# GATE Questions & Answers of BJTs and MOSFETs

## What is the Weightage of BJTs and MOSFETs in GATE Exam?

Total 22 Questions have been asked from BJTs and MOSFETs topic of Analog Circuits subject in previous GATE papers. Average marks 1.64.

For a narrow base PNP BJT, the excess minority carrier concentration ($\triangle n_E$ for emitter,$\triangle p_B$ for base,$\triangle n_C$ for collector) normalized to equilibrium minority carrier concentration ($n_{E0}$ for emitter,$n_{B0}$ for base,$n_{C0}$ for collector) in the quasi-neutral emitter, base and collector regions are shown below. Which one of the following biasing modes is the transistor operating in? ##### Show Answer

In the figure shown, the npn transistor acts as a switch. For the input Vin(t) as shown in figure, the transistor switches between the cut-off and saturation regions of operation, when T is large. Assume collector-to-emitter voltage at saturation VCE(sat)=0.2V and base-to-emitter voltage VBE(sat)=0.7V. The minimum value of the common-base current gain($\alpha$) of the transistor for the switching should be___________.

##### Show Answer

An npn bipolar junction transistor (BJT) is opening in the active region. If the reverse bias across the base-collection junction is increased, then

##### Show Answer

Consider an n-channel MOSFET having width W, length L, electron mobility in the channel µn and oxide capacitance per unit area Cox. If gate-to-source voltage VGS = 0.7 V, drain-to-source voltage VDS = 0.1 V, (µn Cox) = 100 µA/V2, threshold voltage VTH = 0.3 V and (W/ L) = 50, then the transconductance gm(in mA/V) is_________.

##### Show Answer

A MOS capacitor is fabricated on p-type Si (Silicon) where the metal work function is 4.1 eV and election affinity of Si is 4.0 eV. E- EF=0.9 eV, where EC and EF are the condition band minimum and the Fermi energy levels of Si, respectively. Oxide , oxide thickness tox=0.1 µm and electronic charge =1.6 × 10-19 C. If the measured flat band voltage of this capacitor is -1 V, then the magnitude of the fixed charge at the oxide-semiconductor interface, in nC/cm2, is_________.

##### Show Answer

Two n-channel MOSFETs, T1 and T2, are identical in all respects except that the width of T2 is double that of T1. Both the transistors are biased in the saturation region of operation, but the gate overdrive voltage (VGS-VTH) of T2 is double that of T1, where VGS and VTH are the gate-to-source voltage and threshould voltage of the transistors, respectively. If the drain current and transconduction of T1 are ID1 and gm1 respectively, the corresponding values of these two parameters for T2 are

##### Show Answer

Assuming that transistors M1 and M2 are identical and have a threshold voltage of 1 V, the state of transistors M1 and M2 are respectively ##### Show Answer

In a MOSFET operating in the saturation region, the channel length modulation effect causes

##### Show Answer

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) ##### Show Answer

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 ##### Show Answer

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

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 ##### Show Answer

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 ##### Show Answer

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

##### Show Answer

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? ##### Show Answer

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

##### Show Answer

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

##### Show Answer

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) ##### Show Answer

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

##### Show Answer

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

##### Show Answer

The DC current gain ($\beta$) of a BJT is 50. Assuming that the emitter injection efficiency is 0.995, the base transport factor is

##### Show Answer

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 