Question

Consider the circuit shown in the figure below:


The transistors $Q_1$ and $Q_2$ are identical and operated in active mode such that $I_{C}1=10I_{C2}$ and $V_T=26$ mV. If the value of voltage $V_1=25$ mV, then the value of $V_2$ is equal to ___mV.

• A. -34.86

Answer 1

The correct option is A -34.86
Since, the transistor $Q_2$ will be in active mode only if a positive voltage is applied at it's input. Thus the value of $V_2$ will be (-ve) according to the given polarity
Now, $V_1=V_{T}ln\left(\frac{I_{C1}}{I_{C0}}\right)$
and $V_2=V_{T}ln\left(\frac{I_{C2}}{I_{C0}}\right)$
$\therefore V_1-V_2=V_{T}ln\left(\frac{I_{C1}}{I_{C_2}}\right)=26\times {10}^{-3}ln\left(10\right)=59.86mV$
Now, given $V_1=25mV$
$\therefore V_1-V_2=59.86mV$
$-V_2=59.8-25$
$-V_2=34.86mV$
$therefore{V}_2=-34.86mV$