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For a specific application, I need to determine minimum Vds(in other form min Rds) and need to apply it to inverting input of an opamp. I have a source comes with huge ripple like below:

waveform

Before anyone suggest it, no, I can not simply measure the Vds on steady state and put a needed resistor for measured Rds. Because, supply voltage also varies between 13 and 11V and so ripple. Besides, output impedance also varies. So I need to develop an analog design to first, determine the minimum Vds then try to keep that potential on the opamp's inverting gate.

You can see the concept in the schematic below:

schematic

simulate this circuit – Schematic created using CircuitLab

All the components in the circuit must be analog so please avoid to suggest digitally controlling the voltage.

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  • \$\begingroup\$ The opamp will not be happy if you connect capacitor C1 like that at it's output. If you want to filter the signal you need a series resistor between C1 and the opamp's output. \$\endgroup\$ – Bimpelrekkie Apr 3 '16 at 14:41
  • \$\begingroup\$ Could you open up this a little more please? Why a capacitor suits wrong on the output of the opamp? I do not need to filter anything, I just wanted to represent a MOS gate since opamps output will be tied to one. \$\endgroup\$ – Alper91 Apr 3 '16 at 19:08
  • \$\begingroup\$ OK, in your case, controlling the gate of a MOSFET, the capacitor will be so small (1nF max or thereabout) that it does not matter. You can just do that, no problem. But in general opamps have a low impedance output meaning that if you would connect a large capacitor (100 nF or more) large currents will flow when the output of the opamp changes state (when used as a comparator like in your circuit). In the case of an opamp used in a negative feedback situation a large cap at the output might cause oscillations, see the opamp's datasheet to see how large a cap is allowed. \$\endgroup\$ – Bimpelrekkie Apr 3 '16 at 19:32
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In the past I have designed similar circuits, and I have made an envelope detector that was used to identify the minimum voltage. You can do this with a positive reference and a diode, like demodulating AM, or you can just use a RC circuit to GND; however, this approach is dependent heavily on the time constants, and frequency of the input signal. For the sake of simulation, you'd take a unity gain buffer from you "input" signal from Vds and then attach it to a RC network to identify the floor, and then feed that network into Vds_min.

On the academic side, here are a few auto zeroing structures out there. If you want a good mathematical analysis, I would suggest looking up "Autozeroing Floating-Gate Amplifier".

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    \$\begingroup\$ A schematic would make this answer much more clear. \$\endgroup\$ – The Photon Apr 3 '16 at 14:58
  • \$\begingroup\$ I agree, but I'm on a bus on my cellphone. I couldnt get it working. I'll edit it tonight when I get to a mouse. \$\endgroup\$ – b degnan Apr 3 '16 at 16:17
  • \$\begingroup\$ I still could not find a way to do it. Could you sketch it please? It would be very helpful for me. \$\endgroup\$ – Alper91 Apr 4 '16 at 15:26
  • \$\begingroup\$ I added a 100 Hz notch filter and it worked. Thanks. \$\endgroup\$ – Alper91 Apr 19 '16 at 9:26

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