1 March 2020

Transistor Amplifier Interview Short Question Answer - 1

1
Explain the term : Single stage transistor amplifier


Single stage transistor amplifier
  • When only one transistor is used to amplify the input signal, it is called as single stage amplifier.


2
Explain the term : Multistage transistor amplifier


Multi stage transistor amplifier
  • When more than one transistor is used for amplifying the input signal, it is called as multistage transistor amplifier.


3
Which are the main components of the single stage transistor amplifier?


Transistor, Bias circuit and auxiliary components

4
What is necessary of multistage transistor amplifier?


Necessary of multistage transistor amplifier
  • If the output of the single transistor is not sufficient to drive the circuit, the multistage amplifier circuit is used in which output of the first stage is connected to the input of the next stage with different coupling device ( RC coupling, transformer coupling, direct coupling ) and so on...


5
Describe the function of the following in the transistor amplifier circuit : ( a ) Coupling capacitor ( b ) Emitter bypass capacitor and ( c ) Biasing resistor


Coupling capacitor ( CC )
  • The function of the coupling capacitor is to couple output of the one stage to the input of the next stage. 
  • It allows only AC signal to pass from one stage to the other but preventing DC signal. 
  • The collector resistance is in parallel with upper biasing resistance without coupling capacitor therefore the biasing condition is changed.

Emitter bypass capacitor ( CE )
  • The emitter bypass capacitor is shunted to the emitter resistance and it provides low signal path for the AC signal.
  • The AC signal flows through emitter resistance causing voltage drop across it without emitter bypass capacitor  resulting output voltage is reduced. 
  • The voltage gain of the amplifier greatly reduces without emitter bypass capacitor.

Biasing resistor
  • The function of the biasing resistor ( R1, R2 and RE ) is to stabilize the operating point enables largest possible output.


6
What is phase of the output from (a) CE junction and (b) Collector terminal in the CE transistor amplifier?


  • When the output is taken from collector to emitter terminal in the CE transistor amplifier, it is 1800 phase reversal with respect to the phase of the input signal. 
  • When the output of the CE transistor amplifier is taken from collector terminal, it is same phase as that of input phase signal. *There is no phase reversal in the CB and CC transistor amplifier.


7
Explain : ( a ) DC load line and ( b ) AC load line


DC load line
  • It represents the line on the output characteristics of the transistor circuit which gives the value of collector current  ( IC ) and collector to emitter voltage ( VCE ) when dc or zero signal is given to the transistor amplifier circuit.

AC load line
  • It represents the line on the output characteristics of the transistor circuit which gives the value of collector current  ( IC ) and collector to emitter voltage ( VCE ) when only ac signal to given to the transistor amplifier circuit.


8
How to obtain operating point from AC and DC load line?


Operating point
  • The intersection point of the AC load line and DC load line is known as operating point.


9
Describe the effect of loading on the voltage gain of the amplifier.


As the load on the amplifier increases, the voltage gain reduces and vice versa.

10
What is swamping amplifier? Describe its function.


Function of swamping amplifier
  • It is an amplifier in which the emitter resistance is partially bypassed by the emitter capacitor. 
  • The function of the swamping amplifier is to stabilize the voltage gain. 
  • However the swamping reduces the voltage gain of the amplifier.


11
Describe the name of the different type of amplifier


Types of amplifiers
  • According to frequency range : Audio frequency amplifier and Radio frequency amplifier
  • According to amplification of signal : Voltage amplifier and Power amplifier
  • According to stage coupling methods : RC coupled amplifier , Transformer coupled amplifier and Direct coupled amplifier
  • According to input signal cycle operation : Class A amplifier , Class B amplifier and Class C amplifier


12
What are the different coupling devices are used in multistage transistor amplifier?


The following are the coupling device is used in multistage transistor amplifier :
  • Transformer coupling
  • RC coupling and
  • Direct coupling


13
What is function of the audio amplifier?


Audio amplifier
  • It is an amplifier which raises the voltage level of audio signal.


14
Which type of transistor amplifier configuration has effect on current gain, voltage gain and power gain?


Effect of amplifier configuration on gain of the amplifier
CE transistor amplifier
  • It increases current gain, voltage gain and power gain.

CB transistor amplifier
  • It increases voltage gain as well as power gain.
  • The current gain of this amplifier is less than unity.

CE transistor amplifier
  • It increases current gain as well as power gain.
  • It has no voltage gain.


15
Describe the function of cascading of two amplifier stages.


Cascading of the amplifiers
  • To transfer AC output of first stage in to next stage.
  • To isolate DC condition of first stage from next stage.


16
Explain the terms :  Frequency response


Frequency response
  • The change in the voltage gain with respect to the frequency is called as frequency response of the amplifier.


17
Explain the term : Voltage gain


Voltage gain
  • The ratio of the output voltage to the input voltage of the transistor amplifier is known as voltage gain.


18
Explain the term : Bandwidth


Band width
  • The range of frequencies between lower cut off frequency and upper cut off frequency is known as band width

  Band width (BW) = f2 – f1
   Where f2 = Upper cut off  frequency
              f1 = Lower cut off frequency
                              OR
  • It is range of frequencies in which the gain of the amplifier is equal or greater than 70.7% of the maximum gain.

OR
  • It is range of frequencies at which the voltage gain of the amplifier falls by 3 db from its maximum gain. ( If the gain of the amplifier falls from 100 to 70.7%  means

= 20 log10 ( 100 ) -  20 log10 ( 70.7 )
= 20log10 ( 1.414 )  db
= 3 db

19
What is unit of gain of the amplifier?


Unit of gain of the amplifier
  • Generally the gain of the amplifier is expressed in a number, but due to very large number or gain it is also expressed in bel or decibel (db).

       1 bel = 10db
       Power gain = log10 ( Pout / Pin ) db

20
What is ‘ Half power frequency ’ ?


Half power frequency
  • It is frequency at which voltage gain of the amplifier reduced 70.7% from its maximum gain.  
  • When voltage is down 70.7% of its maximum value, the power is reduced 50%  ( P α V2 ) of its maximum value therefore it is called as half power frequency.





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