# Why wont my opamp gain properly? (schematic attached)

I am trying to take a input signal which will shift between 1-5Hz and amplify only the AC portion of the signal (0.02V pk-pk, ~3 volt offset). The whole circuit has to run on 3.3v single supply DC. I am passing the input signal through a Highpass filter at f=0.5Hz to do ac coupling and then though an amplified low pass filter f=5hz. I am them amplifying the output of the unity gain using a regular inverting amplifier but I am not seeing any amplification. I am attaching the output:

Can anyone explain why I am not seeing any output wave at all? My input signal is ~7mV and I would like as large of an amplification as I can in my 3.3 voltage constraints. It looks like my filters are working fine since the AC analysis shows the expected results and I am able to read a sin wave after the filtration.

Also is this the proper way of AC-coupling the signal?

[EDIT: fixed the resistor designators and set R6 to go to VG instead of G. Still not seeing final signal]

You don't need that Sallen-Key filter before your amplifier if you are already filtering with the input RC high-pass filter.

You can simply AC couple the input, bias the amp and gain it up.

simulate this circuit – Schematic created using CircuitLab

Some key things to note from this circuit:

The input RC filter in your original circuit had a cutoff $F_c$ of 1 Hz but your text said 0.5Hz. 0.5Hz is actually better, so I kept that and changed the 0.1μF input capacitor to 0.22μF.

$$F_o(R1C1) = {{1}\over{2 \pi R1 C1}} = {{1}\over{2\pi(220nf)(1.6M)}} \approx 0.5Hz$$

Note that the RC combination of R2C2 with the opamp forms an active high-pass filter which should equal or be close to R1C1:

$$F_o(R2C2) = {{1}\over{2 \pi R2 C2}} = {{1}\over{2\pi(68μF)(5K)}} \approx 0.5Hz$$

Unfortunately, this means a fairly large C2 capacitor, because you need to balance that against R2 and the gain of the op-amp that you want (G=100).

You might be able to get around that by using two gain stages x10 of equal design. This way you can use larger R2 values and thus a smaller C2. For example, x10 would be R3=200k, R2=22k, C2=15μF, and you need two of them.

Also note that the LM324 cannot reach the top rail, so it will clip with such a high gain. You should use a better rail-to-rail output amp when working with 3.3V

EDIT: Oh, on further review, I think I see you want to follow the input high-pass filter @ $F_c$ = 0.5Hz with an active low-pass filter with $F_c$ = 5Hz, with the goal of making a bandpass filter out of the two.

If that's the case, then you will want to just AC couple the second stage again and bias it at Vbias. Furthermore, you'll want to change the Sallen-Key cutoff point to something higher than 5Hz if you want to pass 1-5Hz. Remember these are the 3dB down points, so you want some extra bandwidth. So if you want to pass 1-5Hz, then you should use a high pass at 0.5Hz or less and a low pass at 15Hz or more.

Change your 68K resistors on your Sallen-Key low pass filter to 22k and you will have a $F_c$ of 15Hz. This way you won't roll off and attenuate your 5Hz in-band signal too early.

simulate this circuit

• Thank you very much for this great answer! Couple things: you seem to accidentally mention a sallen-key high pass when it should be low pass near the end. I understand it, but for future readers it might get confusing. Also to AC couple the gain stage, do you need the 1.6M resistor? In addition, would it be wise to design this with R1 and R4 being lower values and increasing the C1 and C4 to help reduce resistor noise? – din Feb 7 '14 at 21:55
• Also is the second AC coupling (C4 + R4) necessary? When I am simulating the results it seems to be reducing the signal very much and it seems to have a fine AC analysis without it. – din Feb 7 '14 at 22:27
• @din oops, you're right. Seems like I got the description back to front. I'll fix that. The circuit is drawn correct, though. As for the 2nd AC coupling, I could not get it to simulate without it. You might not need it in practice. If you are building a real PC board then I recommend you place a 0-ohm where C4 is and leave pads for R4 but Do No Install (DNI). That way you can add it later if you need it. In theory you should not need it since the first stage is just a buffer and has no gain. For me it might have been just a simulation issue. – Brian Onn Feb 7 '14 at 23:52

First of all, you need to put reference designators on all of your components consistently, so I can talk about a particular resistor without having to say "the 1.6M resistor" (fortunately, that value is only used once).

You need to connect the bottom end of "the 1.6M resistor" to VG, not ground, in order to have U1B biased to the center of its operating range. This also affects U1D.

Also, do you understand why channel 3 of the scope can't show you anything? You should connect this channel directly to the output of U1B if you want to see the output of the filter.

• It doesn't look like power is connected to the final op-amp either. And, the input is 0V biased into an LM324 which has 0V as its most negative rail. Why oh why do folk still keep making this basic mistake? You don't need to answer dave LOL. – Andy aka Feb 1 '14 at 23:42
• @Andyaka: Presumably, the quad opamp has just one set of power pins, which should be sufficient. It's just sloppy on the part of the person who created the component library to show power pins on every section. – Dave Tweed Feb 1 '14 at 23:54
• Please check the edits I have made to the schematics. I am still unable to see a signal? Am I understanding how to bias the input signal incorrectly? Could someone explain how to properly amplify a AC signal on a single supply? – din Feb 2 '14 at 0:00
• Perhaps Andy is right after all -- it could be that the simulator doesn't think U1C or U1D are powered. Try connecting power explicitly to all three opamps. – Dave Tweed Feb 2 '14 at 0:05
• It wont actually let connect anything to the other sources since I believe they are interconnected. I even tried to use a separate opamp and there is still no output. I am very unsure on how to progress from here. – din Feb 2 '14 at 0:20