Question

How can you calculate the small-signal output conductance of a common-base BJT amplifier?

Answer 1

To calculate the small-signal output conductance (


g
o
    ​

) of a common-base bipolar junction transistor (BJT) amplifier, you can follow these steps:

Identify the Small-Signal Equivalent Circuit: First, you need to determine the small-signal equivalent circuit of the common-base BJT amplifier. The small-signal equivalent circuit represents the transistor's behavior around its operating point when the input and output signals are small enough to be linearly approximated.

Short Circuit AC Input: For calculating the output conductance, you need to short-circuit the AC input signal (voltage or current source) in the small-signal equivalent circuit. This is done by replacing the AC voltage or current source with a short circuit.

Apply Output Voltage and Measure Output Current: After short-circuiting the AC input, apply a small AC voltage (




v
out
    ​

) at the output and measure the resulting AC output current (




i
out
    ​

).

Calculate Output Conductance: Finally, calculate the output conductance (


g
o
    ​

) using Ohm's law. Output conductance is the ratio of the change in output current to the change in output voltage when the AC input is shorted.

The formula for the small-signal output conductance (


g
o
    ​

) is given by:



=








g
o
    ​

=
v
out
    ​

i
out
    ​

    ​


Please note that the output conductance (


g
o
    ​

) is typically expressed in units of siemens (S) or mhos (℧).

Keep in mind that the small-signal analysis assumes that the signals are small enough for linear approximation, and it helps in understanding the amplifier's behavior in the frequency domain. For a more accurate and complete analysis, you might also need to consider the input conductance (


g
i
    ​

) and transconductance (


g
m
    ​

) along with other parameters such as capacitances, load resistance, etc.