A Square Wave Generator

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Direct links to other pages:
Basic Summing:	[Setting the Gain Coefficient] [Analog Addition] [Adding a Fixed Constant]
Variations in Feedback Circuits:	[Integrators] [Differentiators] [Logarithmic Amplifiers] [Non-Inverting Amplifiers] [A Difference Amplifier] [Increasing the Output Current Capacity] [A Half-Wave Rectifier] [A Full-Wave Rectifier]
Mixing Analog and Digital Technologies:	[Comparators] [Digital to Analog Conversion] [Analog to Digital Conversion]
Generating Waveforms:	[A Square Wave Generator] [A Triangle Wave Generator] [A Sine Wave Generator]
Operational Amplifiers:	[Characteristics of Operational Amplifiers] [Inside the 741]

A Square Wave Generator

One requirement in a wide range of applications is a spontaneous source of some continuous signal, having a regular and definable wave shape. One of the most important of these is a squarewave.

The circuit to the right uses a comparator with both positive and negative feedback to control its output voltage. Because the negative feedback path uses a capacitor while the positive feedback path does not, however, there is a time delay before the comparator is triggered to change state. As a result, the circuit oscillates, or keeps changing state back and forth at a predictable rate.

Because no effort is made to limit the output voltage, it will switch from one extreme to the other. If we assume it starts at -10 volts, then the voltage at the "+" input will be set by R₂ and R₁ to a fixed voltage equal to -10R₁/(R₁ + R₂) volts. This then becomes the reference voltage for the comparator, and the output will remain unchanged until the "-" input becomes more negative than this value.

But the "-" input is connected to a capacitor (C) which is gradually charging in a negative direction through resistor R_f. Since C is charging towards -10 volts, but the reference voltage at the "+" input is necessarily smaller than the -10 volt limit, eventually the capacitor will charge to a voltage that exceeds the reference voltage. When that happens, the circuit will immediately change state. The output will become +10 volts and the reference voltge will abruptly become positive rather than negative. Now the capacitor will charge towards +10 volts, and the other half of the cycle will take place. The output frequency is given by the approximate equation:

f_out =
	1

	2R_fC ln (	2R₁	+ 1 )

		R₂

In practice, circuit values are chosen such that R₁ is approximately R_f/3, and R₂ is in the range of 2 to 10 times R₁.