Basics of Schmitt Trigger Circuits – Part 2

A Schmitt trigger is a simple concept, but it was not invented until 1934, while an American scientist by the name of Otto H. Schmittwas still a graduate student. He was not an electrical engineer, as his studies were focused on biological engineering and biophysics.
He came up with the idea of a Schmitt trigger as he was trying to engineer a device that would replicate the mechanism of neural impulse propagation in squid nerves. His thesis describes a “thermionic trigger” that allows an analog signal to be converted to a digital signal, which is either full on or off (‘1’ or ‘0’).

Little did he know that major electronics companies like Microsoft, Texas Instruments, and NXP Semiconductors could not exist as they are today without this unique invention. The Schmitt trigger turned out to be such an important invention that it is used in the input mechanisms of virtually every digital electronic device on the market.

The concept of a Schmitt trigger is based around the idea of positive feedback, and the fact that any active circuit or device can be made to act like a Schmitt trigger by applying the positive feedback such that the loop gain is greater than one. The output voltage of the active device is attenuated by a determined amount and applied as positive feedback to the input, which effectively adds the input signal to the attenuated output voltage.

This creates a hysteresis action with upper and lower input voltage threshold values. Most of the standard buffers, inverters, and comparators use only one threshold value. The output changes state as soon as the input waveform crosses this threshold in either direction. A noisy input signal or a signal with a slow waveform would appear on the output as a series of noise pulses. A Schmitt trigger cleans this is up - after the output changes state as its input crosses a threshold, the threshold itself also changes, so now the input voltage has to move farther in the opposite direction to change state again. Noise or interference on the input would not appear on the output unless its amplitude happens to be greater than the difference between the two threshold values. Any analog signal, such a sinusoidal waveforms or audio signals, can be translated into a series of ON-OFF pulses with fast, clean edge transitions.

There are three methods of implementing the positive feedback to form a Schmitt trigger circuit. In the first configuration, the feedback is added directly to the input voltage, so the voltage has to shift by a greater amount in the opposite direction to cause another change in output. This is commonly known as parallel positive feedback.  In the second configuration, the feedback is subtracted from the threshold voltage, which has the same effect as adding feedback to the input voltage. This forms a series positive feedback circuit, and is sometimes called a dynamic threshold circuit. A resistor-divider network usually sets the threshold voltage, which is part of the input stage. The first two circuits can easily be implemented via use of a single op amp or two transistors along with a few resistors. The third technique is a little more complex, and is different in that it doesn’t have any feedback to any part of the input stage.  This method uses two separate comparators for the two threshold limit values and a flip-flop as a 1 bit memory element. There is no positive feedback applied to the comparators, as they are contained within the memory element. Each of these three methods is explained in more detail in my next article.

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