Hartley oscillator, Operation in Hartley oscillator, Advantages & Disadvantages of Hartley oscillator
Hartley oscillator
Hartley oscillator are inductively coupled, variable frequency oscillators where the oscillator may be series or shunt fed. Hartley oscillators have the advantage of having one centre tapped inductor and one tuning capacitor.
If we have X1 and X2 to be inductance and X3 to be a capacitance then the oscillator is called as the Hartley oscillator. The LC acts as the feedback network.
The amplifier used along with the LC network can use a transistor or OP-AMP as the active device.
Operation of the circuit
The feedback circuit is formed by components L1,L2 and C.
The amplifiers is in CE configuration. Hence it provides 180’ phase shift. The feedback circuit provides an additional 180’ phase shift .
The frequency of oscillations is given by :
f = 1 .
2π √ (C (L1 +L2 ))
If Inductance L1 and L2 are equal then,
f = 1 . = 1 .
2π √2 L C 2 √ 2π √ (L C)
Where L1 = L2= L
The feedback factor of the LC feedback network is given by
ß = L1 / L2
Therefore to obtain sustain oscillations, the amplifier gain should be,
A > L2 / L1
This will satisfy the condition A ß > 1.
Advantages & Disadvantages
Advantages of Hartley Oscillators
The frequency is simply varied by the net value of C in the tank circuit.
The output amplitude remains constant when tuned over the frequency range.
The feedback ratio of L1 to L2 (figure 1) remains constant.
Disadvantage of Hartley Oscillators
The output is rich in harmonic content and therefore not suitable where a pure sine wave is required.
Applications
It is used as local oscillator in radio and TV receivers.
In the function generators.
In RF sources.