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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:

enter image description here

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.

enter image description here

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.

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  • \$\begingroup\$ You have both scope channels set for AC coupling - that will remove any DC component from the signal, so you won't know where the signal is relative to Ground. The two vertical position sliders have different non-zero settings which will also change the vertical position of the traces. \$\endgroup\$
    – Peter Bennett
    Commented Nov 14, 2018 at 17:28
  • \$\begingroup\$ Also you have not said what the input signal level is not have you said whether it is adequately biased at 2.5 volts. \$\endgroup\$
    – Andy aka
    Commented Nov 14, 2018 at 17:43
  • \$\begingroup\$ Thanks, updated images of oscilloscope. @Andyaka, input signal level is adequately biased at 2.5V, with an amplitude of 2V. \$\endgroup\$
    – rrz0
    Commented Nov 14, 2018 at 17:45
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    \$\begingroup\$ LM311 has enormous gain, and large output-stage current surges during transitions. You may have oscillations. Use a Ground plane. Use a very small package, so inductance in the VDD and GND pins are minimal. \$\endgroup\$
    – analogsystemsrf
    Commented Nov 15, 2018 at 3:56

1 Answer 1

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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): -

enter image description here

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  • \$\begingroup\$ Many thanks for your answer. This very strangely yields no output on my oscilloscope. I will try other comparators such as the LM397. \$\endgroup\$
    – rrz0
    Commented Nov 14, 2018 at 18:06
  • \$\begingroup\$ I must be using the wrong simulator. Got an output 10 minutes after I started the simulation, which has never happened before. This was it. When connecting the output to pin 1 I immediately got an incorrect output, but when connected to pin, I got an output waveform appearing after 10 minutes. Thanks \$\endgroup\$
    – rrz0
    Commented Nov 14, 2018 at 18:21
  • \$\begingroup\$ Ten minutes is an awfully long time to wait for an output! \$\endgroup\$
    – Andy aka
    Commented Nov 14, 2018 at 18:33

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