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I am a beginner in op-amp design and would like to get an explanation to a circuit that has a diode wired in between the output of op-amp-1 (stage 1) and the input of op-amp-2 (stage 2).

The inverting input of op-amp-1 takes a voltage from 0V <= x <= 5V, while the non-inverting input is wired to a 1kOhm potentiometer. The output has a 100kOhm||0.001uF for feedback.

Then, that output is connected to a 10kOhm in series with a 1N4149 diode. That signal is sent through a 0.1uF||100kOhm pot before being used as an input to the second stage (op-amp-2).

Furthermore, if someone could explain the the inclusion of the 0.1uF in parallel with the pot, that would be very beneficial to my understanding.

Thank you for looking this over.

circuit op-amp

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    \$\begingroup\$ Can you tell us what this circuit is supposed to do? That would help someone explain the reason for the diode and other components. \$\endgroup\$ – Peter Bennett Nov 5 '18 at 17:04
  • \$\begingroup\$ The output T1OUT carries a voltage, which is the processed voltage of saline, blood and air - each with their own expected resistances and voltages (1.5 for saline, 0.9 for blood, 0.0V for air). The green parts U17 and U18 stand in for this silicon tube configuration that allows these fluids and air to be present for detection. When saline or blood flow through the tube, the op-amps will process this and allow the T1OUT output to provide information to a computer program to process the voltage. I did not create this schematic, although my job is to interpret it. \$\endgroup\$ – Anthony the Kid Nov 5 '18 at 18:21
  • \$\begingroup\$ That is where I am currently stuck..getting to the nitty-gritty of the reasoning behind the stages and particular components used along the way. \$\endgroup\$ – Anthony the Kid Nov 5 '18 at 18:21
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Very interesting. It looks like an input stage to something equivalent to a theremin although it could just be a capacitive sensor used for other applications. I'm guessing that U17 and U18 (the green plates) are in fact sensor plates and are used to detect the presence of hand movement or some other fairly conductive material that might bridge the plates.

Anyway, enough speculation; the plates (and whatever bridges them) and C16/C17 will form a variable capacitance and inject current into the inverting node of the 1st opamp. The diode (D1) and capacitor (C19) will attempt to resolve the amplitude of the disturbance of this "variable capacitor" and in doing so will also filter high frequency noise.

D1 and C19 form what is known as an envelope detector and this sort of circuit is used in AM radios to demodulate a radio frequency carrier and deliver the audio. I don't think audio is being demodulated but I do suspect that there is a lot of RF that could be present in the signal.

The pot (R17) might be used to set the decay time of the envelope detector.

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  • \$\begingroup\$ I wrote a description of what the U17 and U18 parts were above in the comments, but that may be irrelevant anyway with respect to the usage of the other components. Thank you for your reply, my understanding of the in-depth portions of this circuit are a hell of a lot better than before. A follow up question for you: decay time of the envelope detector..what is it and why is it necessary for the op-amp-2 input? \$\endgroup\$ – Anthony the Kid Nov 6 '18 at 13:29
  • \$\begingroup\$ Why is it necessary - that depends entirely on what the circuit is intended for and I've guessed at some form of hand control detector but now it's your turn to explain the intended use of the circuit. \$\endgroup\$ – Andy aka Nov 6 '18 at 13:37

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