How to connect amplifier and not lose gain | applied electronics engineering

How to connect amplifier and not lose gain

By Applied Electronics - Sunday, September 4, 2016 No Comments
An amplifier must be eventually connected to some source and some load. While connecting or rather coupling amplifier to a circuit you don't want to lose the amplifier gain. Here is short description that helps you take theoretical consideration while connecting an amplifier to a circuit.

For the purpose of understanding amplifier connection effect we replace the amplifier with equivalent circuit model. Then we analyze the voltage gain equation at the input and output circuit.

Voltage Amplifier as Circuit Model

A voltage amplifier circuit model is shown below.

This amplifier circuit is a voltage controlled voltage source circuit model of an amplifier. The Avo is the open loop voltage gain of the amplifier. Ri is the input resistance and Ro is the output resistance. vi is the input voltage and vo is the output voltage.

Connecting the amplifier to Source and Load

Now consider the following circuit diagram where the above amplifier is connected to a source of internal resistance Rs and voltage vs. The output is connected to a load of vo and load RL.

The output voltage vo, that is the voltage across RL is given by,

The overall voltage is given by,

This equation tells us that in order get maximum signal amplification one must design the amplifier with much smaller value of internal output resistance Ro.

An ideal amplifier would have Ro equal to 0Ohm. But a realistic amplifier has some finite internal output resistance Ro. However, by making Ro much smaller than the load resistance we can couple amplifier to the output circuit without losing much gain.

Similarly at the input circuit, the voltage gain relation is given by,

So in order not to lose amplifier gain while coupling at the input, the source resistance Rs must be much less than the input resistance of the amplifier Ri.

The overall gain after connecting the source and load across the amplifier is,