Question

Consider the circuit shown in the figure below:

The diode 'D' can be modeled as a battery of voltage $V_{\gamma }$ in series with a resistor ${}^{\prime }{r}_f^{\prime }$ when biased in forward direction. If the value of resistance $R=1k\Omega$, then the value of resistance $r_f$ is equal to

• A. $83.33\Omega$
• B. $0.923\Omega$
• C. $101.33\Omega$
• D. $7.133\Omega$

Answer 1

The correct option is A $83.33\Omega$

The small signal equivalent can be drawn as

$\therefore$ The output can be expressed as,

$V_0=\frac{R}{R+r_f}{V}_{in}-\frac{R}{R+r_f}{V}_{\gamma }$ .... (i)

Thus, the slope of line in the graph of the input output curve can be written

Slope $=\frac{R}{R+r_f}=\frac{1.2}{2-0.7}=\frac{1.2}{1.3}$ ... from equation (i)

Thus $r_f=83.33\Omega$