Question

In a Colpitts oscillator circuit
A) Capacitive feedback is used.
B) Tapped coil is used.
C) No tuned LC circuit is used.
D) No capacitor is used.

Answer 1

Hint: An oscillator in electronics is a device which is used to produce a particular frequency as per the use. There are different types of oscillators as per need and type of feedback they used for the operation. Here we have asked about Colpitts oscillator, we can draw a circuit diagram to understand its working and the type of feedback it uses.

Complete step by step answer:
In a Colpitts oscillator a transistor is used to produce an undamped oscillation of a desired frequency. A circuit diagram for the Colpitts oscillator is shown below.

As we can see that two capacitors are connected with a common inductor and the center of the two capacitors are tapped or grounded. The tuned circuit consists of two capacitors and one inductor. The RF choke connected to the collector terminal of the transistor decouples any AC signal on the power line affecting the output signal which is obtained across the tuned circuit and the ground terminal shown as \[{{V}_{out}}\].
When the circuit is turned on the capacitor starts charging and it discharges through inductor L, which results in setting up oscillation of frequency which can be given as
\[f=\dfrac{1}{2\pi \sqrt{L{{C}_{T}}}}\]
Where \[{{C}_{T}}\] is the total capacitance for the two capacitors \[{{C}_{1}}\text{ and }{{C}_{2}}\]which are connected in series. \[{{C}_{T}}\]is given as
\[{{C}_{T}}=\dfrac{{{C}_{1}}{{C}_{2}}}{{{C}_{1}}+{{C}_{2}}}\]
Further the output voltage will appear across \[{{C}_{1}}\]and the feedback voltage is produced across \[{{C}_{2}}\].
Hence, the circuit is using capacitive feedback.
Option A is correct.

Note:
As we can see that tuned circuit is used and capacitors are used too, so option C and D are incorrect and the capacitors are tapped not the coil so option B is wrong too. The circuit is using positive feedback as \[180{}^\circ \] phase shift is produced by the transistor and further \[180{}^\circ \]phase shift is produced by the voltage divider of the two capacitors.