LM 7171 Op Amp Oscillation - Page 1
Forum section: electronics / projects

author: flyingphoenix
author_rank: Newbie
timestamp: June 11, 2025, 10:55:26 pm
content: Hi guys, I used LM7171 (non inverting configuration) to amplify my DC source but it's showing sign of oscillating.Feedback resistor of 500ohm was used with another 100ohm resistor, hence expected gain of 6V. Supplied with +-10V. Checked with multimeter and op amp works and gives a gain of 6. However when I used probe (https://www.rigol-uk.co.uk/product/rigol-rp3500a-500mhz-passive-oscilloscope-probe/) and see the output at oscilloscope, it's oscillating,Added bypass capacitor of 2.2 uF (tantalum) and 0.1 uF (ceramic) from both V+/V- to GND.Added 3.3 pF feedback capacitor, and try to increase up until 300pF, doenst help to reduce oscillation.Added 1kohm resistor at input, and 68ohm resistor at output. But these don't work as well.Anyone has any idea why?Orange wire = GNDWhite wire = +10VBlack wire -10VGrey wire = inputGreen wire = output

author: TimFox
author_rank: Super Contributor
timestamp: June 11, 2025, 10:58:35 pm
content: Your circuit, as drawn, does not have a gain of 6 V/V.The 100 ohm resistor needs to go from the inverting input to ground, while the 500 ohm goes from output to inverting input.

author: flyingphoenix
author_rank: Newbie
timestamp: June 11, 2025, 11:02:55 pm
content: Hi, thanks for your reply.My bad, i didnt put the GND in the drawing.However, on the board, you can see that I connected one of the 100ohm (blue resistor) leads to ground, while the other one to the inverting input (pin no 2). Then, the 500ohm is connected from pin no 2 (inverting input) to the output (same row as green wire).

author: moffy
author_rank: Super Contributor
timestamp: June 11, 2025, 11:41:20 pm
content: I don't think I would be building it on perf board, 200MHz BW opamps are sensitive to stray capacitance and inductance, and need really good supply decoupling. Maybe using the 'dead bug' approach of building it over a ground plane e.g.https://spectrum.ieee.org/with-the-dead-bug-method-hobbyists-can-break-through-the-highfrequency-barrier

author: D2236
author_rank: Regular Contributor
timestamp: June 12, 2025, 12:03:24 am
content: Pay attention to section 6.4 in the data sheet about layout and component selection for R and C.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 12, 2025, 08:34:56 am
content: Quote from: flyingphoenix on June 11, 2025, 10:55:26 pmUsing an RF amplifier to amplify DC voltages is unreasonable. The DC accuracy of this amplifier is poor, and unnecessary speed will create difficulties in building the prototype. Circuits with this amplifier should be built in the same way as radio circuits. 100nF THT capacitor with 1" leads have SRF close to 1MHz it doesn’t work as capacitor >1MHz. At 200MHz this capacitor act as low quality inductor. It's just one example of an element that doesn't work in the VHF range, but there are others as well.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 12, 2025, 11:31:39 am
content: Quote from: MariuszD on June 12, 2025, 08:34:56 amThis.That opamp is too fast for a purpose (?) and for breadboarding. It's much easier to use LM358 instead (or some average opamp in between, like OP07), I guess.

author: flyingphoenix
author_rank: Newbie
timestamp: June 12, 2025, 09:14:30 pm
content: Thanks for the insight. I tried changing the input source to a pulse (light blue) and I didnt see the oscillation anymore.However, when I tried this, although the expected gain is 6, but the output is lower. And there's noise in the output waveform. (dark blue).Is it due to the board it self or there might be other reason?

author: Andy Watson
author_rank: Super Contributor
timestamp: June 12, 2025, 09:52:34 pm
content: Quote from: flyingphoenix on June 12, 2025, 09:14:30 pmYou are wasting your time! These op-amps claim to have a bandwidth of 200MHz. At this frequency the circuit on the perf-board bears no resemblence to the schematic that you have drawn (and I don't just mean the missing ground). A few centimetres of conductor will have significant inductance. The photo suggests that you have used perf-board with copper tracks on the reverse, unless you have milled away large protions of conductor you will find stray capacitances, measured in 10s of pF, where you don't expect it! Even using a scope-probe will add a few pF.As has been mention above, look at section 6 of the datasheet. There are reasons why it advises against using sockets. The layout of components at these high frequencies is critical. If I was experimenting with these op-amps, I would consider a ground plane to be the starting point.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 13, 2025, 08:32:34 am
content: It is possible to make working prototype with high speed amplifier but not on perf-board. Do it using method "dead bug on ground plane" using SMD components as possible. Especially power supply decoupling capacitors should be connected close to IC