S3 B.Tech University Questions and answers for Circuits

Here include a sample question and answer which is common in various university question papers.

This as model, you can prepare for other questions also. Some tips are also added, if you are not interested, scroll down until 'Sample question and Answer'.

Few Tips: While answering a question, use headings, sub-headings, minor-headings ... accordingly. It will show off your answer paper. This is important because all invigilators are provided with an answer key during valuation. They evaluate your paper according to this answer key.

For example, read the question below. Suppose the question is for 12 marks.

Q1. Sketch hybrid pi-model of CB amplifier. Derive relations of gain, input and output impedance. (Scroll down to see this question and it's answer.)

The answer key to this question may be like this:

•    circuit diagram(2)
•    pi model of amplifier(4)
•    Derivations: Voltage gain(2)   Current gain(2)  Input impedance(1)  Output impedance(1)

So all together 12 marks. By default, a small description is usually expected for all circuit diagrams about components in that circuit and their roles(scroll down to see example- with fig1and fig2, a small description is added).
Whenever you prepare for an examination: 

• Collect as many as question papers. 

• Write down all the questions for the particular module. You can see that many questions are repetitions or are related to some others. 

• During the very first read/look, read/see the entire thing(or as much as you can) - this is for getting an idea about the whole content. 
• But before you approach it seriously, keep the question related to it in your mind. Either during this step or after it, quickly prepare an-answer key based note(it may be in your mind or in note book).

While writing the examination:

• After reading a question, quickly prepare an answer key. This is important because this will prevent you from over describing unnecessary parts. 
• Use answer key for sub-titles(the examiners can easily locate similar things in your paper as in their scrap) and complete the answer.

If you have confusions about answer key preparation, then read below,otherwise skip it. 

 

Read our sample question, divide it into parts and then assign marks for each division.(Don't take much time-we can only guess, it may vary with examiners). 

1. 'Sketch hybrid pi-model of CB amplifier' ---pi-model of amplifier(*)+to draw model, we need a circuit diagram(*)
2. 'Derive relations of gain, input and output impedance.'----voltage gain(*)+Current gain(*)+input impedance(*)+output impedance(*)
3.  * -assign marks according to importance(here, the question is to sketch pi-model-so assign more marks to it) and content(here derivations of current and voltage gains are more difficult).

 Sample Question and Answer:

I. Low frequency analysis of amplifiers

Q1. Sketch hybrid pi-model of CB amplifier. Derive relations of gain, input and output impedance.

Ans:

Introduction

In a CB amplifier, base is common to both input and output sections. Hybrid pi-model or T-model is used for the analysis of low frequency analysis of CB amplifier.

1.  CB amplifier circuit.

The figure below shows a Common base amplifier. Emitter is the input terminal - an input source Vs is placed through Rs and C1. Output is taken across R_L, connected to the collector of the transistor. DC voltage Vcc provides biasing at the output terminal and a constant current biasing is provided at the input terminal.

fig 1

2.Hybrid π  model.

fig 2

 where, g_m - Transconductance and r_e is the small signal resistance between emitter and base when looking into the emitter.   

3. Derivation  
      a): Input resistance (R_in)

Input resistance of the circuit can be calculated by 'looking into the input side of the circuit' from the mark as given in fig 2. This is because R_S is taken as a source resistance and usually not consider for input resistance calculation.

 From the above circuit,    R_in  = r_e        ---------(1)  
CB amplifier has a low input resistance because the value of r_e is usually a few ohms to few tens of ohms.
b): Output resistance (R_o )
Again to get the output resistance,  look into the circuit from right side of arrow in fig 2.
Then from the circuit, R_o = Rc    ----------(2)
 c): Voltage gain (Av)
Voltage gain (Av) is defined as

                                Av = Vo / Vi               where,
                                Vo = output voltage    and
                                Vi = input voltage appear across the input terminal(here Base) of  the amplifier.

Let's find out expression for Vi :

                                Vi = -i_e r_{e        -----------(3)}

Output voltage(Vo) is the voltage across the parallel combination of Rc and R_L.
                               Vo = -αi_e(Rc  ‖ R_L)     ------------(4)
Now we can write an expression for Av.
                               Av = Vo / Vi

                               -----------(5)  
For T-model,   α/re = g_{m .}   Using this equ(5) can be written as 

                               Av = g_m (Rc  ‖ R_L)     ------------(6)
Here, Av has a positive sign.It means the output and input waveforms have the same polarity. So CB is a non inverting amplifier. 

Another parameter related to voltage gain is overall voltage gain. The overall voltage gain for the circuit in fig 2 is given as  Gv = Vo / Vs . We can easily get it as

Gv = Vo / Vs = (Vo / Vi ) x (Vi / Vs) . 
                      Here Vo/Vi is in our hand. Now find an equ for Vi/Vs.


        --------------(7)

Overall voltage gain is Gv is obtained by multiplying equ (5) and (6).

                  --------------(8)

d): Current gain (A _is)
Here short circuit current gain is considered. That is the current gain when  R_L is shorted. When R_L is shorted, output current is

                        i_os =  - α i_e .

The input current is marked in fig 2 as i_i = - i_e  . 
Short circuit current gain   (A _is) =  i_os  / i_i  .
                                           
                                             A _is =   α   ------(9)

Since α ≈ 1, we can conclude that, short circuit current gain of CB amplifier is nearly unity. Because of this property it can act as a current buffer. 
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