# Questions & Answers of BJTs

## Weightage of BJTs

Total 14 Questions have been asked from BJTs topic of Analog Circuits subject in previous GATE papers. Average marks 1.71.

The small-signal resistance (i.e., dVB/dID) in k $\Omega$ offered by the n-channel MOSFET M shown in the figure below, at a bias point of VB = 2 V is (device data for M: device transconductance parameter kN = ${\mu }_{n}{C}_{ox}^{\text{'}}\left(W/L\right)=40\mu A{V}^{2}$, threshold voltage VTN = 1 V, and neglect body effect and channel length modulation effects)

The current ib through the base of a silicon npn transistor is 1+0.1 cos(10000$\pi t$) mA. At 300 K, the r$\pi$ in the small signal model of the transistor is

The voltage gain ${A}_{V}$ of the circuit shown below is

In the circuit shown below, capacitors C1 and C2 are very large and are shorts at the input frequency. vi is a small signal input. The gain magnitude |vo/vi| at 10Mrad/s is

For the BJT Q1 in the circuit shown below, $\beta =\infty ,$ VBEon = 0.7v, VCEsat=0.7v, The switch is initially closed. At time t = 0, the switch is opened. The time t at which Q1
leaves the active region is

In the silicon BJT circuit shown below, assume that the emitter area of transistor Q1 is half that of transistor Q2.

The value of current I0 is approximately

The amplifier circuit shown below uses a silicon transistor. The capacitors Cc and CE can be assumed to be short at signal frequency and the effect of output resistance r0 can be ignored. If CE is disconnected from the circuit, which one of the following statements is TRUE?

Consider the common emitter amplifier shown below with the following circuit parameters: β=100, gm=0.3861 A/V, r0=∞, rx=259 Ω, RS=1KΩ, RB=93KΩ, RC=250Ω, RL=1kΩ , C1=∞ and C2=4.7μF.

The resistance seen by the source Vs is

Consider the common emitter amplifier shown below with the following circuit parameters: β=100, gm=0.3861 A/V, r0=∞, rx=259 Ω, RS=1KΩ, RB=93KΩ, RC=250Ω, RL=1kΩ , C1=∞ and C2=4.7μF.

The lower cut-off frequency due to C2 is

A small signal source ${v}_{i}\left(t\right)=A\mathrm{cos}20t+B\mathrm{sin}{10}^{6}t$ is applied to a transistor amplifier as shown below. The transistor has β = 150 and hie = 3kΩ. Which expression best approximates vo(t)

In the following transistor circuit VBE = 0.7V, re = 25mV / IE, and $\beta$ and all the capacitances are very large

The value of DC current IE is

In the following transistor circuit VBE = 0.7V, re = 25mV / IE, and $\beta$ and all the capacitances are very large

The mid-band voltage gain of the amplifier is approximately

The DC current gain ($\beta$) of a BJT is 50. Assuming that the emitter injection efficiency is 0.995, the base transport factor is
For the BJT circuit shown, assume that the $\beta$ of the transistor is very large and VBE = 0.7V. The mode of operation of the BJT is