## Amplifier

An amplifier circuit amplifier it's A.C input into D.C output.
The process, in which a larger output is obtained for a smaller input signal, is termed as amplification. The device which performs this function is called amplifier.

In this fig increase the amplitude of its input signal without changing the other signal characteristics . The amplifier must be supplied with D.C operating power . Although the source of this is not shown.
Example of amplifier use are given below:

The amplification of the small signal normally generated by oscillator.

## Classification of amplifier

#### According to the function:

1. Voltage amplifier
2. Power amplifier
3. Current amplifier

#### According to the frequency to be amplified:

1. Audio frequency (A.F).
3. D.C amplifier.

1. Class A
2. Class B
3. Class AB
4. Class C

#### According to coupling method.

1. R.C coupled
2. Impedance coupled
3. Transformer coupled.
4. Direct coupled.

#### 1. Voltage amplifier ;

If the magnitude of the output voltage is appreciably greater than the input voltage, it is known as a voltage , it is known as a voltage amplifier.

#### 2. Current amplifier;

A current amplifier is an electronic circuit which amplifier the input current by a fixed factor and feeds it to the succeeding circuit . A current amplifier amplifies the current without finding with the voltage levels.

#### 3. Power amplifier:

These are similar to voltage amplifier. Their are main function is to supply a sufficient amount of power to the output or load circuit even if the input power is negligibly current.

### 1. Class A;

Most voltage amplifier operate in class A and this mode may also used for the power amplification. In class A amplifiers the grid bias and input voltage are so choose that the value operates over the linear portion of the dynamic characteristics as shown in figure the following points are worth nothing ;

• The output wave form is exactly similar to the input wave form.
• Plate current flows over whole cycle.
• The voltage amplification is very high.
• The power amplification is almost high.

#### 2. Class B;

Class B amplifier 180° output only.
Figure shows the operating characteristics of the class B amplifiers. The following points are worth nothing.-

• Since the output wave form does not have the -ve loop the distortion in comparison to that in class A amplifiers is high .
• The plate current , being to a half sine wave form contain very pronounced harmonic.
• The plate efficiency is of the order of about 50%
• The voltage amplification is reduced due to increase input voltage required.

#### 3. Class AB :

In these amplifier the grid bias and input voltage are so choosen that the characteristics obtained are in between those of the class A and class B i.e, plate current flow for appreciably more than half but less than the entire cycle.

#### 4. Class C :

In this class the bias is about as twice the cut off value so that the value operates continuously beyond the cut off point. Hence, the plate current flows for much less than half of the period . The grid signal voltage is usually very large .

The operating characteristics are shown in figure.

The following points are worth nothing ;
• The distortion and harmonic content is very high in the output.
• The voltage amplification is considerably reduced .
• The power output is considerably increase.