Question

Consider an amplifier circuit shown in the figure below:

The voltage gain of the circuit is equal to
$\text{(Assume}{V}_T=25\text{mV and}{V}_{BE}=0.7V)$

• A. -200
• B. -400
• C. 415
• D. -210

Answer 1

The correct option is B -400

Applying KVL in base emitter loop, we get,

$I_B=\frac{10-0.7}{372\text{k}\Omega }=25\mu A$
$\text{To draw the small signal we need to calculate}{r}_{\pi }$

$\therefore r_{\pi }=\frac{V_T}{T_{B_{DC}}}=\frac{25\times {10}^{-3}}{25\times {10}^{-6}}={10}^3\Omega$

$\text{now,}{g}_m=\frac{\beta }{r_{\pi }}=\frac{100}{{10}^3}=0.1A/V$
$\text{now,}{A}_o=\frac{V_0}{V_{in}}=-g_m{R}_C=-0.1\times 4\times {10}^3=-400$