Confusion on: High Pass Filter at Op Amp Input

  • Thread starter decaf14
  • Start date
23
7
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.
1571592718497.png
1571592738918.png

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.
1571592834459.png
1571592840858.png

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.
1571592887977.png
1571592893956.png


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.
1571593014263.png
1571593028702.png


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.
1571593271268.png


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.
 

berkeman

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I'm not sure what the problem is (I need to look at your simulations more), but in general you should not try to run that opamp with a single power supply and put a ground-referenced 1Vac signal into its input.
 
23
7
What's the input capacitance of the opamp?
I'm having trouble finding it as the pspice model simply shows this for the lm358.
1571593753780.png


I'm fairly certain it should be negligible compared to the 10u capacitor in series, though. If I simply test the opamp with a series resistance, I should be able to intuit the capacitance to ground based on the low-pass cutoff frequency of the system. Using a voltage source now, I see a -3db at roughly 5 MHz, which should correspond to a capacitance of 32 pF.
1571594085149.png
1571594099509.png


If I analyze the same system, now with a 2.4 gig-ohm resistor in parralel with a 32 pF capacitor, I still see the same lower-than-expected impedance in my frequency range of interest. Although now there are two poles which is what I expect as well because there are two reactive components. You don't see this in the plots I posted first because I did not go out to that frequency - I'm not interested in it.
1571594217297.png
1571594232721.png


Thanks for the response and let me know what you think of this. Also, if you have a better way to determine the input capacitance of an op amp in OrCAD pspice, I'm all ears.
 

berkeman

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Thanks for the response and let me know what you think of this. Also, if you have a better way to determine the input capacitance of an op amp in OrCAD pspice, I'm all ears.
It should be in the SPICE model netlist. I'm at home now without access to my work MicroCAP SPICE software, but you should be able to look at the netlist for that opamp model to see what they are specifying the input capacitance as. It's probably only a few pF, though, so that may not account for the low input impedance you are seeing.

Did you see my post about the power supplies for the opamp? You might be clipping the input signal against the ground rail to the opamp...
 
23
7
It should be in the SPICE model netlist. I'm at home now without access to my work MicroCAP SPICE software, but you should be able to look at the netlist for that opamp model to see what they are specifying the input capacitance as. It's probably only a few pF, though, so that may not account for the low input impedance you are seeing.

Did you see my post about the power supplies for the opamp? You might be clipping the input signal against the ground rail to the opamp...
Dang it. That's exactly it. I don't know why I didn't think of that. Thank you! Sometimes it helps to get an extra set of eyes.

Here's the simulation output that verifies this (I set vsupp to 5 gig-volts and v- is -5 gig-volt). Impedance drops to 56k like expected.
1571595715845.png
1571595737882.png
 

berkeman

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I use +/-5GV in my circuits all the time... :smile:
 

berkeman

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That's exactly it.
BTW, I didn't notice until just now, but somewhere along the way your 1Vac input source turned into a 1Aac source. Not sure why that happened. Just an FYI in case it was unintentional.

1571698501378.png
 

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