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1.

Determine the conductivity of pure germanium at `27^(@)C`. The concentration of the carriers at this temperature is `2.2 xx 10^(19)m^(-3)`. The mobility of electrons = 0.36 and that of holes = 0.17

Answer» Correct Answer - `1.87 Omega^(-1)m^(-1)`
Discussion
2.

The plate resistance of a triode is `7.7 kOmega`, and the transconductanace is 2.6 millimho. If only the plate voltage is increased by 50 V, what is the increase in the plate current ? What change in grid voltage will now bring the plate current to its former value ?

Answer» Correct Answer - `6.5 mA, 2.5 V`
Discussion
3.

The voltage of a triode amplifier with a resistive load of `5 kOmega` is 10 while with a resistive load of `12 kOmega`. It is 15. Calculate the amplification factor and plate resistance of the triode.

Answer» Correct Answer - `23.3, 6667 Omega`
Discussion
4.

In the accompanying circuit (Fig. 5.9) the value of `beta` is `100`. Find `I_(B),V_(CE),V_(CE)` and `V_(BC)`, when `I_(C) =1.5 mA`. Is the transistor in active, cut-off or saturation state ?

Answer» Correct Answer - `I_(B) = 15 muA,V_(CE) =16.95 V,V_(BC) = 3.75 V,V_(BE) = 20.7`, saturation state
Discussion
5.

In a transistor collector load is `4 kOmega` whereas the zero signal current is 1 mA. (a) What is the operating point if `V_(C C) = 10 V` ? (b) What will be the operating point if `R_(L) = 5 kOmega` ? [Hint : Operating point is zero signal collectorm current and `V_(CO)`]

Answer» Correct Answer - `1 mA and 6 V, 1mA and 5V`
Discussion
6.

In triode valve, for a grid voltage `V_(g) = -1.2 V`, the plate current `I_(p)` (in mA) ad the plate voltage are given by the relation `I_(p) = -50 +0.1 V_(p)`, when the grid voltage is changed to `-3.2 V` and the plate voltage is kept at 150 V, plate current of 5 mA is observed. Calculate the valve constant and the voltage amplification for `20 k Omega` load in the plate circuit.

Answer» Correct Answer - `10^(4) Omega, 2.5 xx 10^(-3) Omega^(-1),25 ; 16.67`
Discussion
7.

A transistor connected in common emitter configuration has `V_(C C)=8V, V_(L) = 0.5 V` and `R_(L) = 800 Omega`. If its `alpha = 0.96` determine (i) collector emitter voltage, (ii) its base current.

Answer» Correct Answer - `7.5 V,0.026 mA`
Discussion
8.

The base current of a transistor is `105 muA` and the collector current is 2.05 mA. Determine the alue of `beta,I_(C) and alpha`. If a change of `27 muA` in the base current produces a change of 0.65 mA in the collector, find `beta_(ac)`.

Answer» Correct Answer - `beta = 195,alpha = 0.95, I = 2.55 mA, beta_(ac) = 24.07`
Discussion
9.

Assume that the silicon diode in the given circuit requires a minimum current of 1 mA to be above the (knee-point voltage = 0.7 V) of its `I-V` characteristics. Assuming that the voltage across the diode is independent of current above the knee-point, find the maximum value of R so that the voltage is above the knee-point if `V_(B) = 5A`. Also find the value of R to establish a current of 5 mA in the circuit for the since voltage.

Answer» Correct Answer - `4300 Omega,300 Omega
Discussion
10.

In a silicon transistor the base current is changed by `20 mu A`. This results in a change of 0.02 V in the base to emitter voltage and a changed of 2 mA in the collector current (a) Find the input resistance `r_(b),beta_(ac)` and transconductance `g_(m)` of the transistor (b) The transistor is used as an amplifier with a load resistance of `5 kOmega`. What is the voltage gain of the amplifier ?

Answer» Correct Answer - (a)`r_(b) 1000 Omega beta = 100, g_(m) = 0.1 Omega^(-1)`, (b) `A_(v) = 500`
Discussion