33
Experiment 7: The Hartley Oscillator
Purpose and Discussion
The purpose of this simulation is to demonstrate the characteristics and operation of a
Hartley oscillator. The Hartley oscillator is characterized by the inductive voltage
divider made up of L1 and L2. This feedback voltage is used to sustain the oscillations.
Once again, the parallel LC resonant circuit is responsible for the oscillator
frequency.
As with other LC oscillators, the Barkhausen criteria must be met in order for
oscillation to take place. Specifically the gain from input to output must be one and
the net phase around the loop must be zero. In the design in Figure 7-1, the BJT must
have a voltage gain greater than the ratio L1 /L2 in order to sustain oscillations. In
other words, the gain of the BJT must make up for the attenuation created by the
feedback fraction:
Parts
DC 12 V Supply
Transistor: Ideal BJT
Resistors: 500 Ω, 10 kΩ
Inductor: virtual 0.5 mH, 2.5 mH
Capacitor: virtual 1 µF
Test Equipment
• Oscilloscope
Formulae
Frequency of Oscillation
Equation 7-1
Gain
Equation 7-2
BL
LSince AvB then Av B
L
L
=>>=
2
1
1
2
11
.,
fRC
C
=
1
26
π
Av L
L
>
1
2
