Circuit Models for Amplifiers

Amplifier are generally put in four categories

1. voltage amplifier,

2. current amplifier,

3. trans resistance amplifier and

4. transconductance amplifier.

These models apply irrespective of the complexity of the internal circuit of the amplifier. The values of the model parameters can be found either by analyzing the amplifier circuit or by performing measurements at the amplifier temlinals.

The model for a voltage amplifier is shown in figure 1. Real amplifiers have input and output resistance. This is reflected in the model.

                Figure 1: A Voltage Amplifier.

Gain

Gain is the increase in the strength of a signal and is often expressed in decibel1 s (dB). An increase of 3 dB is about equal to doubling in a linear scale. A gain of more than 1 is called amplification, while a gain of less than 1 is called Attenuation.

Gain is given different symbols depending of the type. For No load gains

Using the model, the gain with a load can be calculated.