Mumbai University > Electronics and telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 5M

Years: Dec 15

In LC oscillators the feedback network consists of inductors (L) and capacitors (C), instead of resistors and capacitors in case of RC oscillators.

These LC components determine the frequency of oscillations of the LC oscillator.

The operating principle of LC oscillators is based on the Barkhausen conditions. These oscillators can operate at high frequencies typically from 200 kHz to few GHz. They are not suitable for low operating frequencies because the values of L and C will be large at low frequencies.

•Large value inductors and capacitors are bulky (large in size) and expensive as well.

• But as the operating frequency is increased we need small value inductors and capacitors which are small in size and less expensive.

General Form of LC Oscillators:

LC tuned circuit forms the feedback network while an op-amp, FET or bipolar junction transistor can be active device in the amplifier stage.

The Fig8 (a) shows the basic form of LC oscillator circuit with gain of the amplifier as Av. The amplifier output feeds the network consisting of impedances Z1, Z2 and Z3. Assume an active device with infinite input impedance such as FET or op-amp. Then the basic circuit can be replaced by its linear equivalent circuit as shown in the Fig8 (b).

Amplifier provides a phase shift of 180°, while the feedback network provides an additional phase shift of 180°, to satisfy the required condition.

Table shows the various types of the LC oscillator depending on the design of the reactances $X_1, X_2$ and $X_3$.