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Driving N-channel mosfet with op-amp, mosfet switch higher voltage then op amp supply

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Newbie level 4
Aug 30, 2014
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Hello everyone,

I have one question.

When I simulate circuit below it shows error if I start simulation while signal voltage is set to be lower then reference voltage (mosfet should be OFF).

But if I start simulation while signal voltage is set to be greater then reference voltage everything works fine (mosfet ON), I can then during simulation decrease signal voltage to be lower then reference voltage and mosfet will turn OFF and then if I again increase signal voltage above reference voltage mosfet again turns ON and there is no error.

So error occurs only in one case and I don't know whether this part of circuit will work fine in real life? Is this some error in Proteus or I am maybe doing something wrong?

Best regards


What is the signal level?

You don't use the opamp as a regular amplifier because you miss the input resistor, so in theory a very little input voltge would cause output voltage on the opamp go to max.

Thanks for fast response,

Yeap it should work like comparator, Image bellow shows signal levels. Signal goes from 2.5V to 0V. Mosfet is logic level, supply for op-amp can be 9V from battery. Signal is slow changing DC and 680k resistor is for hysteresis


I had a simulator do something similar. I was testing a circuit with the same semi-hysteresis aspect which we don't usually do with an op amp. The output fooled me but it was not the real-life response.

I tried increasing the input signal so that its amplitude was greater than the supply rails. When it reached the supply rail, then the simulator started to create the proper response. After that the output was sensible.

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I suggest that you install an input resistor for 'Signal'. That will create a hysteresis loop as it is normally made.

The LM358 opamp is one of the slowest ever made. The 1k gate resistor at the Mosfet charges and discharges its high gate capacitance slowly. Then the Mosfet output will ramp instead of making fast switching.

The signal source must have some series resistance or your circuit will not have any hysteresis.

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