84-op-amp-inverting-schmitt-trigger-simulation

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Inverting Schmitt trigger realized with op-amp is given in figure bellow.

Simulate op-amp inverting Schmitt trigger.

inv-schmitt-trigger

Positive feedback is used with inverting Schmitt trigger realized with op-amp. The feedback is applied from the output to the non-inverting input of the op-amp.

Comparator is inverting since the output voltage always has an opposite sign to the input voltage when it is out of the hysteresis cycle. Inside hysteresis cycle output voltage depends on the last state and the circuit behaves as an elementary latch. Hysteresis transfer function for inverting Schmitt trigger is given in figure bellow.

inv-schmitt-hysteresis

If input voltage is lower then low level voltage threshold (VTL), V+ is higher then V and output is saturated to +VCC. If output is +VCC, potential of V+ can be calculated by combining nodal analysis and superposition theorem, as:

inv_schmitt_sch_eq01

Input voltage has to be higher than that, so that V becomes higher then V+, and output is going to be saturated to –VCC. It means that high level voltage threshold (VTH) is

inv_schmitt_sch_eq02

On the other hand, if input voltage is higher then high level voltage threshold (VTH), V is higher then V+ and output is saturated to –VCC. If output is -VCC, potential of V+ can be calculated by combining nodal analysis and superposition theorem, as:

inv_schmitt_sch_eq03

Input voltage has to be lower than that, so that V becomes lower then V+, and output is going to be saturated to +VCC. It means that low level voltage threshold (VTL) is

inv_schmitt_sch_eq04

Input impedance is very high, since series positive feedback is applied.

Go to op-amp inverting Schmitt trigger simulation.

External links:

Op-amp Schmitt trigger Inverting Comparator on Wikipedia

Op-amp Schmitt trigger Inverting Comparator on CircuitsToday

Op-amp Schmitt trigger Inverting Comparator on TalkingElectronics

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