Designing a charge amplifier in Pspice simulation - Page 1
Forum section: electronics / beginners

author: meadow
author_rank: Newbie
timestamp: October 17, 2023, 06:12:38 pm
content: I'm currently working on understanding the functionality of a charge amplifier and verifying the component values required to achieve the desired output. My target gain is 10, as I'm working with a piezoelectric sensor that generates a relatively small voltage signal, so I assume that input voltage peak to peak is 0.1V. This means I need the output to be at least 0.5V.In my recent PSpice simulation, I encountered unexpected results, where the output voltage was significantly higher, around 89.32V. I'm seeking assistance in identifying where I may have made an error in my circuit design or calculations.I would appreciate any insights or advice regarding this issue, and if you could confirm the equations related to charge amplifiers, it would be very helpful.Thank you for your assistance!

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2023, 06:21:09 pm
content: The generator section of your drawing is not a charge source.To make a charge source, you want the small capacitance to be in series with your voltage generator:the charge flowing into your virtual ground input will be governed bydQ=Cx dVAlso, you should not drive a virtual ground directly from a voltage source, as in that drawing.Of course, the virtual ground of the 741 is not perfect.Your piezo source would normally be characterized as an open-circuit voltage (measured by a high-impedance meter or scope) in series with the measured capacitance.If your voltage measurement device has a substantial capacitance compared with the device capacitance, you need to compensate the measured value for the capacitive voltage divider produced there.

author: meadow
author_rank: Newbie
timestamp: October 17, 2023, 06:35:58 pm
content: Quote from: TimFox on October 17, 2023, 06:21:09 pmyou mean that I should make C2 connected between the generator and the amplifier? I am sorry, I am a bit confused.May you draw on what you mean?Quote from: TimFox on October 17, 2023, 06:21:09 pmCould you explain further? I'm not entirely sure I understood.

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2023, 06:42:56 pm
content: Yes, C2 should go from the generator to the 741 input.For an ideal op-amp circuit, a feedback network is required from the output to the input to get a well-defined result.That feedback network tries to make the voltage zero between the + and - inputs of the op amp.Therefore, if the + input is connected to "real ground", the - input is called a "virtual ground", since the voltage there will be zero (or really small) for a reasonable current driving that point.Connecting an ideal voltage source (whose voltage is independent of the load current) directly will therefore overdrive the - input.Your PSpice results, with a voltage generator amplitude set to 0.1 V, indicates a calculatedvoltagegain of 832 = 83.2/0.1.This is basically the open-loop voltage gain of a 741 at 1000 Hz.This is an ".AC" simulation in Spice, where the circuit is assumed to be linear: obviously, a 741 with +/- 15 V power can't put out 83.2 V in reality.

author: meadow
author_rank: Newbie
timestamp: October 17, 2023, 11:34:43 pm
content: I made a little change, but now instead of getting -9.3461 mV I got 93.461mv. Why?

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2023, 11:38:22 pm
content: Wrong change!Your piezo device is a voltage sourcein series witha capacitance.Your 160 Megohm feedback resistor is far, far too large for a 741, which has an input bias current somewhere around 80 nA (typical), which gives a DC error at the output of over 12 V.Did you understand my statement about what the equivalent circuit for a piezo device is?You are still connecting a voltage source directly to a virtual ground: that never works.

author: meadow
author_rank: Newbie
timestamp: October 17, 2023, 11:42:41 pm
content: sorry, forget to change, but I got an output (-554.190 μV)

author: meadow
author_rank: Newbie
timestamp: October 17, 2023, 11:45:22 pm
content: Quote from: TimFox on October 17, 2023, 11:38:22 pmThanks for your help, unfortunately I tired to understand your second reply but I couldn't grasp it at allthen, may you explain it to me what should I do for the virtual ground?

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2023, 11:45:56 pm
content: The 1 uF series capacitor is, I believe, much larger than the capacitance of your piezo: what is the spec?Again, the feedback resistor is too large.A 1 uF capacitor at 1 kHz is only 159 ohms reactive, so you are almost directly connecting the voltage source to the virtual ground.

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2023, 11:54:54 pm
content: Quote from: meadow on October 17, 2023, 11:45:22 pmPlease refer to any elementary textbook that handles op amps.Virtual ground: it's not a wire to ground, but, because the op amp tries to force the voltageVbetween - and + inputs to zero, for reasonable input currentI,the apparent resistance from the - input to ground isR=V/I= 0 /I= 0.Driving a virtual ground works for acurrent(or charge) input, and the output voltage of the op amp is the current multiplied by the feedback impedance.Again: what is the spec for your piezo device?In your first post, your target gain is "10".Is that a voltage gain (10 V/V) or a charge gain (which would be in V/C, not dimensionless)?

author: meadow
author_rank: Newbie
timestamp: October 18, 2023, 11:49:36 am
content: Quote from: TimFox on October 17, 2023, 11:54:54 pmstill I don't have any piezo device, my purpose in this simulation is to see how charge amplifier worksSo i used function generator instead of piezo sensorso, I don't know any spec since I don't have the deviceas for the gain, should be voltage gain in the output, since charge amplifier convert the charge to voltageso the output must be negative voltagehow should I know the suitable value of components for 741?

author: TimFox
author_rank: Super Contributor
timestamp: October 18, 2023, 01:00:20 pm
content: If you want to make a charge source to simulate a piezo device, set your generator to sine or square wave voltage output and put it through a small capacitor to the summing point or virtual ground (low impedance to ground) of the op amp.Since the virtual ground (- input of your op amp) will have very low voltage compared with the generator voltage, the charge is given by the equation dQ=Cx dV.If you really want a "charge amplifier", where the output voltage is proportional to the input charge, you need a feedback circuit where the feedback capacitor dominates the result:You then get an output voltage change dVout= dQin/CfbI can't write an elementary textbook on op amps, including on how to read the data sheet for individual devices, in this short space.The 741 has, by modern standards, mediocre performance.I can only tell you what you need to look up.Do you have "Art of Electronics" or a similar elementary textbook?

author: RoGeorge
author_rank: Super Contributor
timestamp: October 18, 2023, 01:13:53 pm
content: Quote from: meadow on October 17, 2023, 06:35:58 pmWould be easier if you use LTspice instead of PSpice. LTspice is free, and most of EEVblog users might have LTspice installed already.So, if you redraw your schematic in LTspice, then attach the .asc file here, others can change it then post back the modified schematic.