Op amp microphone circuit gain limited? - Page 1
Forum section: electronics / projects

author: pacmann
author_rank: Contributor
timestamp: October 01, 2025, 04:49:28 am
content: Using LM321 I want to amplify a cheap mems microphone. I built a band pass circuit like this pic.R3 is 100 ohms and R4 is a 50k pot. At 50K I get the most amplification but still not enough. I replace R4 with a 100k then 1M but the gain stays about the same as the 50k. Is that a limit of the LM321? Am I using too small resistance for R3?PS. Output of the circuit goes to a ADC input of a micro controller

author: Weston
author_rank: Regular Contributor
timestamp: October 01, 2025, 05:05:16 am
content: Looking at the LM321 datasheet the gain-bandwidth product is 1MHz (1E6). The datasheet also lists a typical large signal gain of 100V/mV (1E5). Additionally, opamps have a low frequency gain limit, you can not extrapolate the gain-bandwidth product down to infinite gain at DC. A slightly dubious graph in the datasheet shows a low frequency gain limit of 100dB (1E5).A non-inverting opamp has a gain of 1+ R4/R3 (using the notation of your schematic). Your circuit would have a gain of 1 + 1E6/100 ~= 1E4. What frequency are you testing at? The gain-bandwidth product is a bit of a rough approximation, but your gain would be limited by the gain-bandwidth product at 100Hz. Human voice is roughly 300 Hz to 3.4KHz (or at least that is the range used for telephones), so it is likely that you are limited by the gain bandwidth product of the LM321.You could either cascade two stages or use an opamp with a higher gain-bandwidth product.https://www.ti.com/lit/ds/symlink/lm321.pdf

author: pacmann
author_rank: Contributor
timestamp: October 01, 2025, 06:06:22 am
content: Quote from: Weston on October 01, 2025, 05:05:16 amThanks for explaining! Yes the band pass caps and resistors are set up to pass between 300Hz and 3KHz. The cascaded circuit works but I thought it will be possible to rid the other op amp if I can make it work with 1 stage.MCP6021 has a 10MHz gain-bandwidth product.https://ww1.microchip.com/downloads/en/DeviceDoc/20001685E.pdfWill even that struggle to amplify 3KHz with a gain of 1E4? The maths points to 1KHz cutoff with 1E4 gain....

author: HumbleDeer
author_rank: Contributor
timestamp: October 01, 2025, 09:53:16 pm
content: Honestly, a two stage unit is desirable in a gain this large. Set the first stage to amplify most of the way so you don't amplify the inherent noise of the first stage too much.If you don't like having two op-amp chips, just use a dual op amp IC.

author: pacmann
author_rank: Contributor
timestamp: October 02, 2025, 06:23:45 am
content: Quote from: HumbleDeer on October 01, 2025, 09:53:16 pmIs it best to put the band pass filter on the second or first stage?

author: SteveThackery
author_rank: Super Contributor
timestamp: October 02, 2025, 09:23:53 am
content: Quote from: pacmann on October 02, 2025, 06:23:45 amI'd put it on both, so you get steeper sides to the response curve.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 02, 2025, 10:52:56 am
content: My advice: never buy a single opamp when you can get a dual for about the same price.Two stages will do the job. Its a good idea to set the DC gain to unity and set the AC gain as required.Why? Lets say you have a gain of 1000x and an opamp with an input offset voltage of 7mV thats 7V of DC at the output.The unity gain DC achieved by blocking the dc path with capacitors to ground.This configuration also makes it easy to convert to single supply operation.Note that the blocking capacitors set the low frequency response too.From a noise point of view, bandwith limiting is always a good idea.See attached

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 02, 2025, 11:22:07 am
content: Terry C6 C7 you dont need ?!?!and also it is better to have as much gain as possible in the first stage = less noise

author: HumbleDeer
author_rank: Contributor
timestamp: October 02, 2025, 09:40:46 pm
content: Quote from: oz2cpu on October 02, 2025, 11:22:07 amFor context: this is because the second stage amplifies the same way -- a ratio. So if you make the signal 10 times louder, so does the noise floor introduced by the op amp. That means that using the first stage as a buffer would be negatively impacting your signal, because you're then enormously amplifying the noise floor introduced by the first op amp. :^)

author: Terry Bites
author_rank: Super Contributor
timestamp: October 03, 2025, 11:23:11 am
content: Yes you're right, schematic error I'm affraid.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 03, 2025, 11:27:38 am
content: DC gain should be unity single or dual supply so C6 and C7 are needed in a high gain amplifier and they increase the slope of the LF roll off, not a bad thing.Yes, You are right about the gain distrubution. Schematic error I'm affraid.