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Oscillators Questions and Answers

121.

For a given unijunction transistor (UJT) circuit peak voltage (Vp) is ..........

Diode Voltage VD= 0.6V
Intrinsic stand of ratio (η) = 0.6

UJT circuit: VBB = 3 V between base B2 and base B1 (B1 grounded) and a supply VE between the emitter E and ground

Symbol carrier usual meaning.

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

The peak voltage is 2.4 V.

The UJT fires when the emitter voltage exceeds the voltage at the internal junction of its two base resistances by one diode drop. That junction sits at the fraction \(\eta\) (the intrinsic standoff ratio) of \(V_{BB}\):

\[ \begin{aligned} V_{p} &= \eta V_{BB} + V_{D} \\ &= 0.6 \times 3 + 0.6 = 2.4\ \text{V} \end{aligned} \]

Leaving out the diode drop would give 1.8 V, which is not offered. The other values do not fit \(\eta V_{BB} + V_{D}\) with \(\eta = 0.6\): 3.4 V would need \(\eta \approx 0.93\), while 1.4 V and 1.0 V would need \(\eta\) of only about 0.27 and 0.13.

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122.

The following are the oscillators :

1. Crystal
2. Weinbridge
3. Colpitt's
4. Tunnel diode oscillator
The correct sequence of decreasing order of the frequency of oscillation will be :

Exam Year(s)

UGC NET - 2012,

Answer: A

The tunnel diode oscillator works in the gigahertz range, a crystal oscillator reaches tens of megahertz, an LC (Colpitts) oscillator works up to a hundred megahertz or so, and a Wien bridge RC oscillator is used at audio and low radio frequencies, so the order of decreasing frequency is 4, 1, 3, 2 (option A).

Options B and C put the Wien bridge or the Colpitts above the crystal, and D has the Colpitts below the Wien bridge, which is the wrong way round.

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123.

The phase shift introduced by feed network or amplifier in Wein bridge oscillator is

Exam Year(s)

UGC NET - 2012,

Answer: A

The phase shift introduced by the feedback network of a Wien bridge oscillator is zero degrees, option (A). At the frequency of oscillation, \(f = 1/(2\pi RC)\), the series-parallel RC network gives a phase shift of 0 degrees and an attenuation of one third. The non-inverting amplifier that goes with it also gives 0 degrees, so the total phase around the loop is 0 and the Barkhausen condition is met when the amplifier gain is 3. A shift of 180 degrees, option (C), is typical of the RC phase-shift oscillator, and 90 and 270 degrees, options (B) and (D), are not needed in the Wien bridge.

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