We define a parameter βac, which is similar to the βdc as
which is also known as the ac current gain Ai. Usually βac is close to βdc in the linear region of the output characteristics.
From Eq. (1), we get
v0 = ΔVCE = – βac RL ΔIB
The voltage gain of the amplifier is
The negative sign represents that output voltage is out of phase with the input voltage.
The power gain Ap can be expressed as the product of the current gain and voltage gain. Mathematically
Ap = βac × Av
Since βac and Av are greater than 1, we get ac power gain. However it should be realised that transistor is not a power generating device. The energy for the higher ac power at the output is supplied by the battery.
