muti stage amplifier

8/15/2019 muti stage amplifier

1/28

J Srinivasa Rao-Multi Stage Transistor Amplifiers Page 1

Chapter 3 : Multistage Amplifiers

Q 1. What do you understand by multi-stage transistor amplifier?Mention its need.

OR

What are multi-stage amplifiers?

Ans.

A transistor circuit in which a number of amplifier stages are connectedone after another is called as multistage transistor amplifier.

When the output of one amplifier acts as an input to the next amplifierwith suitable coupling device and so on then it is called as cascadedamplifier.

The voltage gain of a multistage amplifier is equal to the product of the

gains of the individual stages. If and A1 ar&thjrhaLt guins,stLpziiaiagaini1inei then overall voltage gain is given by:

Thus if we want to increase the voltage gain., current gain or power gain weneed multistage transistor amplifiers.


2/28


3/28


(i) voltage

(ii) current and

(iii) power.

Ans.

(i) Decibel for expressing voltage : It is 20 times the common logarithm ofvoltage gain

(ii) Decibel for expressing current : It is 20 time.s the common logarithm ofcurrent gain

(iii) Decibel for power: It is 10 times of the common logarithm of power gain

Q 4. Explain in brief the function of a tank circuit in a tuned-voltageamplifier.

Ans.

Tuned voltage amplifier amplify a seleted frequency signal. In this tank circuit very high impedance at resonant frequency (i.e. XL =

Xc) and very small impedance at er frequencies. The resonant frequency of tank circuit can be selected by adjusting the of

variable capacitor connected in the circuit.


4/28


Q 5. State what will happen to the voltage gain of an amplifier if the bypassitor is open circuited.

Ans.

If the by pass capacitor is not put in the circuit, a.c. emitter current willflow through using an ac voltage drop across it.

This voltage produce a feedback effect and there by tie output voltage.

Thus for proper amplification, an emitter bypass capacitor is essential.

Q 6. A multistage amplifier consists of two stages. The voltage gain ofthe s are 30 and 80. Calculate the overall voltage gain in dB.

Ans.

Voltage gain of one stage = 20 log10 30

and Voltage gain of seôond stage = 20 log10 80

Overall voltage gain is 201og10 30 + 20 tog10 80,

Q 7. Which configuration of bipolar transistor is called as emitterfollower r why for what purpose is it used?

Ans.


5/28


Common collector amplifier configuration is called as emitter follower 1this circuit the output voltage developed across the emitter resistor justfollows voltage .

Hence the name emitter follower and it can be redrawn to form collectoras terminal.

Hence the circuit i.also called a common collector amplifier. The voltage gain of this circuit is one, but input impedance is very high

impedance is very low. Therefore, this circuit is basically employed for imJeud1I(. IIIdL

Q 8. What are the different types of configurations used in multistageam circuits?

Ans.

Different types of configurations used in multistage amplitude circuits are

1.Class-A

2.Class-B

3. Class-C

4.Class-D

Q 9. What is the advantages of slagger tuner amplifier?


6/28


Ans.

Stagger tuner amplifier have high selectivity, high gain and desirable band

Q 10. Explain the statement : The cut-off frequencies of single stageamp are influenced by R-C combinations. .

Ans.

The cut-off frequecies of single stage amplifiers are influnced by R-Ccombinatic because :

The first cut off frequency lies in low frequency range, in which theimpedance by coupling capacitor Cc is comparable to the collectorresistance Rc. Thus it largely the current amplification.

The second cut-off frequency lies in the high frequency range where the

area offered by coupling capacitor C is very small and it may beconsidered as a short circui Thus with increase in input frequency themagnitude of voltage gain, falls off.

Q 11. State the factors to be considered while designing a biasingcircuit t good transistor voltage amplifier

Ans.


7/28


(i) It should ensure proper zero signal collector current and VCE.

(ii) It should ensure that .VCE does not fall ihe.required limits i.e. 1V for Si and

0.5 V.

(iii) It should ensure the stabilisation of collector current against temperaturevariatic

(iv) It should ensure to make the operating point independent of transistorparameters

Q. 12. What are half-power frequencies?

OR

What is meant by frequency response of an ‘amplifier? What are half -power frequencies?

Ans.

As the signal frequency varies the voltage gain of an amplifier also variesdue to capacitive reactances of the circuit.

The curve drawn between the signal frequency and voltage gain of anamplifier is called• as frequency response of an amplifier. The lowfrequency and high frequency at which the gain falls by 3 db are knownas half‘r frequencies or 3 -db points.

It is termed as half power because the power output is 3db from itsnormal level.


8/28


Q 13. Why is coupling capacitor provided in RC coupled amplifier also

called locking capacitor?OR

Explain the effect of coupling capacitor on low frequency response ofBJT amplifiers.

Ans.

The first stage of RC coupled amplifies is coupled to the base of thesecond stage rough the coupling-capacitor Cc.

The coupling capacitor C blocks thç dc voltage and aows the ac signal toreach the base of the second stage.

Thus, the coupling capacitor Cc is also known as blocking capacitor.

Q 14. Discuss briefly the choice of transistor configuration in a cascadeamplifier ?

Ans.

In a cascade amplifier the most commonly used configuration is commonemitter (CE) because the voltage gain of common emitter amplifier isgreater than unity while common collector (CC) configuration has voltagecain less than unity as well as the voltage gain of common base (CB) isalso less than unity.

Q 15. What are the requirements for efficient coupling in a multistage


9/28


amplifier?

Ans.

The requirements for efficient coupling in a multistage amplifier is

(i) There should be proper impedance matching. (ii) Distortion should be removedduring coupling.

Q 16. Give applications for transformer coupled amplifier.

Ans.

(i) It provide impedance matching due to which maximum power can betransferred.

(ii) It keeps the dc power small.

Due to following reasons transformer coupled amplifier isused in public addresssystems to drive speakers and to drive other power devices.

Q 17. What is the function of tank circuit in tuned voltage amplifier?

Ans.

The tank circuit is parallel combination of L and C. It is used to select aparticular audio frequency for amplification.

The selected frequency depends upon the resonance frequency of thetank circuit In this case the impedance Z, is high and thus current 1r is S


10/28


which reduces the power loss,

Q 18. Draw and label the frequency response of an RC coupled

amplifier?

Ans.

The frequency response of an RC coupled amplifier is as shown:-

Q 19. What are the limitations of single tuned voltage aplifIers?

Ans

Limitations of single tuned voltaged amplifiers:

(i) Single tuned voltage amplifier reauces the bandwidth to large extent to gethigher 10 factor.

(ii) This results in poor reproduction.


11/28


Q 20. A tuned circuit has a resonant frequency of 1600 kHz andbandwidth.What is the value of Q factor ?

Ans.

Q 21. A transformer coupling is used in the final stage of a multistage

amplifier. If the output impedance of transistor is and the speaker

has a resistance of , find the turn ratio of the transformer so thatmaximUm power is transferred to the load.

Ans.

According to maximum power transfer theorem the source impedance should

be equal to load impedance.


12/28


Q 22. State and explain atleast one typical application of each type ofcoupling.

Ans.

(i) RC coupling transistor amplifier : AC coupled amplifiers provideexcellent audio fidelity over a wide range of frequencies. Therefore, they areWidely used as voltage amplifiers.

The function of voltage amplifier is to raise the voltage level of the input signal.As R-C coupling has smaller size, smaller weight, lower cost and occupy lessspace therefore, it is preferred to couple various stages of voltage amplifiers bythis coupling method as shown fig. below,


13/28


14/28


coupling is used

Q 23. Derive the equation for the overall voltage gain of multistageamplifier in terms of the individual voltage gains?

Ans.

Let us consider the block diagram for multistage amplifier as shown:


15/28


Q 24. A multistage amplifier consists of three stages. The voltage gainof stages are 60, 100 and 160. Calculate the overall gain in db.

Ans.

Voltage gain in db for 1st stage 20 log10 60 = 20 X 1.778 = 35.56db

Voltage gain in db for 2nd stage = 20 log10 100-= 20 x 2 = 40db

Voltage gain in 1b for 3rd stage = 20 log10 160 = 20 x 2.04 44.08 db

The overall voltage gain in db = 35.56+ 40 + 44.08

Av(overat() = 119.64 db.

Q 25. What is stagger tuning? Explain in brief

OR

Write note on Stagger tuned amplifier?

Ans.

Stagger tuned amplifier:

It has been observed that if two or more tuned circuits, which aresynchronously tuned are cascaded, the overall bandwidth decreases.

However, if the different tuned circuits are cascaded, are tuned to slightlydifferent frequencies, it is possible to obtain an increased bandwidth witha flat pass-band with steep sides.

This technique is known as stagger-tuning Following fig shows a twostage tuned voltage amplifier

The stagger tunning in this circuit, may be achieved by resonating the tunedcircuits L1 C1 and L2 C2 to slightly different frequencies.


16/28


Q 29. Draw the circuit diagram of a tuned amplifier Discuss itsoperation How are current gain and voltage gain given in this casesName various types of tuned amplifiers

Ans.

Tuned amplifiers are those amplifiers which amplify a selected frequency or aselected band of frequencies Circuit diagram of a single tuned amplifier is as


17/28


shown

Operation:

When high frequency signal is applied at the input terminals the resonantfrequency of parallel tuned circuit select and amplifiers the resonantfrequency by changing the value of C.

Only a particular frequency is selected and others are rejected by thetuned


18/28


Q 30. Find gain in dB for voltage gain = 40.

Ans.

Voltage gain in dB

= 20 log10 40= 20 x 1.602

= 32.04


19/28


Q 31. A multi stage amplifier consists of three stages as voltage gain in

dB 33, 36,40 dB respectively. Then calculate the overall voltage gain in dB.

Ans.

Overall voltage, gain in dB

= 33 + 36 + 40 = 109 dB.

falls

Q. 32. In an amplifier the maximum voltage gain is 2000 and occurs at10 KHz. It to 1414 at 20 KHz and 30 Hz. Determine

(i) lower cut off frequency

(ii) upper cut-off frequency.

Ans.

The frequency response curve is shown in figure.


20/28


21/28


Q 35. In two stage RC coupled amplifier and for each stage

Find

(i) voltage gain of second stage

(ii) Overall voltage gain.

Ans.

(i) As no load is for, second stage

Voltage gain of second stage


22/28


Q 36. Two stage amplifier stages coupled by coupling transformer. foutput impedance of first stage is and input impedance of second

stage is .Calculate the inductance of primary and secondary of

transformer for perfect matching at frequency =


23/28


Q 38. Draw cascode connection as tuned amplifier.

Ans.


24/28


25/28


Q 41. Differentiate between tuned and untuned amplifiers.

Ans.

The main difference between tuned and untuned amplifiers is that the,tuned amplifiers are designed for specific, usually narrow bandwidth butuntuned are not designed by such specifications.

Q 42. Why R-C coupling is the most widely used coupling between thetwo stages cascaded amplifier?

Ans.

R-C coupling gives excellent frequency response i.e. a constant gain overthe audio-frequency range.

Thus, are useful for speech and music. Also R-C coupling is cheaper in cost because of low cost of resistors and

capacitors used in the circuit. By using R-C coupling with circuit becomes very compact because of its

components are very light and small.

Q 43. Compare the characteristics of single, double and stagger tunedamplifiers. Give typical circuits and list their important applications.


26/28


Ans.

1. Single tuned voltage amplifier:

2. Double tuned voltage amplifier:

3. Stagger tuned voltage amplifier


27/28


Application of tuned voltage amplifiers

1. Tuned voitage amplifiers are used in the wireless communication systems.

2. These are used in communication transmitters to transmit the signals.

3 These are used in communication receivers to: receive the desired signals.

Q 44. Write shrt note on: Single turned amplifier.

Ans.

Single tuned amplifier is consists of ransistor amplifier with a paralleltuned circuit as the collector load. The values of capacitance andinductance- ot the tuned circuit are so selected that the frequency to be

amplified is equal to its resonant frequency.

It is as shown in fig.


28/28

The output of the single tuned amplifier can be obtained by using either acoupling capacitor C or by using a secondary coil at the output stage.

Documents

muti stage amplifier