GATE 2025 EC – Question 47
In the circuit shown, the identical transistors Q1 and Q2 are biased in the active region with $\beta=120$. The Zener diode is in the breakdown region with $V_Z=5$ V and $I_Z=25$ mA.
If $I_L=12$ mA and $V_{EB1}=V_{EB2}=0.7$ V, then the values of $R_1$ and $R_2$ (in kΩ, rounded off to one decimal place) are ______, respectively.
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Correct answer: (A) 0.6 and 0.4
Explanation
Q2 is diode-connected, so the node under it is at $20-0.7=19.3$ V. The Zener drops 5 V, so the base of Q1 is at $19.3-5=14.3$ V and the emitter of Q1 is at $14.3+0.7=15$ V. Hence $R_2$ has 5 V across it. $I_E=I_L\dfrac{\beta+1}{\beta}=12\times\dfrac{121}{120}=12.1$ mA, so $R_2=5/12.1\approx0.4$ kΩ. $I_B=I_E/(\beta+1)=0.1$ mA flows out of the base into the node above $R_1$, so $I_{R_1}=I_Z+I_B=25.1$ mA and $R_1=14.3/25.1\approx0.6$ kΩ.