OSCILLATORS

An oscillator is a circuit used to generate a periodic signal. Generally an oscillator is used to create a sine wave, although oscillators generating pulsed waveforms, saw tooth waveforms and square waves also appear in many circuits.

The simplest type of oscillator is the ringing tank circuit. Although widely used in the early days of radio; it is not used today. Practical oscillators are amplifiers whose output is fed back into the input in phase with the input signal. An example (albeit unintentional) is the feedback one hears when microphones are being set up in an auditorium. The feedback is provided by sound waves traveling from the loudspeakers back to the microphone.

In order for a circuit to function as an oscillator, it must satisfy the Barkhausen Criteria:

1. The loop gain must be 1 

2. The loop phase must be a multiple of 360 degrees.

The second condition is obvious - the output must be fed back into the input in phase with the input signal so that the amplitude of the input is increased.

The first condition is not so obvious - real devices saturate above a certain input level, which means that further increases in the input signal do not produce a corresponding increase in the output signal. In order to have a sine wave output, it is important that the gain be close to 1, so that the circuit is not driven into saturation.

The diagram below shows a block diagram of a simple oscillator. It consists of an amplifier whose output is fed back to its input through a resonator that provides the appropriate phase shift. The diagram shows the output waveforms for three different values of loop gain.

The ideal oscillator circuit would be one in which the loop gain is initially greater than 1 when the signals are small, and then decreases to 1.0 as the signal reaches the desired amplitude. Believe it or not, it is possible to make just such a circuit with only a single transistor and a few passive components. While there are many types of oscillator circuits, we will consider only the following four in detail:

Hartley 

Colpitts 

Clapp 

Pierce

The first three use resonant LC circuits to determine the frequency of operation. The last uses a quartz crystal resonator to determine the operating frequency.

The Hartley circuit uses a tank circuit with a tapped inductor, as shown below. This circuit's output is rich in harmonics, which makes it unsuitable for some applications. However, this disadvantage is offset by the fact that the output amplitude of a Hartley oscillator is generally relatively constant over its entire operating range and that the feedback ratio, which is determined by the tap position, remains constant. 

The feedback that maintains oscillation comes from the tapped inductor, L. Typically, the tap is located about  one-quarter of the way up the inductor from the grounded end. The operating frequency of this oscillator is given by:

If L is measured in mH and C in pF, then the resulting frequency will be in MHz.

The Colpitts circuit, shown below,  uses a split capacitor to provide a feedback signal, but in other respects it is similar to the Hartley. The diagram below shows a typical Colpitts oscillator circuit using an FET. The capacitor and inductor values are given as reactances, to allow scaling to any desired frequency. If you are unsure how to do this, review the AC Circuits module.

Colpitts Oscilator

The resonant frequency of the Colpitts oscillator can be determined from the following equation:

The Clapp oscillator is a variation of the Colpitts oscillator that uses a series tuned circuit rather than a tank circuit. The Clapp oscillator is one of the easiest to construct and get running because  the series resonant circuit is not heavily loaded. The typical Clapp tuning network has a high L/C ratio which reduces circulating currents and improves stability.

The Pierce circuit uses a quartz crystal resonator to determine the resonant frequency of the oscillator.

 The internal capacitances of the transistor provide the split capacitor and feedback signal. Because the internal capacitances of a well-designed transistor are quite low,  the Pierce circuit is limited to use at frequencies greater than 1 MHz. Note that the Pierce circuit has no frequency determining element other than the crystal.