Is there a simple way to discharge a bipolar capacitor using a 5V signal and a MOSFET/BJT?

Question

I am using an op amp configured as an integrator, as shown in the schematic below. There is an input signal which can go both positive and negative [+1V ... -1V], and then depending on this input signal polarity the output of the integrator will ramp up or down until it reaches the supply rail of the op amp (±12V in my application). This all works fine:

I would now like to use a control signal from a microcontroller (~5V DC) to be able to reset the capacitor charge to zero at any arbitrary time, and keep it shorted out to prevent further charging. I tried to do this using a simple MOSFET, as shown below:

As can be seen in the SPICE calculation, this doesn't work properly - the charge is reset to zero correctly, but the negative part of the waveform is now clipped. I am not sure I understand why this is the case.

Can anyone tell me what mistake I have made, and suggest a way to achieve what I am looking for? Thanks!

EDIT ------------------------------------------------------------------------

After reading Caleb's answer, I have also tried to place a switch across the capacitor (a classic toggle switch here, but in practice could be an analog switch such as the DG417, as suggested). I have also placed a 1 kΩ resistor in series, to keep the discharge current through the switch at around 10 mA, as recommended in the comments (discharge response time is not an issue or requirement for me at all):

However, now a new problem arises as seen above. I didn't realise the need to specify in the original question, but whenever the capacitor is shorted I would also like the integrator output to be zero, regardless if the input signal is still applied. It can be seen here that the output is non-zero, as the input resistor and series discharge resistor effectively form a potential divider. Does this mean that I also need to add some MOSFET to short the output of the op amp down to ground whenever this "discharge control signal" is applied?

Answer 1

Reset Switch

You should not use discrete MOSFETs for this due to the body diode. A single SPST analog switch such as the DG417 or DG9421 would be ideal for this task. These devices have a logic-level threshold and can operate over a wide voltage range due to the fact that they utilize a transmission gate.

Just one caution: you will probably want to place a resistor in series with the switch so that you stay within the maximum power rating of the part.

---

Zeroing the output

I would not recommend grounding the op-amp's output. Although most small op-amps will survive this, you have to consider what happens when you switch off the op-amp. During reset, the output stage will most likely saturate. This saturation will "carry" over into the next integration cycle, causing a large (and unpredictable) voltage spike as the op-amp tries to compensate for the sudden change in output current. Instead, I recommend one of the following options.

Option 1: use a smaller resistor on your reset. This will not actually zero the output. I would recommend a 220Ω resistor, since it limits the current to 100mA (the maximum 1ms pulsed current rating for the DG417) when combined with the switch resistance. This is a worst-case calculation (24V across the capacitor).

\$ I = 100\mathrm{mA} = \frac{24}{20 + 220} \$

Note that the 100mA pulse will be much shorter than 1ms. In this case the time constant during discharge is 240μs. Also, this does not actually zero your output: it forms a low-pass filter with a cutoff at 660Hz and -12dB DC gain.

Option 2: if you really need the output to stay at zero, you can always add an SPDT analog switch (such as the DG419) between the input resistor and inverting input of the op-amp. Your circuit would then look something like this:

^{simulate this circuit – Schematic created using CircuitLab}

If you are very concerned about transients (you probably aren't), you could use a DPDT switch that connects your input resistor to ground separately, so that the previous stage sees a constant load.

---

Component selection

As @Sunnyskyguy EE75 mentioned in the comments, you are going to have trouble finding a 1μF capacitor that works well in an integrator. Due to a variety of factors (dielectric absorption, leakage, temperature variations, etc.) it is best to use either a plastic film (PP/PS/PPS) or Class I ceramic (C0G/NP0) capacitor. These are generally limited to fairly small values (10s of nF max). Thus, you will want to use a smaller capacitor and larger input resistor.

One small warning: as the input resistor value increases, the input bias current of the op-amp becomes more and more significant. Using the OP07C as an example (\$I_B\$ of ±7nA), a 1MΩ input resistor will result in a 7mV offset between the op-amp terminals. This is an order of magnitude higher than the input offset voltage of the OP07.

Answer 2

Although it is often not shown in the MOSFET symbol, they all have a diode between source and drain. This is called the body diode.

If the source goes more positive than the drain by more than a little bit, the MOSFET will conduct regardless of gate voltage.

You might be able to solve this by adding a second MOSFET in series with the source and drain reversed like this:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

The FET body diode has the Anode on Source which on the OA output causes Vout max = 0.7V while the OA inputs are at 0V in the linear mode of operation.

Your design show T= 1ms with R=1k, C=1uF. You have not given any design specs for integration time and discharge time.

A more practical solution might use a Transmission gate or bipolar Analog Switch with logic level control. Or suitable logic conversion.

Also consider a stable low leakage plastic cap, with a much smaller value like 1nF and R much higher value like 1M if you need 1ms using a much higher impedance or low input bias current OA as you have shown. IF you need high accuracy then choose COG/NP0 ceramic caps that do not have memory or microphonic issues like std ceramic caps.