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Dual supply would be so much easier...

AmiguelS

New Member
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.
 
Could you use a negative-voltage generator IC to get -9V from your +9V?
Can you AC couple the monitor to the amp output to eliminate the DC offset?
 
I could use such IC, but I was triyng not to just for "cleaness".
I have tried capacitor coupling but it doesn't work as expected, it distorts the "out" signal on negative voltages.

He's the diagram:

JTlX3Zh.png




All comments are welcome, related to this problem or not. As I said, I'm a newbie on this matter...
 
The problem has been solved by increasing R2, R20 and R21 to 100k and to use a 10uF and 1Kohm filter at input of the bottom circuit.
 
Your bias supply is not very "stiff".

It would be better if you you used an opamp as a voltage follower between the junction of R1/R2 and the bias inputs of your two opamp stages.
Some decoupling capacitors on the bias and supply lines would also help.

JimB
 
You are right, I should buffer the bias voltage, or else if I get too much current out of it the voltage will drop.

On the capacitors, are you talking about one from +9V to ground and one from bias to ground?
 
You are right, I should buffer the bias voltage, or else if I get too much current out of it the voltage will drop.

Two resistors (and a decoupling cap) are fine, but 100K is rather high. You only need to use an opamp buffer if there's a lot of stuff connected to it, and if it takes asymmetrical current from the +ve and -ve rails.

On the capacitors, are you talking about one from +9V to ground and one from bias to ground?

Yes, those are both essential.
 
Two resistors (and a decoupling cap) are fine, but 100K is rather high. You only need to use an opamp buffer if there's a lot of stuff connected to it, and if it takes asymmetrical current from the +ve and -ve rails.



Yes, those are both essential.

What kind of values are we talking about? I'm interested in audio frequencies only.
 
Nigel said:
Two resistors (and a decoupling cap) are fine, but 100K is rather high.
.
.
I'd try 10K for a start. and monitor the voltage.

Once upon a time I would have agreed, and I think that I used 10k resistors.
The circuit was all over the place and misbehaving, a voltage follower fixed it and it worked like a charm after that.

JimB
 
I'd try 10K for a start. and monitor the voltage.
I meant for the capacitors.
I will use a voltage follower, I even have an Opamp to spare, the package has 4 and I'm only using 3 (for the bottom circuit, the top circuit uses a different model.)
 
Last edited:
Nigel said:


Once upon a time I would have agreed, and I think that I used 10k resistors.
The circuit was all over the place and misbehaving, a voltage follower fixed it and it worked like a charm after that.

It depends on what the circuit is actually doing, for most purposes resistors are fine - as I said it depends if your design takes different currents from the -ve and +ve supply rails.

But as he's got a spare opamp in his existing package, he may as well use that.
 
Hello again!
Same circuit new problem.
Picture here

I'm using opamp U8 as a comparator. In the simulation I took that image from, the result is TRUE, allowing Q3 and Q4 to turn on.
The multimeters show the current across the components I talked about previously. Notice how high the source current for the opamp is. Is there any way I can reduce it?
 
Simulations aren't real life, but for the schmitt trigger you really want positive feedback to make it 'snap', add a resistor from output to positive input (try a 470K or something like that).

You also need a resistor on the output, as otherwise you're drawing excessive current through the two transistors and LED - it's also better to have the LED in the collector, NOT the emitter.
 
Simulations aren't real life, but for the schmitt trigger you really want positive feedback to make it 'snap', add a resistor from output to positive input (try a 470K or something like that).

You also need a resistor on the output, as otherwise you're drawing excessive current through the two transistors and LED - it's also better to have the LED in the collector, NOT the emitter.

I redesigned that stage and everything works fine now, with low current and all. Thanks.
 

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