Question

A transistor is used in common-emitter mode in an amplifier circuit. When a signal of 20mV is added to the base-emitter voltage, the base current changes by $20\mu A$ and the collector current changes by 2mA. The load resistance is $5k\Omega$. Calculate
(a) the factor $\beta$,
(b) the input resistance $R_{BE}$,
(c) the trans conductance and
(d) the voltage gain.

Answer 1

(a) $\beta =\frac{{△|}_C}{{△|}_B}=\frac{2mA}{20\mu A}=‘100$.
(b) The input resistance $R_{BE}=\frac{△V_{BE}}{△{|}_B}=\frac{20mV}{20\mu A}=1k\Omega$.
(c) Transconductance=$\frac{{△|}_C}{{△V}_{BE}}=\frac{2mA}{20mv}=0.1mho$.
(d) The change in output voltage is $R_L{△|}_C$
$=\left(5k\Omega \right)\left(2mA\right)=10V$
The applied signal voltage=20mV
Thus, the voltage gain is $\frac{10V}{20mv}=500$.