-biased Base–collector junction forward-biased

Question 31

why is emitter bias circuit used

Answer 31

he emitter – bias configuration is therefore more stable than that of fixed bias for the same change in beta

Question 32

why is voltage bias circuit used

Answer 32

In the previous bias configurations, the bias current ICQ and voltage VCEQ were a function of the current gain  of the transistor. ➢ Since,  is temperature sensitive, especially for silicon transistors, and the actual value of beta is usually not well defined. ➢ it is necessary to develop a bias circuit that is less dependent on, or independent of, the transistor beta. ➢ Thus, Voltage divider bias configuration is preferred.

Question 33

what is design operators

Answer 33

is one where a current and/or voltage may be specified and the elements required to establish the designated levels must be determined. ➢ This synthesis process requires a clear understanding of the characteristics of the device, the basic equations for the network, and a firm understanding of the basic laws of circuit analysis, such as Ohm’s law, Kirchhoff’s voltage law, and so on

Question 34

Operational Amplifier

Answer 34

a very high gain differential amplifier. very high input impedance. Ideally its value is infinity has a low output impedance. Ideally its value is zero finds applications in filters and oscillators

Question 35

When the input signal is applied to the inverting ....

Answer 35

the output is phase shifted by 180

Question 36

Input Offset Voltage:

Answer 36

differential DC voltage that must be applied between the input terminals to make the output offset voltage zero. Ideally its value is 0v. Its value is 1 mv to 6mv for IC 741.

Question 37

Output offset voltage:

Answer 37

small DC voltage that appears at its output even when the input terminals are shorted together and no input signal is applied. This offset is caused by internal mismatches in the op-amp, such as differences in the transistor parameters or imbalances in the internal circuitry. Ideally its value is 0v.

Question 38

Input Resistance

Answer 38

effective resistance seen at its input terminals. It is generally measured in open-loop condition. Its value is 2MΩ for IC 741 under open loop condition.

Question 39

Output resistance

Answer 39

is the resistance seen at the output terminal when the op-amp is configured in an open-loop condition ts value is 75Ω for IC 741.

Question 40

Gain Bandwidth

Answer 40

The frequency at which the gain drops by 3 dB is known as the cutoff frequency 𝑓𝑐 of the Op-Amp. Unity-gain frequency = gain–bandwidth product of the op-amp. Ideally = infinity. Its value is 1MHz for IC741

Question 41

Skew Rate

Answer 41

The maximum rate at which amplifier output can change in volts per microsecond when driven by a large step-input signal. Ideal value of slew rate is infinity. It is 0.5 V/μs for IC741

Question 42

Common Mode Rejection Ratio

Answer 42

ratio of differential gain to the common mode gain of the amplifier. Ideally, CMRR is infinity. its value is 90 dB for IC741

Question 43

Negative feedback in Op-amp

Answer 43

open-loop gain=very high. 2 x10^5 for IC 741. unsuitable for linear applications. Negative feedback decreases the gain open-loop gain is not stable and varies with temp, supply voltage and f, negative its possible 3. The open-loop bandwidth is very small making it unsuitable for any practical application. With negative BW increases.

Question 44

Virtual ground Concept

Answer 44

Vi is small and taken as 0, thus can imagine its virtual short circuited, or virtual ground

Question 45

inverting amplifier

Answer 45

input given to inverting amplifier, most widely used constant gain amplifier circuit output= input*fixed constant gain by input resistor