Question

In the voltage regulator circuit given below, $V_t$ is unregulated at 15 V. Assume $V_{BE}=0.7$ V and the base current is negligible for both the transistors. If the regulated output $V_0$ is 9 V, the value of $R_2$ will be

• A. 400 $\Omega$
• B. 1000 $\Omega$
• C. 1600 $\Omega$
• D. 800 $\Omega$

Answer 1

The correct option is D 800 $\Omega$

For given circuit we can use potential divider relation

$9\times \frac{R_2}{R_2+1k\Omega }=4$

$9R_2=4R_2+4k\Omega$

$5R_2=4k$

$R_2=\frac{4000}{5}=800\Omega$