Hartley oscillator Definition
Fig. 4 shows the circuit diagram of a Hartley oscillator using a transistor in C.E. configuration.
In hartley oscillator, tank circuit consists of L1, L2 and C1. Stabilized 1self-bias to the amplifier is provided by R1, R2, Re and Ce. Radio frequency choke is used to provide load for the collector and the reactance of RF choke is greater than that of L2.
Hence the reactance of RF choke may be neglected in the equivalent circuit. The capacitor Cc is used to provide path to the A.C. from the collector to the tank circuit. The capacitor in the base circuit Cb can be neglected in the equivalent circuit as it gives low reactance at the frequency of oscillations.
The transistor in common emitter configuration introduces a phase shift of 180o between the output and input voltages. Moreover, a phase shift of 180o between output and input voltages is also introduced by L1 and L2 (a transformer).
Hence the net phase shift of 360o or 0o is introduced between the input and output voltages leading to positive feedback which is essential for the sustained oscillations.
Hartley Oscillator Working
When collector current flows and begins to increase, the charging of capacitor C1 begins. After that capacitor C1 is fully charged, it begin to discharge through L1 and L2. Hence oscillations are produced.
The voltage in coil L1 is induced by this oscillations. This voltage is applied between the base and emitter and appears in the collector circuit in the amplified form. The mutual inductance M between L1 and L2 accomplishes the feedback of energy from the collector circuit to the base circuit. Thus, losses taking place in the tank circuit are compensated and hence undamped oscillations are obtained at the output.
Hartley Oscillator Derivation
The equivalent circuit of hartley oscillator is shown in fig. 5.
Load impedance between output terminals (ZL)
Here hie and Z1 are in parallel, so their equivalent impedance is given by
Now Z‘ and Z3 are in series, so their equivalent impedance is given by
Z” and Z2 are in parallel, so the load impedance is given by
Output voltage is given by
Using eqn. (2), we get
Voltage feedback to the input is given by
Using eqn. (1), we get
The voltage gain of C.E. amplifier (without feedback) is given by
According to Barkhausen criterion,
Using eqn. (3), we get
Using values of Z1, Z2, and Z3 as given in fig. 24, we get
Comparing the imaginary parts of eqn. (9), we get
Eqn. (11) is the expression for the Hartley oscillator frequency formula.
Comparing the real parts of eqn. (9), we get
Using eqn. (10), we get
Which is the condition for sustained oscillation.