Amplifier Circuit – A weak electronic signal makes it impossible to use some of the information it contains. Amplification is the process of boosting a signal’s amplitude. Almost all electronic devices must have some method of signal amplification. Amplifiers are used in a variety of devices, including those used in communication, automation, medical, scientific, and military applications.

1. What is an Amplifier?

An amplifier is a circuit that uses energy from an external source to increase the amplitude of the input current or voltage at the output.

2. Based on the Transistor Configuration how Amplifiers are Classified

Based on Transistor Configuration, the Amplifier are Classified as

1. Common Emitter Amplifier
2. Common Collector Amplifier
3. Common Base Amplifier

3. Draw a CE Amplifier Circuit & its hybrid Equivalent circuit.

4. Draw a CC Amplifier Circuit & its hybrid Equivalent circuit.

5. Write the Hybrid Parameters Equation for Transistor Amplifier?

Vi = hi Ii + hrVo

Io = hf Ii + hoVo

6.      Draw a CB Amplifier Circuit & its hybrid Equivalent circuit

7. Which Amplifier is called as voltage Follower? Why?

Common collector transistor amplifiers are referred to as having voltage followers. It has a unity voltage gain and an extremely high input impedance. It does not at all make use of the input current of the signal. This results in a distortion-free coupling of the output circuit to the input signal.

8. Write the input impedance, output impedance, voltage gain and current gain of the common emitter amplifier in terms of h parameters for the fixed bias condition?

Current gain Ai = -hfe

Voltage gain Av = (hfeRC)/hie Input Impedance Zi = hie

Output Impedance Zo = RL ||RC

9. What are the Limitations of H-parameters?

The h parameters has the following limitations,

The accurate calculation of h parameters is difficult.

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

10. Why hybrid Parameters are called so? Define them?

The dimensions of the hybrid parameters are not alike, that is they are hybrid in nature so they are called hybrid parameters.

h11 = [ V1/I1] at V2=0; h11 = Inpu,t impedance with output port short circuited.

h12 = [ V1/V2] at I1=0; h12 = Reverse voltage gain with input port open circuited.

h21 = [ I2/I1] at V2=0; h11 = Forward current gain with output port short circuited.

h22 = [I2/V2] at I1=0; h11 = output impedance with input port open circuited.

11. What is Transresistance Amplifier?

In voltage shunt feedback amplifier the sampled signal is a voltage and the feedback signal (Which is fed in shunt) is a current.

Rm = Vo / Ii (or) Vo = Rm. Ii Where, Rm = Amplifier gain.

Vo = Output voltage. Ii = Input current.

12. What does Bootstrapping mean? Why Bootstrapping is done in a buffer Amplifier?

In the emitter follower amplifier AV tends to unity. If a resistor is connect 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.

13. Draw the Darlington emitter Follower circuit.      

14. How can a DC Equivalent circuit of an Amplifier Obtained?

The analysis of Transistor Circuits for small signal Behaviour can made by following simple Guidelines. These Guidelines are,

1. Draw the actual circuit diagram
2.  Replace Coupling Capacitors and emitter bypass Capacitor by short circuit
3. Replace D.C. source by a short circuit
4. Mark the points B, E, C on the circuit diagram and locate these points as the start of the Equivalent circuit
5. Replace the Transistor by its H-parameter model

15. 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.

Z1 = Z/(1-K)

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.

Z2 = Z-K / (K-1)