# Analog Circuits Questions and Answers – Colpitts Oscillator-2

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Colpitts Oscillator-2”.

1. The frequency of Colpitts oscillator is expressed as __________
(Where L is inductance and C is the effective capacitance)
a) 1/(4ᴨ√LC)
b) 1/(2ᴨ√LC)
c) 1/(3ᴨ√LC)
d) √3/(2ᴨ√LC)

Explanation: The frequency of colpitts oscillator is expressed as 1/(2ᴨ√LC) where L is the inductance and C is effective capacitance.

2. Colpitts oscillator uses which type of feedback ________
a) Negative feedback
b) Positive feedback
c) No feedback
d) Positive or Negative depends upon frequency

Explanation: Colpitts oscillator uses positive feedback for oscillation. The feedback is provided through a capacitor voltage divider tank circuit to the amplifying element.

3. To ensure constant feedback which method is used in Colpitts oscillator ________
a) Gang tuning
b) Using same capacitor
c) No method is used
d) Voltage divider always gives constant feedback

Explanation: Frequency of oscillation is determined by the tank circuit and is varied by the gang tuning of two capacitors/center tapped capacitors. As they are tuned, thus as tuning is varied then values of both capacitors varies, and the ratio of both capacitors remains same.

4. If C1 and C2 are the capacitance used in Colpitts oscillator the effective capacitance in the equation of frequency calculation is equal to ___________
a) (ᴨ×C1×C2)/(C1+C2)
b) 3 (C1×C2)/(C1+C2)
c) (C1×C2)/(2ᴨ(C1+C2))
d) (C1×C2)/(C1+C2)

Explanation: In the tank circuit of Colpitts oscillator there are two capacitors connected in series and hence effective capacitance is (C1×C2)/(C1+C2).

5. Which configuration of transistor amplifier is used for Colpitts oscillator?
a) Common emitter amplifier
b) Common collector amplifier
c) Common base amplifier
d) Combination of both common emitter and common collector

Explanation: Common emitter configuration is used for amplifying distorted oscillatory signal to a perfect oscillation. The transistor provides a 180° phase shift and tank circuit another 180°, a total of 0 phase shift.
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6. Phase shift provided by overall tank circuit is ___________
a) 0 degree
b) 90 degree
c) 180 degree
d) -90 degree

Explanation: The overall phase shift provided by tank circuit is 180 degrees. Since the active element produces a phase shift of 180 degrees the tank circuit should create 180 degrees to obey Barkhausen’s criteria.

7. The gain device in the colpitts oscillator act as a ___________
a) Low pass filter
b) High pass filter
c) Band pass filter
d) Band rejection filter

Explanation: The gain device in Colpitts oscillator act as a band pass filter and thus limits frequency of oscillation to a certain range.

8. Which of the following equation gives amplitude of oscillation in Colpitts oscillator using the describing function method? (Where Ic is the is the bias current RL is the load resistor and C1 & C2 are the upper and lower capacitance of voltage divider bias)
a) 2ICRL/C1+C2
b) 2ICC2/C1+C2
c) 2ICRL C2/C1
d) 2ICRL C2/C1+C2

Explanation: Amplitude of oscillation in Colpitts oscillator using describing function method is egual to
V=(2IC RL C2)/C1.

9. The improvement of Colpitts oscillator over Hately oscillator is, Colpitts oscillator’s performance in ___________
a) All frequency region
b) Mid frequency region
c) High frequency region
d) Low frequency region

Explanation: Colpitts oscillator shows better frequency stability in high frequencies than Hartley oscillator.

10. Colpitts oscillator provides more performance than Hartley oscillator because of its ________ elements.
a) Capacitive
b) Resistive
c) Inductive
d) Active

Explanation: Colpitts oscillator provides more performance than Hartley oscillator because of its capacitive feedback elements. Due to less self and mutual inductance in the circuit, frequency stability of the oscillator is improved along with a more simple design.

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