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The OPA172 is connected in a constant current source configuration. I'm having a hard time understanding what the impedance will be when measuring between R3 at 10kHz. If you measure the impedance between R3 at 10kHz, will the impedance be:
Output Impedance of Opamp at 10kHz (1 ohm) to ground,
up R1 from ground (100 ohms). Total impedance at 10kHz is 101 ohms at 10kHz?
Hi EE_Matt,
The impedance looking to the left across R3 will be 100k for low frequencies up to ~1kHz where loop gain is large. Beyond this frequency, the circuit's loop gain starts diminishing and the impedance lowers as a result.
At 1kHz, loop gain is 20dB (loop gain = (10MHz / (1+(100k/100))) / 1kHz = ~10V/V = 20dB) and it's down to 0dB at 10kHz. 10MHz is the OPA172 Gain Bandwidth product and (1+100k/100) is the applicable noise gain for this circuit.
Here is an estimate of the output impedance looking to the left showing 71.1kohm:
Here is the TINA-TI simulation file for reference:
OPA172 Output Impedance E2E Hooman 1_23_17.TSC
Regards,
Hooman
