Hello everyone!
In the last couple of days I've been designing a really simple circuit just to amp something up (lets call it AMP). To do that I'm using op amps. The ideal would be to supply them with +-4.5V, but since I only have a 9v supply, I'm just biasing them at 4.5v and then using a high pass on the exit.
But here comes the problem, I have another circuit monitoring the output of this previous (lets call it MONITOR). The MONITOR loads the AMP in a way that the output gains a DC level, neglecting the presence of the high pass.
After a few tries I noticed that if I further filtered the input of the MONITOR this offset would go away. Another problem is that I need -40dB at 0Hz and near -1dB at 15Hz. A second order filter would give me these specifications, but a double RC filter doesn't seem to be working. I tried a Sallen-Key and it worked wonders, the thing is, I don't have the negative supply to power it in real life.

tl;dr: Only have a single supply to bias amps, need to filter output with -40dB/dec. Can't use active filters, double RC is not working.

Can you guys help me out? I'm new to designing circuits, it has always been "assemble this and it does that" kind of thing.

Thank's in advanced!


EDIT: The monitor circuit is really sensitive to DC levels since it contains a rectifier.

 

I don't fully understand your circuit without seeing a schematic. Is this strictly an AC signal you are amplifying? What is the frequency range?

You could use a switched-cap circuit to generate a negative voltage from your positive supply. How much negative current do you need?

 

I am using on of those designs, but when I connect the monitor a DC level apears on the "out" signal.

Here's the diagram.

 

I see a few problems with this circuit. First is this just a simulation, or have you built it?

Any how the source, V1 is 4V peak. The gain in your upper circuit ranges from 1 to 20 depending on S1. In the high gain the gain stage would go rail-rail, depending on which opamp you are using. Maybe thats what you want.

The reason that you are getting a DC offset when you connect the monitor is that it's feeding an unbalanced DC current back through R3 and or R20, and your output cap (C3) is charging up. I think you need a buffer amp on the input of your monitor circuit.

As for your filter, you need a two to three pole Butterworth highpass filter. A Sallen-Key is a two pole filter, so it should have been close. You could try two in cascade. Make sure you are using the Sallen-Key high pass topology.

You can see the current that I mention. Connect V1 with a 10 K resistor in series to the monitor circuit input, and look at the current through the resistor. I was a bit puzzled by the Monitor circuit. I didn't recognize it as a full wave rectifier. Where did you get it?

 

I see a few problems with this circuit. First is this just a simulation, or have you built it?

Any how the source, V1 is 4V peak. The gain in your upper circuit ranges from 1 to 20 depending on S1. In the high gain the gain stage would go rail-rail, depending on which opamp you are using. Maybe thats what you want.

The reason that you are getting a DC offset when you connect the monitor is that it's feeding an unbalanced DC current back through R3 and or R20, and your output cap (C3) is charging up. I think you need a buffer amp on the input of your monitor circuit.

As for your filter, you need a two to three pole Butterworth highpass filter. A Sallen-Key is a two pole filter, so it should have been close. You could try two in cascade. Make sure you are using the Sallen-Key high pass topology.

You can see the current that I mention. connect V1 with a 10 K resistor in series to the monitor circuit input, and look at the current through the resistor. I was a bit puzzled by the Monitor circuit. I didn't recognize it as a full wave rectifier. Where did you get it?

Thank you for your reply!

The output is not supposed to go from rail to rail, but this circuit is supposed to allow me to level a variety of input signals to a somewhat standard output level, hence being able to amplify or attenuate, depending on the switch.

When I implemented the Sallen-key filter, everything worked properly, as you say, probably because the amp I was using in it was buffering the input. The problem is that to implement the buffer I need to bias the amp, and I go back to the problem that is supplying a no offset signal to the rectifier.

Maybe a simple voltage follower to buffer the signal and then 2/3 RC passive high pass will do the trick. Let me check.

I saw that rectifier circuit here. I was amazed with it, it's quite simple, works wonders, and no isolation problems as in diode bridge.
The monitor circuit rectifies the signal and then looks for a voltage greater than a predetermined level. By rectifying the signal I can check both positive or negative swings of the signal for opamp clip.

EDIT1: Just checked, there is a current of 20-30uA from R3 and R20.
EDIT2: Kids, always read your manuals!! I just re-read the page I got the rectifier from and they warn about some limitations of the circuit, one of them being the one dougc314 pointed out. It says not to use a coupling circuit with a resistance of more that 1/100 of the resistances used on the rect. I increased R3, R20 and R21 to 100k, then coupled it to "Split" (utilize "U2" as buffer) through a 10uF capacitor and a 1k resistance and works just fine and smooth. Thank you for you advice! It was spot on!

 

That precision full-wave rectifier is my favorite design as it uses only 2 op amps, 3 resistors, and 1 diode. And with single supply op amps it will rectify both positive and negative inputs with a single supply. The op amp with the diode prevents the (+) input of the top op amp from going negative with means the top op amp is a follower for positive inputs and a gain-of-one inverter for negative inputs.

 

Yup, that circuit is a gem!