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Simple Frequency Generator Circuits Explained

Updated on May 9, 2013

The adjoining diagram shows a fundamental astable multivibrator circuit using a couple of transistors, capacitors and a few resistors. This transistor multivibrator circuit is perhaps the most popular type of oscillator circuit due to its simplicity, reliability and the incorporation of very ordinary components. The circuit functioning may be understood as follows:

The circuit is basically made up of two identical stages, however the conduction of each transistor depends on the state of the other transistor. These states are controlled by the condition of the opposite capacitors.

Since the conditions of the two stages can never be perfectly identical, one of the stages switches ON first when powered, initially.

It remains switched ON until the capacitor connected across its collector and base of the other transistor is fully charged through the above conducting transistor.

The situation now reverts and the other transistor is forced to conduct and the cycle repeats and keeps on repeating forever as long as power remains connected to the circuit.

The simple configuration also allows easy adjustment of the duty cycle of the oscillations, which is simply done by adjusting either the capacitor values or the relevant biasing resistor values of the two sections.

Simple IC 555 Oscillator Circuit

This is yet another popular oscillator circuit incorporating the ubiquitous 555 IC rigged as an astable multivibrator which is one of its most common modes of operation.

The duty cycle is set by adjusting the pot and the associated resistors. The capacitor decides the overall frequency of the astable.

The advantage of a 555 IC over the above transistor type oscillator is the availability of high current (upto 200 mA) across its output pin#3. This output can be utilized for operating heavy loads directly without incorporating intermediate buffer stage such as a transistor driver stage.

CMOS Oscillator Circuits

With CMOS ICs, oscillator designing becomes very easy, because the gates can be easily wired using just a single resistor and a capacitor. Basically to make an oscillator from CMOS gates, a NOT gate is what one may require specifically.

A NOT gate is a signal inverter, which will invert a potential at its input, and produce exactly the opposite potential at its output. An “inverter” gate is obtainable either from a conevtional NOT gate, like in IC 4049, or other gates like a NAND or a NOR gate may also be modified into an “inverter” by joining their input terminals together, making them as a single input.

While making an oscillator using NOT gates, or modified NOT gates as explained above, one would require two of them if they are ordinary types, and only a single would suffice if Schmidt trigger type of gate is employed.

The first figure shows how two NOT gate inverters can be wired up together to generate a well regulated oscillation pattern. The same effect can be achieved with only a single gate if a Schmidt trigger is used.

The next figure shows how a NAND gate is configured into an oscillator. Again if it’s an ordinary gate we may require a couple of them and if it’s a Schmidt trigger, just one serves the purpose as desired.

In all of the above circuits, the product of the capacitor and the resistor decides the overall frequency rate, and therefore any one of the values may be altered for fixing the desired frequency of the particular oscillator.


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