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Depends on the operating point. You can assume that the 2nd op-amp input is high impedance and draws no current.
It seems like too many parts to simply limit the current in an LED. A single resistor will do it.
Are you attempting to clamp Vin to a certain voltage?
Or are you attempting to change the Vin range to a fraction of the original range?
Or are you attempting to generate a reference voltage?
Sorry for all the questions, but like AG mentions, you might be over-thinking the circuit.
The zener diode circuit can't precisely clamp the voltage, it will cause nonlinear behavior far below the zener voltage. You already spending two OPs for your circuit, a third OP plus a diode can precisely clamp the control voltage to an external reference.Op amp #2 should see ~150 mV to ~1.8 V
No. RI senses the LED current and the opamp #2 adjusts the current to be the zener voltage divided by the resistance of RI.Isn't the RI Limit resistor limiting the current through the LED?
Wouldn't the zener be trying to clamp the voltage going into the + of
the opamp and causing the - side to compensate?
By the way, I'm not too clued up on opamp operations.
Regards,
Relayer
Embarrassed to admit am unclear about "operating point" meaning, even after quick online search. Op amp #2 should see ~150 mV to ~1.8 V at non-inverting input (and inverting input) but no higher, theoretically.
Isn't the RI Limit resistor limiting the current through the LED?
Wouldn't the zener be trying to clamp the voltage going into the + of
the opamp and causing the - side to compensate?