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Electronic Devices and Circuits written test Questions & Answers on “Oscillators – 2”.
1. For a phase-shift oscillator, the gain of the amplifier stage must be greater than ________
A. 19
B. 30
C. 29
D. 1
Answer: c
Clarification: Since the resistor-capacitor combination in the RC Oscillator circuit also acts as an attenuator producing an attenuation of -1/29th ( Vo/Vi = β ) per stage, the gain of the amplifier must be sufficient to overcome the circuit losses.
Therefore, in our three stage RC network above the amplifier gain must be greater than 29.
2. What is the minimum frequency at which a crystal will oscillate?
A. fundamental
B. seventh harmonic
C. second harmonic
D. third harmonic
Answer: A
Clarification: The physical size of the quartz crystal affects the final or fundamental frequency of oscillations. The fundamental frequency is generally called the crystals “characteristic frequency”. The crystals characteristic or characteristic frequency is inversely proportional to its physical thickness between the two metalized surfaces.
3. A circuit that can change the frequency of oscillation with an application of a dc voltage is sometimes called___________
A. a crystal oscillator
B. a voltage-controlled oscillator
C. an astable multivibrator
D. a Hartley oscillator
Answer: b
Clarification: A Voltage controlled oscillator is an oscillator with an output signal whose output can be varied over a range, which is controlled by the input DC voltage. It is an oscillator whose output frequency is directly related to the voltage at its input.
The oscillation frequency varies from few hertz to hundreds of GHz.
4. In order to start up, a feedback oscillator requires______
A. unity feedback equal to 1
B. negative feedback less than 1
C. positive feedback greater than 1
D. no feedback
Answer: c
Clarification: In oscillators using positive feedback it is important that amplitude of the oscillator output remains stable. Therefore the closed loop gain must be 1 (unity). In other words, the gain within the loop; provided by the amplifier, should exactly match the losses (caused by the feedback circuit) within the loop.
5. The Nyquist plot combines the two Bode plots of gain versus frequency and phase shift versus frequency on a single plot.
A. True
B. False
Answer: A
Clarification: Bode plots show the frequency response of a system. There are two Bode plots one for gain (or magnitude) and one for phase.
The Nyquist plot combines gain and phase into one plot in the complex plane. It is drawn by plotting the complex gain g(iω) for all frequencies ω. That is, the plot is a curve in the plane parameterized by ω.
6. An input signal is needed for an oscillator to start.
A. True
B. False
Answer: B
Clarification: The oscillator is a device which gives an AC output without any input. Oscillator needs an input signal for start. But once the capacitor is charged then, Oscillator can effectively start work without an input signal.
7. The lead-lag circuit in the Wien-bridge oscillator has a resonant frequency at which the attenuation is_________
A. 1/2
B. 1/4
C. 1/5
D. 1/3
Answer: D
Clarification: The response curve for the lead-lag circuit indicates that the output voltage peaks at a frequency called the resonant frequency, fr. At this point, the attenuation (Vout/Vin) of the circuit is 1/3.
Vout/Vin = 1/3
The lead-lag circuit in the Wien-bridge oscillator has a resonant frequency fr, at which the phase shift through the circuit is and the attenuation is 1/3.
8. At series resonance, the impedance of a crystal is________
A. minimum
B. maximum
C. equal
D. Zero
Answer: A
Clarification: The slope of the crystals impedance suggests that as the frequency increases across its terminals. At a particular frequency, the interaction between the series capacitor and the inductor creates a series resonance circuit reducing the crystals impedance to a minimum and equal to Resistance.
9. The twin-T oscillator produces a ________ response.
A. low-pass
B. high-pass
C. band -pass
D. band-stop
Answer: d
Clarification: Twin T-oscillator is an RC oscillator consists of Twin T-network and an op-amp. Twin-T is actually a combination of low pass and high pass filter combined parallel response provides a band-stop filter with a center frequency equal to
fr=1/2πRC
This is resonant frequency, Oscillations can occur only at this frequency as the filter provides significant negative feedback at frequencies below or above the resonant frequency which is not good for oscillation.
10. Which of the following improvements is (are) a result of the negative feedback in a circuit?
A. Higher input impedance
B. Better stabilized voltage gain
C. Lowered frequency response
D. Higher input impedance & Better stabilized voltage gain
Answer: d
Clarification: The applied negative feedback can improve its performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, many amplifiers and control systems use negative feedback.