There are three types of high frequency oscillators:

1.Hartley Oscillator.

2.Colpitt Oscillator.

3.Clapp Oscillator.

Hartley Oscillator:

Figure 1 shows the circuit diagram for hartley’s oscillator. It consist of tank circuit which is made up of two inductors connected in series and an capacitor is connected parallel to this series combination. The frequency of oscillator is determined by the value of capacitor and inductor.

RFC (Radio Frequency Choke) is used to prevent high frequency oscillations passing through power supply.

In this circuit resistor R1 and R2 provides voltage divider biasing to the transistor Q1.Cin is the input DC decoupling capacitor while Cout is the output decoupling capacitor. Re is the emitter resistor which is bypassed by capacitor Ce. The bypass capacitor is not used then AC signal will drop across RE , hence it will alter DC bias condition of transistor and reduces gain.

When the power supply is switched ON the transistor starts conducting and the collector current increases. As a result the capacitor C starts charging and when the capacitor C is fully charged it starts discharging through coil L1. This charging and discharging creates a series of damped oscillations in the tank circuit.

The oscillations produced in the tank circuit is fed back to the base of Q1 and it appears in the amplified form across the collector and emitter of the transistor.

The output voltage of the transistor (voltage across collector and emitter) will be in phase with the voltage across inductor L1. Since the junction of two inductors is grounded, the voltage across L2 will be 180° out of phase to that of the voltage across L1.

The voltage across L2 is actually fed back to the base of Q1. The feedback voltage is 180° out of phase with the transistor and also the transistor itself will create another 180° phase difference.

So the total phase difference between input and output is 360° and this is a condition for creating sustained oscillations.

The frequency of oscillations of the colpitt’s oscillator is given by,

$F=\frac{1}{2 \pi \sqrt{LC}}$

Where, C= Capacitance of capacitor in tank circuit

L= Effective inductance of inductors of tank circuit=L1+L2. If they are wound on same core then consider the mutual inductance M, hence L=L1+L2+2M.

This is used for Radio Frequency application (Frequency range from 20KHz to 30MHz).

Advantages:

Easy to tune by using variable capacitor.

Operate over a wide frequency range typically from few Hz to several MHz.

Disadvantages:

Poor frequency stability.

Applications:

RF application.

Local oscillator in radio and TV receivers.