Important Short Questions and Answers: Electronic Circuits - BJT Amplifiers

BJT AMPLIFIERS

1. What are the advantages of Representation of Gain in Decibels.

Logarithmic scale is preferred over linear scale to represent voltage and power gains because of the following reasons :

•      In multistage amplifiers, it permits to add individual gains of the stages to calculate overall gain.

•      It allows us to denote, both very small as well as very large quantities of linear, scale by considerably small figures.

For example, voltage gain of 0.0000001 can be represented as -140 dB and voltage gain of 1,00,000 can be represented as 100 dB.

•      Many times output of the amplifier is fed to loudspeakers to produce sound which is received by the human ear. It is important to note that the ear responds to the sound intensities on a proportional or logarithmic scale rather than linear scale. Thus use of dB unit is more appropriate for representation of amplifier gains.

2. What is the coupling schemes used in multistage amplifiers?

In multistage amplifier, the output signal of preceding stage is to be coupled to the input circuit of succeeding stage. For this interstage coupling, different types of coupling elements can be employed.

These are :           

1. RC coupling

2. Transformer coupling

3. Direct coupling

3. Define Common Mode Rejection Ratio.

Common Mode Rejection Ratio is the figure of merit of a differential amplifier to reject common mode signal and is given by,

4. What does bootstrapping mean? Why bootstrapping is done in a buffer amplifier?

In the emitter follower amplifier A_V tends to unity. If a resistor is connected between input and output of the emitter follower, the change in the voltage at one end of the resistor changes the voltage at the other end of the resistor by same value. It is as if resistor is pulling itself up by its bootstraps. Such effect is known as boot strapping.

5. Draw the Darlington emitter follower circuit. 

6. How can a DC equivalent circuit of an amplifier be obtained?

The analysis of transistor circuits for small signal behaviour can be made by following simple guidelines. These guidelines are,

•      Draw the actual circuit diagram

•      Replace coupling capacitors and emitter bypass capacitor by short circuit

•      Replace D.C. source by a short circuit

•      Mark the points B, E, C on the circuit diagram and locate these points as the start of the equivalent circuit

•      Replace the transistor by its h-parameter model

7. State Miller’s Theorem.

It states that the effect of resistance Z on the input circuit is a ratio of input voltage to the current which flows from the input to the output.

It states that the effect of resistance Z on the output circuit is the ratio of output voltage to the current which flows from the output to input.

8. Define i) Differential gain ii) Common mode gain

The gain with which differential amplifier amplifies the difference between two input signals is called differential gain of the differential amplifier denoted as A _D. The gain with which it amplifies the common mode signal to produce the output is called common mode gain of the differential amplifier denoted as A _C.

9. What are practical limitations in selecting very high R _E?

1.     Large R _E  needs higher biasing voltage to set the operating point of the transistors.

2.     This increases the overall chip area. Hence practically R _E can not be selected very high.

10.What are the limitations of h parameters?

The h parameters has the following limitations,

a.     The accurate calculation of h parameters is difficult.

b.     A transistor behaves as a two port network for small signals only, hence h parameters can be used to analyze only the small signal amplifiers.

11. What are the advantages of Darlington amplifier?

A Darlington transistor connection provides a transistor having a very large current gain, typically a few thousand. The main features of the Darlington connection is that the composite transistor acts as a single unit with a current gain that is the product of current gains of the individual transistors.

βD= β1β2

βD = Darlington connection current gain

β1 and β2 – Current gain of the transistors 1 & 2 in the Darlington pair

12.            Methods of coupling multistage amplifiers

•         RC coupling

•         Transformer coupling

•         Direct coupling

13.            Features of differential amplifier.

•         High differential voltage gain

•         Low common mode gain

•         High CMRR

•         Two input terminals

•         High input impedance

•         Large bandwidth

•         Low offset voltages and currents

•         Low output impedance

14.            List the configuration of differential amplifiers.

•         Dual input, balanced output differential amplifier

•         Dual input, unbalanced output differential amplifier

•         Single input, balanced output differential amplifier

•         Single input, unbalanced output differential amplifier

15.            State Bisection Theorem.

A particular network which has mirror symmetry with respect to an imaginary line. If the entire network is denoted as N then it can be divided into two half networks N/2 about the line of symmetry is called bisection theorem or Bartlett’s bisection theorem.

16. Methods of improving CMRR

To improve the CMRR, the common mode gain Ac must be reduced. The common mode gain Ac approaches zero as RE tends to infinity. This is because RE introduces a negative feedback in the common mode operation which reduces the common mode gain Ac. Thus higher the value of RE, lesser is the value of Ac and higher is the value of CMRR. The differential gain Ad is not dependent on RE

17.            What are the other methods to improve CMRR without RE?

•      Constant current bias method

•      Current mirror circuit.

18.            List the advantage of current mirror circuit?

•      Provides very high emitter resistance RE.

•      Requires fewer components than the constant current bias.

•      Simple to design

•      Easy to fabricate.

•      With properly matched transistors, collector current thermal stability is

•      achieved.

19.            Draw the small signal equivalent circuit of CE amplifier.

20. Define Miller effect input capacitance.

For any inverting amplifier, the input capacitance will be increased by a miller effect capacitance, sensitive to the gain of the amplifier and the inter electrode capacitance connected between the input and output terminals of the active device. CMi = (1- AV) Cf CM0 = Cf Cf = Inter electrode capacitance between input and output.