1. Vtg gain AV and current $A_1$
2. Input resistance Ri
3. O/P resistance $R_0$

Voltage Gain and current:

The gain of an amplifier is defined as the ratio of o/p quantity to the i/p quantity.

So the ratio of o/p vtg to i/p voltage will be called as “voltage gain AV” of the amplifier.

Similarly the ratio of the o/p current to i/p current is called as ‘current gain’ of the amplifier.

$A_I=\frac{I_0}{I_i} \\ A_V=\frac{V_0}{V_i}$

The voltage gain and current gain of amp should be as large as possible. Gain is a written quantity.

Input Resistance:

It is the resistance seen looking into the i/p terminal of an amplifier.

More generally on i/p impedance $Z_i$ is defined, however $Z_i$ is resistive for the transistorized amplifiers hence it is replaced by $R_i$

Ideally $R_i$ should be infinite and practically as large as possible.

O/P resistance $(R_0 )$:

It is the resistance seen looking into the o/p teminals of an amplifier when the input signal source Vs is short circuited $V_i=0$ and o/p terminals are open circuited.

$R_0$ Should be ideally equal to zero and practically it should be as small as possible.

Thus current gain, voltage gain, i/p resistance and o/p resistance are 4 important parameters of amplifiers.