Schmitt Trigger -Open Loop OpAmps - Operational Amplifiers Types Tutorials Series

Non-Linear Op-amp Circuits

•Op-amps are sometimes used in non-linear open-loop configurations where the slightest change in vIN will force the op-amp into saturation (VPOS or VNEG)–Such non-linear op-amp uses are often found in signal processing applications
•Two examples of such non-linear operation are shown at the left
–In below fig is an open-loop polarity indicator

•If vIN is above or below GND by a few mV, vOUT is forced to either positive or negative rail voltage
–Left-bottom is an open-loop comparator
•If vIN is above or below VR by a few mV, vOUT is forced to the positive or negative rail voltage

–Below is an open-loop comparator
•If vIN is above or below VR by a few mV, vOUT is forced to the positive or negative rail voltage

Schmitt Trigger Op-amp Circuit

•The open-loop comparator from the previous two slides is very susceptible to noise on the input
–Noise may cause it to jump erratically from + rail to – rail voltages
•The Schmitt Trigger circuit (at the left) solves this problem by using positive feedback
–It is a comparator circuit in which the reference voltage is derived from a divided fraction of the output voltage, and fed back as positive feedback.
–The output is forced to either VPOS or VNEG when the input exceeds the magnitude of the reference voltage
–The circuit will remember its state even if the input comes back to zero (has memory)

•The transfer characteristic of the Schmitt Trigger is shown at the left 

–Note that the circuit functions as an inverter with hysteresis
–Switches from + to – rail when vIN > VPOS(R1/(R1 + R2))
–Switches from – to + rail when vIN< VNEG(R1/(R1 + R2))



Schmitt Trigger Op-amp Example for Practice:

•Assume that for the Schmitt trigger circuit shown at the left,
VPOS/NEG = +/- 12 volts, R1 = R2, and vIN is a 10V peak triangular signal. What is the resulting output waveform?
•Answer:

–The output will switch between +12 and –12 volts
–The switch to VNEG occurs when vIN exceeds VPOS(R1/(R1 + R2)) = +6 volts
–The switch to VPOS occurs when vIN drops below VNEG(R1/R1 + R2)) = -6 volts
–See waveforms above

•Consider the case where we start out the Schmitt Trigger circuit with vIN = 0 and vOUT = 0 (a quasi-stable solution point for the circuit) 

–However, any small noise spike on the input will push the output either in the + or – direction, causing v+ to also go in the same direction, which will cause the output to move further in the same direction, etc. until the output has become either VPOS or VNEG.