(a) Current gain (a): Current amplification factor ‘a’ is defined as a ratio of small change in the collector current to the small change in the emitter current at constant collector voltage.
Thus, a = \(\frac{\delta I_e}{\delta I_e}\) with Ec constant
(b) Resistance gain: The resistance gain of a transisor is defined as the ratio of collector-base internal resistance to the emitter-base internal resistance. Thus
Resistance gain = \(\frac{R_0}{R_i}\)
(c) Voltage gain: The collector circuit, being reverse biased, has very high internal resistance. This permits a high load resistance R, to be inserted in the collector circuit, without affecting the output current and this is the basis of voltage amplification in a transistor.
For the sake of explanation, let us consider a p-n-p common base junction transistor having a current gain a. Let an input signal voltage be fed to the emitter-base circuit having the low input.
Emitter Current, Ie = \(\frac{V_i}{R_i}\)
(d) Power gain: It is defined as the ratio of the output power to input power.
Power gain = \(\frac{\text {Output power }}{\text {Input power}}\)
= \(\frac{I_e \times R_e}{I_e \times R_i}\)
= a2 x Resistance gain
(e) Transistor conductance: Transistor conductance is defined as the ratio of change in collector current to change in base emitter voltage.
gm = \(\frac{\bigtriangleup I_e}{\bigtriangleup V_{bc}}\)
The quantity gm has units of conductance and is called the transfer conductance or transconductance of the transistor. The transconductance of a transistor depends upon the geometry, doping levels and biasing of the transistor.
