Bias Circuit

Equation C.17
We note that the magnitude of the result is the dc voltage drop across the bias resistor divided
by VT. For example, for VRc = 5 V and VT = 26 mV (room temperature), the gain magnitude is
about 200. The BJT circuit is capable of providing very substantial voltage gains.
C.3.2 Overall Gain Magnitude from Signal Source Voltage to Output
The circuit input resistance looking into the base of the transistor, Rb, is the transistor input
resistance, rπ (still neglecting rb), in parallel with the bias resistor RB, that is,
Equation C.18
The gain from the signal source to the output at the collector of the transistor is thus
Equation C.19
When the input-signal source resistance is large (Rs >> Rb), a good approximation for av is
Equation C.20
Further approximation can be made using RB >> rπ, to obtain
Equation C.21
Finally, using βac = gmrπ [(C.15)],
Equation C.22
This result is intuitive on the basis of Ib Is, Is Vs/Rs, and Ic = βacIb. The sequence of
approximations for the gain magnitude is