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Supose I have an opamp with pnp input stage (constant current flowing out the input pin), like the following:

schematic

simulate this circuit – Schematic created using CircuitLab

Would the input from OA2 be susceptible to noise since the current flowing through it is low?

Could I 'fix' this problem by inserting a pulldown very close to OA2 and let OA1 source this current?

MdxBhmt
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  • From the related, I believe this confirms question1: http://electronics.stackexchange.com/questions/84831/why-are-high-impedance-circuits-more-sensitive-to-noise – MdxBhmt Jun 22 '15 at 05:22

1 Answers1

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The impedance that the input "sees" determines the amount of noise that will be present. The only possibly noise sources on that net are the output of OA1, the specified input noise of OA2 and anything getting onto the net via the transmission line.

Assuming that OA1's output is quiet and there is no external noise, the only noise is the internal noise generated by the op-amp's input but this only becomes a problem when the source impedance it sees is significantly high i.e. tens of kohms and above. This is not the case here because OA1 has a very low source impedance of a few ohms or tens of ohms at audio frequencies rising to hundreds of ohms as frequencies get higher.

This op-amp source impedance continues to rise but the transmission line saves the day because it will have capacitance to ground.

All of this has NOTHING to do with input bias currents.

Andy aka
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  • What do you mean by: "...but the transmission line saves the day because it will have capacitance to ground"? How does capacitance ralate to noise? – Golaž Jun 22 '15 at 11:36
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    The possible current noise source is the noise produced by the inputs of the LM324. This is a well-known phenomena for ALL op-amps. if that input connects to a low impedance then that current noise gets shunted away and virtually no noise voltage is created because of it. The op-amp output (OA1) has a low output impedance from DC to several tens of kHz so this does the shunting at low frequencies. At higher frequencies the transmission line capacitance to ground is relevant and it shunts the current away. – Andy aka Jun 22 '15 at 11:41
  • Ohh, because the capacitance is really small and it only "works" at higher frequencies? – Golaž Jun 22 '15 at 11:43
  • Thanks for clearing my confusion. I was kinda expecting it had some relation to current going through a wire. So if I understand this correctly, if I make the source impedance higher, I'm making the net more susceptible to low frequency noise - because OA1 can't shunt it away? – MdxBhmt Jun 22 '15 at 14:44
  • Is there a resource where I could look more into it? – MdxBhmt Jun 22 '15 at 14:53
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    ADI and TI both have good papers/application notes on op-amp noise – Andy aka Jun 22 '15 at 18:02