Complete Electronics Self-Teaching Guide with Projects

The Hartley Oscillator

25 Figure 9.27 shows a Hartley oscillator circuit. In this type of circuit, the feedback is taken from a tap on the coil, or from a connection between two inductors.

Figure 9.27

Capacitor C_L stops the emitter DC voltage from being pulled down to 0 volts through the coil. C_L should have a reactance of less than R_E/10, or less than 160 ohms at the oscillator frequency.

Questions

Work through the following calculations:

A. What is the resonance frequency?

f_r = _____

B. What is the approximate impedance of the load? Z = _____

C. What missing information prevents you from calculating the fraction of the voltage drop across the coil that is fed back to the emitter?

Answers

A. 80 Hz (approximately).

B. 7.7 kΩ (approximately).

C. The number of turns in the coil and the position of the tap are not known.

Figure 9.28 shows a Hartley oscillator with the parallel LC circuit connected between the collector and the supply voltage. As with the circuit shown in Figure 9.27, this circuit provides a feedback signal to the emitter from a tap in the coil, in the correct phase to provide positive feedback.

Figure 9.28

Project 9.2: The Hartley Oscillator

Objective

The objective of this project is to demonstrate a Hartley oscillator using two inductors in ...