Soldering On (Part 7)
This month Tim Edwards concentrates on a fundamental circuit — the oscillator.
Oscillator circuits are fundamental to much electronic music, so this month we will look at a few of the many different oscillators that can be either built or bought complete in chip form.
A square wave or rectangular wave oscillator is produced by causing some kind of amplifier to switch alternately between its maximum and minimum output. Fig 1 shows a basic relaxation oscillator built using an op-amp. The positive feedback (to the non-inverting input) ensures rapid switching from one state to the other. The capacitor charges through R1 until the inverting input voltage exceeds that on the non-inverting input. The op-amp now changes state and the capacitor discharges through R1 until the voltage across C1 drops sufficiently for the op-amp to switch back again.
Figure 2 shows the LM555 timer chip configured in astable or free running mode. The operation is very similar to the previous circuit but the frequency range and drive capability (200mA) are much greater. The related timing equations are:
Charge time (output high) t1 = 0.693 (RA+RB)*C
Discharge time (output low) t2 = 0.693 *RB*C
Period T = t1 + t2 = 0.693 (RA + 2RB) * C
The capacitor is charged through RA and RB until the threshold voltage is reached (2/3 VCC). The internal discharge transistor now turns on as the output goes low and the capacitor discharges through RB and pin 7. When the trigger voltage is reached (1/3 VCC), the discharge transistor turns off and the output goes high. The resultant output is a rectangular wave, and as the equations show, a 50% duty cycle can never be obtained. If, however RA is small compared to RB then a 50% duty cycle can be approached.
For sinewave generation, different types of circuits are required. Fig 3 shows the popular Wien Bridge circuit sinewave oscillator. C1, C2, R2 and R3 form the phase shift feedback network. Only sinewaves will be self perpetuating through this type of circuit so the output will be sinusoidal. D1 and D2 are amplitude regulating diodes. Without some form of regulation the amplitude will increase regeneratively until the output clamps at its limit. R5 is the gain set preset that must be adjusted until the circuit starts to oscillate.
Single chip function generators have been available for some time (eg, XR2206 and ICL 8038), and provide a simple way of building oscillators. Although a little more expensive than using op-amps they can, as Fig 4 shows, produce a low component count circuit that give square, triangle and sinewave outputs. Fig 4 shows one simple circuit using the ICL 8038. There are many variations. The thing to remember here is that the outputs are high impedance, so if you wish to drive a load resistance of less than 10k then an op-amp buffer should be used.
Since there are so many different forms of oscillator and function generator, this introduction to the subject is only designed to give the flavour of the type of circuits possible. For a fuller discussion there are many good textbooks available.
Feature by Tim Edwards
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