Electrical – Inverting, single-supply comparator not working as expected

comparatoroperational-amplifieroscilloscopevoltage

I am trying to convert an input sine wave to a square wave. The sine wave is centered at 2.5V and has an amplitude of 2V, with a frequency of 100kHz.

I am looking for a hysteresis of 100mV centered around 2.5V. (ie. 50mV below and 50mV above 2.5V)

I am trying to achieve something like the following (except that the output is inverted), where both waves are centered about 2.5V:

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To do so I looked through Design with Operational Amplifiers and Analog ICs, and I thought that I could work with a VTC offsetting single-supply inverting Schmitt trigger.

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I then calculated the resistance values according to my parameters.

VTH = 2.55V
VTL = 2.45V
VCC = 5V

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Assuming R3 = 100kΩ and R4 = 2.2kΩ, I calculated R1 and R2 both being 4.1kΩ`.

I proceeded to construct the circuitry on Proteus software using the LM311 comparator.

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And here is my output.

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Yellow is Channel A, Blue is Channel B.
I zoomed in on the square wave can be seen from the dials.

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The output is not only attenuated, but also unevenly centered. The book mentions none of these effects, so I suspect that I am missing something more fundamental here. Any suggestions on what I am doing wrong would be appreciated.

Best Answer

You've tied the collector of the NPN output to ground and you are trying to take an output from the emitter - you have this the wrong way round. Tie emitter to ground and use a pull-up on the collector like most folk (who still use this ancient part): -

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