SSathya Priyadharshini. Asst. Prof./EEE 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

DEPARTMENT: EEE SEMESTER : III

SUBJECT CODE: EE2203 SUBJECT NAME: ELECTRONIC

DEVICES &CIRCUITS

UNIT 4-AMPLIFIERS AND OSCILLATORS

PART A (2 Marks)

1. Name the types of feedback amplifiers. (AUC MAY 13, DEC06)

Voltage Series feedback amplifier

Voltage shunt feedback amplifier

Current series feedback amplifier

Current shunt feedback amplifier

2. State piezo electric effect. (AUC AMY 13)

When a mechanical stress is given to a crystal, a potential difference is developed

across a crystal or when a potential difference is applied across a crystal, the

mechanical structure of the crystal is deformed. This effect is called as piezoelectric

effect 7 such a crystal is called piezoelectric crystal.

3. State the condition for oscillators. (AUC MAY 12)

The essential conditions for oscillations are,

1. ǀ A β ǀ = 1, the magnitude of open loop gain must be unity.

2. The total phase shift around the closed loop is zero or 360o.

4. What are the advantages of negative feedback. (AUC MAY 12, MAY 10)

The advantages of negative feedback are,

Stabilization of gain

Increased bandwidth

Decreased distortion

Decreased noise

Increase in input impedance

Decrease in output impedance

5. State Bar Khausen criterion for sustained oscillation(AUC NOV 12, DEC 11, APR09)

Same as Q3

6. List the advantages of crystal oscillator (AUC NOV 12)

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The advantage of crystal is its very high Q as a resonant circuit, which results in good

frequency stability for the oscillator. However since the resonant frequencies of

crystal are temperature dependent, it is necessary to enclose the crystal in a

temperature controlled oven to achieve the frequency stabilization in order of one

part in 1010

7. What are the advantages of differential amplifier?(AUC DEC’11)

8. Define CMRR and write its significance in differential amplifiers. (AUC MAY10, APR

09)

The figure of merit called Common Mode Rejection Ratio(CMRR) defines the ability

of the differential amplifier to reject common mode signal.

9. Mention the operating modes of Differential amplifier. (AUC NOV’10)

Differential amplifier with resistive loading.

Differential amplifier with active loading.

The four differential amplifier configurations are following:

Dual input, balanced output differential amplifier.

Dual input, unbalanced output differential amplifier.

Single input balanced output differential amplifier.

Single input unbalanced output differential amplifier.

10. Mention any two high frequency LC oscillators.(AUC NOV’10)

11. What is positive feedback? (AUC MAY’04)

If the feedback signal φf is inphase with the input signal φs the net effect of feedback

will increase the input signal given to the amplifier φi. Hence φi= φs+ φf. Hence the

input voltage applied to the amplifier is increased thereby increasing the output φo.

This type of feedback is called positive or regenerative feedback.

12. What is negative feedback? (AUC MAY’06)

If the feedback signal φf is out of phase with the input signal φs the net effect of

feedback will decrease the input signal given to the amplifier φi. Hence φi= φs - φf.

Hence the input voltage applied to the amplifier is increased thereby decreasing the

output φo. This type of feedback is called negative or degenerative feedback.

13. Define feedback factor β. (AUC MAY’05)

Feedback factor β is defined as the ratio of φf to φo.

14. What is a differential amplifier?

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An amplifier, which is designed to give the difference between two input signals,

is called the differential amplifier.

15. What is the function of a differential amplifier ?

The function of a differential amplifier is to amplify the difference of two

signal inputs,

i.e., where Ad is the differential gain

16. When two signals V1 and V2 are connected to the two inputs of a

difference amplifier, define a difference signal Vd and common-mode signal Vc

The difference signal Vd is defined as the difference of the two signal inputs, i.e., Vd

= V1 – V2

The common-mode signal Vc is defined as the average of the two signals, I.e., Vc

=1/2 (V1 + V2)

17. What is the differential-mode voltage gain of a differential amplifier? It is given by

Ad = ½ (A1-A2)

18. What is the common-mode gain AC in terms of A1 and A2? It is given by Ac = A1

+ A2

19. Define CMRR.

The common-mode rejection ratio(CMRR) of a differential amplifier is defined as

the ratio of the differential-mode gain to common-mode gain.

CMRR = |Ad|Ac|

20. What are the ideal values of Ad and Ac with reference to the differential

amplifier?

Ideally, Ac should be zero and Ad should be large, ideally infinite.

21. Express CMRR in d B.

CMRR (dB) = 20 log Ad – 20log Ac.

22. What are advantages of differential amplifier? It has high gain and high CMRR.

23. List some applications of differential amplifiers? Used in IC applications, AGC

circuits and phase inverters.

24. Define (i) feedback (ii) positive feedback and (iii) negative feedback.

i. Feedback: The process of combining a fraction of the output (of a

Device-amplifier) back to its input is called feedback.

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ii. Positive Feedback: If the feedback is in phase to the input, it is called positive

feedback. Here

iii. Negative Feedback: When the feedback is in opposition (out of phase) to the

input, it is called negative feedback. Here

25. What loop gain of a feedback amplifier.

In a feedback amplifier, when the signal passes through an amplifier

26. Mention the four connections in Feedback. 1. Voltage series feedback.

2. Voltage shunt feedback 3. Current series feedback. 4. Current shunt feedback.

27. Explain the voltage series feedback.

In this case, the feedback voltage is derived from the output voltage and fed in series

with input signal. The input of the amplifier and the feedback network are in series is

also known as series parallel in parallel, hence this configuration is also known as

series parallel feedback network.

28. Explain the voltage shunt feedback.

The input of amplifier and the feedback network are in parallel and known as parallel

– parallel feedback network. This type of feedback to the ideal current to voltage

converter, a circulating having very low input impedance and very low output

impedance.

29. Explain the current series feedback.

When the feedback voltage derived from the load current and is fed in series with the

input signal, the feedback is said to be current series feedback, the inputs of the

amplifier and the feedback network are in series and the output are also in series.

This configuration is also called as series-series feedback configuration.

30. Explain the current shunt feedback.

When the feedback voltage is derived from the load current and a fed in parallel with

the input signal, the feedback is said to be current shunt feedback. Herein the inputs

of the amplifier and the feedback network are in parallel and the outputs are

in series. This configuration is also known as parallel series feedback

31. Write the effects of negative feedback.

1. The gain becomes stabilized with respect to changes in the amplifier active device

parameters like hfe.

2. The non-linear distortion is reduced there by increasing the signal handling

capacity or the dynamic range of the amplifier.

32. Write the conditions for a circuit to oscillate.

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1. The oscillator circuit should consist of an amplifier and a portion of the output

should be feedback to the input. For sustained oscillations, the feedback voltage

must be in phase with the input, i.e., total phase shift around the loop must be 360°.

2. The amount of energy or power feedback to the input must be sufficient to the

input circuit.

33. Mention the classification of oscillators. According to the frequency determining

networks, 1. RC oscillators

2. LC oscillators

3. Crystal oscillators

34. List the advantages of phase shift oscillator.

1. The phase shift oscillator does not required conductance or transformers.

2. It is suitable for the low frequency range i.e., from a few hertz to several hundred

KHz. The upper frequency is limited because the impedance of RC network may

become so small that it loads the amplifier heavily.

35. Write the disadvantages of Phase shift oscillator.

1. It is necessary to change the C or R in all the three RC networks simultaneously

for changing the frequency of oscillations. This is practically difficult.

2. It is not suitable for high frequencies.

36. Write the main drawback of LC oscillators. 1. The frequency stability is not very

good.

2. They are too bulky and expensive and cannot be used to generate low

frequencies.

37. Define Piezo electric effect.

Certain crystal, when suitable wt, develop a potential difference between opposite

faces, magnitude and polarity of which depends on pressure or tension applied to the

other pair of perpendicular faces. This effect is called Piezo electric effect.

38. Define differential amplifier.

When two or more input is applied to the amplifier and receive the single output is

called differential amplifier.

VO = V1 - V2

40. Define stability.

The variation of input is applied to the system; to get the constant output is called

stability.

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41. Define switch.

A switch is a device which opens or closes the electrical circuit, i.e., can turn on

or off current in an electrical circuit. An ideal switch has zero internal resistance

when it is closed an infinite leakage resistance when it is open

PART B(16 Marks)

1. What do you understand by differential amplifier? Draw the circuit diagram and

explain the working of differential amplifier. Explain the circuit operation at CM & DM

(AUC NOV 12, MAY 13, NOV10)

Differential amplifier

A differential amplifier is a type of electronic amplifier that multiplies the difference

between two inputs by some constant factor (the differential gain).

Differential amplifier symbol

The inverting and non-inverting inputs are distinguished by "−" and "+" symbols

(respectively) placed in the amplifier triangle. Vs+ and Vs− are the power supply

voltages; they are often omitted from the diagram for simplicity, but of course must be

present in the actual circuit.

Differential Amplifiers:

Differential amplifier is a basic building block of an op-amp. The function of a

differential amplifier is to amplify the difference between two input signals.

How the differential amplifier is developed? Let us consider two emitter-biased

circuits as shown in fig. 1.

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Fig. 1

The two transistors Q1 and Q2 have identical characteristics. The resistances of the

circuits are equal, i.e. RE1 = R E2, RC1 = R C2 and the magnitude of +VCC is equal

to the magnitude of –VEE. These voltages are measured with respect to ground.

To make a differential amplifier, the two circuits are connected as shown in fig. 1. The

two +VCC and –VEE supply terminals are made common because they are same.

The two emitters are also connected and the parallel combination of RE1 and RE2 is

replaced by a resistance RE. The two input signals v1 & v2 are applied at the base of

Q1 and at the base of Q2. The output voltage is taken between two collectors. The

collector resistances are equal and therefore denoted by RC = RC1 = RC2.

Ideally, the output voltage is zero when the two inputs are equal. When v1 is greater

then v2 the output voltage with the polarity shown appears. When v1 is less than v2,

the output voltage has the opposite polarity.

The differential amplifiers are of different configurations.

The four differential amplifier configurations are following:

1. Dual input, balanced output differential amplifier. 2. Dual input, unbalanced output

differential amplifier. 3.Single input balanced output differential amplifier. 4. Single

input unbalanced output differential amplifier.

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2. Draw the circuit diagram & explain the principle of operation of following oscillators (i)

RC Phase shift oscillator (ii) any one LC oscillator. (AUC NOV 12, APR09)

The RC Phase Shift Oscillator:

At low frequencies (around 100 KHz or less), resistors are usually employed to

determine the frequency oscillation. Various circuits are used in the feedback circuit

including ladder network.

A block diagram of a ladder type RC phase shift oscillation is shown in fig. 4. It

consists of three resistor R and C capacitors. If the phase shift through the amplifier

is 180º, then oscillation may occur at the frequency where the RC network produces

an additional 180 phase shift.

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A block diagram of a ladder type RC phase shift oscillation is shown in fig. 4. It

consists of three resistor R and C capacitors. If the phase shift through the amplifier

is 180º, then oscillation may occur at the frequency where the RC network produces

an additional 180 phase shift.

For phase shift equal to 180o between Vx and VO, imaginary term of Vx / VO must

be zero.

Therefore,

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This is the frequency of oscillation. Substituting this frequency in Vx / VO expression.

In order to ensure the oscillation, initially |Aβ| >1 and under study state Aβ =1. This

means the gain of the amplifier should be initially greater than 29 (so that Aβ >1) and

under steady stat conditions it reduces to 29.

The Colpitts Oscillator:

Wein bridge oscillator is not suited to high frequencies (above 1MHz). The main

problem is the phase shift through the amplifier.

The alternative is an LC oscillator, a circuit that can be used for frequencies between

1MHz and 500MHz. The frequency range is beyond the frequency limit of most

OPAMPs. With an amplifier and LC tank circuit, we can feedback a signal with the

right amplitude and phase is feedback to sustain oscillations. Fig. 3, shows the circuit

of colpitts oscillator.

The voltage divider bias sets up a quiescent operating point. The circuit then has a

low frequency voltage gain of rc / r'e where rc is the ac resistance seen by the

selector. Because of the base and collector lag networks, the high frequency voltage

gain is less then rc / r'e.

Fig. 4, shows a simplified ac equivalent circuit. The circulating or loop current in the

tank flows through C1 in series with C2. The voltage output equals the voltage across

C1. The feedback voltage vf appears across C2. This feedback voltage drives the

base and sustains the oscillations developed across the tank circuit provided there is

enough voltage gain at the oscillation frequency. Since the emitter is at ac ground the

circuit is a CE connection.

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Most LC oscillators use tank circuit with a Q greater than 10. The Q of the feedback

circuit is given by

Because of this, the approximate resonant frequency is

This is accurate and better than 1% when Q is greater than 1%. The capacitance C is

the equivalent capacitance the circulation current passes through. In the Colpitts tank

the circulating current flows through C1 in series with C2.

Therefore C = C1 C2 / (C1 +C2)

3. Describe the construction of Hartley oscillator and explain the working principle.

Derive the expression for frequency of oscillation & condition for starting of

oscillation. (AUC MAY 12, MAY 13,NOV10, APR09)

Hartley Oscillator:

The figure shows Hartley oscillator when the LC tank is resonant, the circulating

current flows through L1 in series with L2. Thus, the equivalent inductance is L = L1

+ L2.

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4. Explain the operation of voltage series feedback amplifier & current shunt feedback

amplifier. (AUC MAY 12, MAY09)

Voltage Series Feedback

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Current Series Feedback:

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5. Explain the concept of negative feedback in amplifier. Derive the expression for

voltage gain, input impedance and output impedance. (AUC DEC’11)

6. (i)Draw the block diagram of a voltage series feedback amplifier and derive the

equation for input impedance, output impedance and the voltage gain.

(AUC MAY’09)

(ii) Calculate the voltage gain, input and output resistances of a voltage series

feedback amplifier having AV = 300, Ri = 1.5 k, Ro = 50 k and β = 1/15.

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7. Derive the equation for differential mode gain and common mode gain of a

differential amplifier. (AUC NOV’10)

Discuss the various topologies of feedback amplifier. (AUC APR’09)

Voltage Series Feedback

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Current Shunt Feedback:

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Voltage Series Feedback:

Voltage Shunt Feedback:

8. Explain the effects of negative feedback in amplifiers.(AUC APR’09)

9. Draw the circuit of crystal oscillator & explain its operation. (AUC NOV 13)