Op amp inverter amplifier clamping of signal. - Page 1
Forum section: electronics / projects

author: VAO1
author_rank: Newbie
timestamp: October 15, 2025, 04:04:30 pm
content: Good day,I am trying to shorten a pulse coming from a sensor to reduce the chance of pulse pile-up between different sensors that all share the same readout.My approach is to add a capacitor in parallel with a resistor at the output of the signal. This modified signal is then fed into another amplifier (configured as an inverter, biased between Vcc and GND). The purpose of this amplifier is to invert the signal and clamp the negative output.In simulation, the circuit works as expected. However, in my physical implementation, the negative part is not clamped, and I don’t understand why.Any help with this issue would be greatly appreciated.I’ve attached images of the simulation (Image1 shows the simulation layout) and photos of the real setup.Thanks a lot in advance,Best regards,

author: jwet
author_rank: Frequent Contributor
timestamp: October 15, 2025, 04:35:14 pm
content: Anything that is AC coupled (goes through a cap) will have to have equal areas above and below zero unless you clamp it with something non linear.Its common to see this kind of shaping in nuclear pulse processing. Is this the application? In those circuits, there is usually a charge amp that takes a pulse input and outputs a votage output that has a lot of extraneous frequency components. Nucleonics guys think in the time domain usually though. The charge amp output is usually followed by a chain of RC and CR type filters. Typically its a single RC (low pass) and then multiple (3 is common- high pass), CR's to give a pulse shape with optimum SNR. Radiation Detection and measurement by Glenn Knoll is a good text that covers pulse shaping for nucleonics. Once you have shaped pulse, you have extracted the frequency components that you want, you can't do a ton with it, you removed most of the BW. If you want to make a pulse narrower, you need a high pass type response. The difficulty may be that there is not energy in those bands, its already filtered. It also might not be the best SNR shape for the signal. Look at Knoll.

author: VAO1
author_rank: Newbie
timestamp: October 17, 2025 02:02:15 pm
content: Good day Jwet. Thank you for your answer.Yes, I have looked at Knoll, The idea is to do a zero-pole cancellation. In the simulation this gave a narrower pulse, with an overshoot, so the idea was to eliminate this overshoot with the inverter amplifier powered from GND to Vcc (so the overshoot becomes an undershoot, but because the amplifier is powered from GND to Vcc, it cant go under GND, so the pulse should be clamped, only getting the narrower pulse), as it can be seen in figure (salida 3). The idea for DC coupled was to eliminate DC components, about the equal areas above and below zero, I am trying to read some about this, dont really get it now.Best regards!!

author: Terry Bites
author_rank: Super Contributor
timestamp: «Reply #3 on:Todayat 09:14:04 am »
content: Here's a great read, if you like this kind of thing.I think the body diodes in the opamp's cmos ouput stage are allowing the signal to swing below ground.Its hard to know if they are included in the spice model.

author: VAO1
author_rank: Newbie
timestamp: «Reply #4 on:Todayat 06:45:03 pm »
content: Good day Terry Bites,Thank you very much!, yes I will read the document.Difference in model and real component makes sense.Best regards!