# RC high-pass filter is blocking all current instead of just removing the DC offset

I am trying to simulate a simple current mirror, which accepts an AC current with a DC offset making the entire signal positive, so that it is properly mirrored (the signal was being cut off for negative values). At its output I have added an RC high-pass filter to remove the offset and get an AC current centered at 0A.

For clarification: the input signal is an AC current with frequency 1000Hz, amplitude 1mA, and offset 2mA. The biasing voltage Vout is 1.5V. The capacitor and resistor forming the RC filter are 1nF and 300kOhm (arbitrary values to just test out the working of the circuit).

I am implementing the filter in a way that I saw everywhere: capacitor connected in series and a resistor connected to ground, the output is between the capacitor and resistor.

Edit: The main goal of the circuit is to duplicate a current from one channel of an electrical stimulator into three separate channels that would later feed into cell cultures, that is why I am using a CM. I am adding, and later removing, the offset because the CM works only for positive currents and I don't want to use more complicated biasing circuits for the CM. The cell cultures resistance is around 10 Ohm.

However when I simulate the circuit, the filter seems to be completely blocking the signal, and the output current is 0A. The yellow trace is the input, the red trace is the current mirror output/filter input, and the pink trace is the filter output.

I have changed around the component values to make sure the AC component is in the pass-band of the filter, but the output is still 0A.

Is there something I am missing with the implementation of the circuit? Why is my signal completely blocked?

Thank you for any help and advice.

• Not sure what your intention is here. Your filter is connected directly to a DC voltage source. This same source is feeding NM0, so you can see its current varying, but this has no effect on the voltage at its drain. The AC part of the current passes only through V3 and nowhere else. Commented May 4 at 15:55
• @DaveTweed I put the voltage source there to keep the transistor in saturation region. You are right, I have checked and all of the current goes to the voltage source and not the filter. Is there a way to separate the biasing voltage and the actual output of the current mirror? Commented May 4 at 16:18
• If you want to convert the AC current into an AC voltage, you need to put some resistance in series with the bias source. Commented May 4 at 17:06
• You can put a resistor between the voltage source and the drain. Leave the rest the same. Commented May 4 at 17:07
• @RussellH I'm assuming putting a resistor between the voltage source and the drain would make the current flow into the filter as it would choose the path with smaller resistance, is that right? Unfortunately, the resistor decreases the biasing voltage that reaches the transistor, which could only be compensated by greatly increasing the voltage source value, which would not be possible to be done in practise. Commented May 4 at 17:47

## 2 Answers

I'm still not sure I'm following all your requirements, but based on the comment, "My main goal is to duplicate a current from one channel of an electrical stimulator into three separate channels that would later feed into cell cultures", I wonder whether an approach like the following might be more appropriate. No need for bias sources or filters at all; the input is referenced to ground, and each output is referenced to a virtual ground.

simulate this circuit – Schematic created using CircuitLab

• Thank you for the suggestion and the great schematic. I will try to simulate it myself to see how it works. Out of curiosity, would the CM circuit presented in the question, with right modifications, be able to perform this function? I think the main thing that confused me is how to keep the transistors in the CM in the saturation region, while connecting other components, like the appropriate filter, to its output. Commented May 5 at 18:08

Inserting a resistor between the drain and the bias voltage works. The bias voltage may need to be increased to about 5.0 V. I suggested this in the comments.

You may want to simulate using a real FET instead of the ideal one.