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Electronics Components: Oscillator Circuits

An oscillator is an electronic circuit that generates repeated waveforms. The exact waveform generated depends on the type of circuit used to create the oscillator. One of the most commonly used oscillator circuits is made from a pair of transistors that are rigged up to alternately turn on and off. This type of circuit is called a multivibrator.

If the circuit is designed to continuously cycle between the two transistors, it's called an astable multivibrator because the circuit never reaches a point of stability — that is, it never decides which of the two transistors should be on, so it just keeps flipping back and forth between the two.

Here is a schematic diagram for an astable multivibrator made from a pair of NPN transistors.

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When you first power on this circuit, only one of the transistors turns on. You might think that they would both turn on, because the bases of both transistors are connected to +V, but it doesn't happen that way: One of them goes first. For the sake of discussion, assume that Q1 is the lucky one.

When Q1 comes on, current flows through R1 into the collector and on through the transistor to ground. Meanwhile, C1 starts to charge through R2, developing a positive voltage on its right plate. Because this right plate is connected to the base of Q2, positive voltage also develops on the base of Q2.

When C1 is charged sufficiently, the voltage at the base of Q2 causes Q2 to start conducting. Now the current flows through the collector of Q2 via R4, and C2 starts charging through R3. Because the right-hand plate of C2 is bombarded with positive charge, the voltage on the left plate of C2 goes negative, which drops the voltage on the base of Q1. This causes Q1 to turn off.

C1 discharges while C2 charges. Eventually, the voltage on the left plate of C2 reaches the point where Q1 turns back on, and the whole cycle repeats. The dueling capacitors alternately charge and discharge, turning the two transistors on and off, which in turn allows current to flow through their collector circuits.

Here are a few other interesting things to know about astable multivibrators:

  • The time that each half of the multivibrator is on is determined by the RC time constant formed by the capacitor charging circuits. Thus, you can vary the speed at which the circuit oscillates by adjusting the capacitor and resistor values.

  • You can, if you want, create an astable multivibrator from PNP transistors simply by switching the ground with the +V voltage source.

  • Output from the multivibrator circuit can be taken directly from the collector of either transistor. For example, you could place an LED or a speaker in series with R1 or R4 to see or hear the oscillator in action.

  • Alternatively, you can use a third transistor to couple the multivibrator with an output load. Just connect the emitter of one of the multivibrator transistors to the base of the third transistor and connect the load to the collector.

    This arrangement has two advantages. First, the load itself interferes with the multivibrator circuit if you take it directly from the collector of Q1 or Q2. By using a third transistor, you isolate the load from the multivibrator circuit. Second, the output is much closer to a true square wave when the coupling transistor is used; without it, the output isn't a clean square wave because of the effects of the capacitor charging.

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