I have a circuit which I have designed to pull down a voltage via sinking current into the opamp output. When the source signal is of higher output impedance this works fine, but when the source signal impedance is lower the minimum voltage which this signal can be pulled down to is affected by the opamp output impedance. This source signal is supplied at max 20mA.

In the circuit the opamp will pull the input voltage (VF3) down via the diode to the voltage set on VG2, but can not push the voltage above the input voltage due the diode being reversed biased.

Does someone maybe have a suggestion as to how this could be improved via adding a transistor on the opamp output? E.g have the transistor base connected to the opamp output and the collector connected to the diode negative? I hope to be able to pull this voltage down to a much lower value, ideally rail to rail.

Thanks in advance!

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  • \$\begingroup\$ In the circuit the opamp will pull the input voltage (VF3) down via the diode to the voltage set on VG2 Could you please explain how? The shown circuit (whether it is a opamp or comparator) will pull the output as close to 0V as possible. \$\endgroup\$
    – Huisman
    Commented Jul 16, 2019 at 12:19

1 Answer 1


ideally rail to rail

Rail to rail won't happen because of the diode drop.

Use a PNP bipolar transistor with base on op-amp output, collector to ground and emitter connected to VF3 via the diode. If your supply rail is genuinely only 2.5 volts then you probably won't require the series diode. Any supply rail greater than 5 or 6 volts requires a diode to prevent the base-emitter region getting too much reverse bias and exceeding the transistor specification.

  • \$\begingroup\$ Thanks for the info. The Supply is actually 5V - sorry, its not clear on the diagram. The PNP transistor would work, although there would be a 0.7V drop on Vbe which would worsen the rail to rail performance (Vbe + Vdiode drop). Do you know if there is any way this could be improved? \$\endgroup\$ Commented Jul 17, 2019 at 12:44
  • \$\begingroup\$ You will not get rail-to-rail performance with a limiter circuit because it relies on a diode and diodes have a forward volt drop. \$\endgroup\$
    – Andy aka
    Commented Jul 17, 2019 at 14:57
  • \$\begingroup\$ I am trying to achieve as close to rail to rail performance as possible. Even without the diode, there is still the 0.7 Vdrop from the PNP BE voltage. Is there no transistor better suited for this? E.g Mosfet/IGBT. My knowledge on these technologies is limited so any input would be appreciated. \$\endgroup\$ Commented Jul 21, 2019 at 12:22
  • \$\begingroup\$ You won't get rail-to-rail operation with a limiter circuit. \$\endgroup\$
    – Andy aka
    Commented Jul 22, 2019 at 7:03

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