- #1
decaf14
- 23
- 7
- TL;DR Summary
- I'm simulating an op amps impedance in SPICE. I'm confused on why the impedance dips below the impedance of the ground resistor.
Hello,
I am simulating the input impedance of a high-pass filter with the output voltage of the filter input to the non-inverting pin of an op amp. I'm confused as to why the input impedance can possibly dip below the resistance of the high-pass filter resistor. Please see the following circuit diagram and simulation results.
Here, we see that the impedance impedance drops to about 5k ohms despite the resistor being 56k. I theorized that the op amp impedance might be extremely low, so I checked that as well. The impedance of the op amp was roughly 2 G-ohm as shown below.
To make sure I'm not crazy, I simulated the impedance of a normal passive high pass filter and saw that the impedance dropped to exactly 56k as expected.
The way I see it, I can model the op amp as an equivalent resistance of 2.4G-ohm up until about 1 kHz, yielding the system below. I see exactly what I expect: impedance dipping to exactly 56k.
The reason I care so much about this is that I'm modelling the performance of a light detector. The circuit uses a phototransistor to detect light, and an emitter resistor controls the voltage output of the photo transistor. Previously, I was using a two-stage system using two op amps for a single phototransistor. One op amp buffered the signal and the other was an active bandpass filter. This ensured that no parallel loading effect occurred at the emitter of the phototransistor since the buffer input was so large. However, I want to make another version of this design that takes up less space so that I could potentially cram this into a wristwatch design and use it as an educational tutorial. I model my phototransistor simply as an npn device as shown below. Essentially, I want to avoid placing a significantly low resistance in parallel with the 33k resistor so that I mantain exactly 33k of resistance, the "optimal" resistance for the amount of light I detect.
Any input to this issue is appreciated. A couple of my thoughts are that I'm simply simulating it wrong or that the op amp capacitance is significant, but I think neither of those are true based on the curves I've shown above that verify my expectations.
Thanks.
I am simulating the input impedance of a high-pass filter with the output voltage of the filter input to the non-inverting pin of an op amp. I'm confused as to why the input impedance can possibly dip below the resistance of the high-pass filter resistor. Please see the following circuit diagram and simulation results.
Here, we see that the impedance impedance drops to about 5k ohms despite the resistor being 56k. I theorized that the op amp impedance might be extremely low, so I checked that as well. The impedance of the op amp was roughly 2 G-ohm as shown below.
To make sure I'm not crazy, I simulated the impedance of a normal passive high pass filter and saw that the impedance dropped to exactly 56k as expected.
The way I see it, I can model the op amp as an equivalent resistance of 2.4G-ohm up until about 1 kHz, yielding the system below. I see exactly what I expect: impedance dipping to exactly 56k.
The reason I care so much about this is that I'm modelling the performance of a light detector. The circuit uses a phototransistor to detect light, and an emitter resistor controls the voltage output of the photo transistor. Previously, I was using a two-stage system using two op amps for a single phototransistor. One op amp buffered the signal and the other was an active bandpass filter. This ensured that no parallel loading effect occurred at the emitter of the phototransistor since the buffer input was so large. However, I want to make another version of this design that takes up less space so that I could potentially cram this into a wristwatch design and use it as an educational tutorial. I model my phototransistor simply as an npn device as shown below. Essentially, I want to avoid placing a significantly low resistance in parallel with the 33k resistor so that I mantain exactly 33k of resistance, the "optimal" resistance for the amount of light I detect.
Any input to this issue is appreciated. A couple of my thoughts are that I'm simply simulating it wrong or that the op amp capacitance is significant, but I think neither of those are true based on the curves I've shown above that verify my expectations.
Thanks.