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I have setup the below circuit; a basic envelope detector, however I believe that the op-amp is struggling with the load capacitance.

circuit

With the 100nF cap removed, then the output of the op-amp behaves as expected following the input voltage (AC around 1.6v DC). The output of the op-amp is driven higher to make up for the diode voltage drop. (Below scope trace, blue input, red output).

scope1

However when the 100nF is added, then the op-amp is not drving the output high enough. It just stays around 1.6V. In theory the output voltage should rise to around 1.9V.

enter image description here

I added the 2.2K output series resistor to try and get round the issue, but it didn't help. Are there any obvious errors or ways around the issue?

It may be that I need to use a different op-amp.

Thanks

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    \$\begingroup\$ An envelope detector needs to rectify the signal; your circuit does not do that because the diode is always forward biased. If you drive the input ac coupled (with split supplies for the amplifier) then it should work. You could also ac couple the output prior to the diode (and the diode should not be part of the feedback loop). \$\endgroup\$ – Peter Smith Jan 7 at 14:40
  • \$\begingroup\$ The diodes purpose is to rectify the signal, unless Im mistaken. It doesnt seem to be rectifying, so the behavior as shown is correct, just a cap forming a low pass filter \$\endgroup\$ – Sam Gallagher Jan 7 at 14:43
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The opamp will saturate at the negative rail unless you add some more components. The opamp can take a while to come out of saturation affecting the circuit operation. You may need to lower R1 if you are running at a high frequency.

Note that R2 and R3 will form a voltage divider, so if this is undesirable, make R2 very small. I try to avoid opamps hitting their current limit, which it will if R2 is small.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I modified my circuit as per your schematic and that made a big improvement. The output level now rises from 1.6V to about 1.8V when input modulated. Not quite the peak value of the input (~1.95V) however. I will tinker with C1 perhaps it is too big / small. \$\endgroup\$ – Phil Jan 7 at 16:05

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