Question

Consider an amplifier circuit shown in the figure below:

The voltage gain of the circuit is equal to
(Assume $V_T=25mVand{V}_{BE}=0.7V)$

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

Answer 1

The correct option is C -400

Applying KVL in base-emitter loop, we get,

$I_B=\frac{10-0.7}{372k\Omega }=25\mu A$

To draw the small signal we need to calculate $r_{\pi }$

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

$g_m=\frac{\beta }{r_{\pi }}=\frac{100}{{10}^3}=0.1A/V$

$A_V=\frac{V_0}{V_{in}}=-g_m{R}_C=-0.1\times 4\times {10}^3=-400$