The Hartley oscillator is as popular as Colpitt’s oscillator and is widely used as local oscillator in radio receivers. The circuit arrangement is shown in figure. Hartley oscillator circuit is similar to Colpitt’s oscillator circuit, except that phase-shift network consists of two inductors Lr and L2 and a capacitor C instead of two capacitors and one inductor. The output of the amplifier is applied across inductor Lx and the voltage across inductor L2 forms the feedback voltage. The coil L1 is inductively coupled to coil L2, the combination functions as an auto-transformer. However, because of direct connection, the junction of L1 and L2 cannot be directly grounded. Instead, another capacitor CL is used. The operation of the circuit is similar to that of the Colpitt’s oscillator circuit.
Considering the fact that there exists mutual inductance between coils L1 and L2 because the coils are wound on the same core, their net effective inductance is increased by mutual inductance M. So in this case effective inductance is given by the expression
L = L1 + L2 + 2 M and resonant or oscillation frequency is given by the expression
F = 1 / 2∏√[C (L1 + L2 + 2M)]
Hartley oscillator can also be suitably used for generating RF signals. The frequency can be easily varied by varying the inductances which can be done by making the core movable. Another method of varying frequency is of varying capacitance. Hartley’oscillator is not suitable for low frequency work because at low frequency, the value of inductance required becomes large.