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Why Opamp with potentiometer?

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Hi,

Exactly how much do you think changing the output resistor will influence the input resistance?
 
Hi,

Exactly how much do you think changing the output resistor will influence the input resistance?

I think not too much. As I 'm going to use TL081 as op amps which have a high input impedance decreasing 10k to 1k may not affect the input resistance of whole circuit. besides these resistors are in the output of both buffers.
 
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Hi,

Exactly how much do you think changing the output resistor will influence the input resistance?

I think not too much. As I 'm going to use TL081 which has a high input impedance. decreasing 10k to 1k may noe affect the input resistance of whole circuit. besides they are on the output of both op amps.
 
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The TL081 opamp has Jfet input transistors so its input impedance is extremely high (typically one trillion ohms). A trillion is a thousand times a billion or a million times a million.
 
Hello again,


Oh, so the oscillation only occurs with 0.000v input? Well guess what. Once the current mirrors are added, this circuit is no longer a perfectly linear circuit like most op amp circuits, but becomes partly digital in nature. That's because when the input signal goes through zero the current mirror mostly responsible for the output current switches from the upper mirror to the bottom mirror or vice versa. Adding a resistor from the positive supply terminal to the negative supply terminal of the op amp may help this situation because then both current mirrors are always working to some degree, more so than without the extra resistor to draw more quiescent current. The problem may not show up in a simulator.


I forgot to mention here when I breadboard only UPPER current mirror with the op amp of output stage, I mean not the whole circuit, oscillation happens. I guess I should find a way to solve this oscillation at first. As I breadboard the whole circuit It's possible to have this kind of oscillation when I try to
decrease the offset voltage to zero. because the number of op amps is increased. One way is I never ever adjust the offset voltage but I have no idea what is the influence of this action? and How can it affect the whole circuit?
 
To increase the gain there are a couple different things to try.

First, decreasing the 10k resistors on the output of the two input buffers would increase the input voltage to buffere output current ratio so that would mean more current through the input of the current mirrors PER volt of input. Decreasing them to 1k would mean a ten fold increase in current, so that would increase the overall gain by a factor of 10.

Second, the 1k resistor at the output of each current mirror converts the current back to a voltage. If we increase the 1k to a higher value the output voltage will be higher for the same amount of current. Thus raising the 1k to 10k would cause a gain increase by a factor of 10 also.

Third, the output amp gain can be increased by decreasing the two 10k resistors. Deceasing them to 1k would cause a gain increase of 10 again.

Doing all three of these things would increase the gain by a factor of 1000, at least theoretically. You'd have to investigate the best method here. For example, raising the gain of the output stage means imposing more of a limit on the upper frequency range for the circuit.

Also note Part 2 is a little more tricky when one of the inputs is zero because then the zero input still gets a signal from the other section but it gets inverted. We could write an equation to solve for any input though.

I can't decrease 10k to 1k and increase 1k resistors on the output of both currents to 10k or even 2k at the same time. because gain that I got isn't sinusoidal anymore. I don't know what exactly happens but it seems that one of transistors becomes saturated. I would be grateful if you tell me my guess is right or not?
 

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Hi,

I'll have to take another look at this. Theoretically it works, but practically there may be other considerations.
 
I can do one of the things out of 3 things that you mentioned before for increasing gain. Actually for increasing gain I can only increase 1k resistors at the output of current mirrors. when I do change 1k at the output of current mirrors and decrease 10k to 1k at the output stage, I get this error : "Singular matrix: check node u4:24 iteration No.2"
I don't know how can I solve this error?
 

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This circuit is one of the most interesting circuits that I've ever seen. by changing 1k to 20k at the output of current mirror I get another error! :-( In theory
changing resistors at the output of first stage buffer, output of current mirrors and 10k resistors at the input of last stage should give me a high gain, while they make the result worse.
 
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Hi,

Singular matrixes and problems with convergence come up for a number of reasons and most of the time it's not the circuit it's the simulators fault. Sometimes just changing the minimum step size can help solve that issue. It could also be those 1 ohm resistors, as you know i added them just for a simple test earlier but they are not really needed anymore.

If you post the .asc file for the entire circuit i'll take a good look. I drew up part of the schematic but not all of it but since you already did i might as well use the same one you are using.
 
I did simulate this circuit without 1ohm resistors, those kind of errors happened again. By the way I attached .asc file for you to check and test it better.
 

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Hi,

Ok give me a little while to take a look. I'll get back here in a little while from now.

A FEW MINUTES LATER:

Ok your file made it much much easier to see what is going on. This way i get to see first hand what progress you have made.

The first error i got was that "R" was not declared anywhere. This error was found in four places, and turns out it was what used to be the 1 ohm resistors but the value had been changed to simply "R" and not some value like 1 ohm, 2 ohms, etc. So the first thing i did after that was change all of them to zero ohms, "0".
Next, i got an error that said that R11 can not be zero ohms. That was one of the resistors that used to be 1 ohm, so i changed all four to 0.1 ohms.

After that the simulation ran normally, and i clicked on three different nodes in order to observe the voltages there.
In the attachment we see the simulation run. The nodes are:
Yellow: input voltage (1v peak)
Violet: output voltage of one of the current mirrors (i think it was R5).
Blue: output voltage of the final stage.

Comparing the input voltage (yellow) to the output voltage (blue) i find that the gain is 10, as expected with the new 10k resistors on the outputs of all the current mirrors.

So try it again with the new resistor values of 0.1 and see what happens. Remember there are four of them to change.
 

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Thanks. My main problem is when I change 10k resistors on the output of first buffers to 1k and I increase 1k resistors on the output of current mirrors to 10k I don't get sinusoidal waveform at the output it seems one of transistors becomes saturated. Other problem is when I ONLY change the resistors of output stage (100k to 10k and 10k to 1k ) I have "Singular matrix" error again. It's really strange why doesn't this circuit work when we have 1k and 10k at the final stage?
 
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Hello again,

Ok, when you increase the gain past a certain point the output of the op amps (one or more of them) may saturate, which could cause problems. This means when you change the two 10k resistors on the output of the two input buffers to 1k then you should use an input test voltage of 0.1v instead of 1.0v. That means we should still get 10v peak on the output even with resistors 10k and 100k for the output stage resistor pairs.

BTW i did verify that a lone LM358 with the two current mirrors does in fact get it's apparent slew rate increased as per our previous discussion, where the voltage gain is increased because of the current mirrors and choice of output resistor and current mirror output resistor.
This means that i was able to pass a 50kHz signal through the amplifier with a voltage gain of 10. This would be unheard of for just an LM358 with no additional parts (other than gain resistors) as that would only go up to about 8kHz with 10v peak output, and anything higher in frequency would cause severe distortion.
So we see one benefit from using the current mirrors, the slew rate has increased 10 fold. I did not try any higher but i have a feeling we could go up more yet and still get acceptable operation.
Of course there is still the DC input offset but that's another issue which may or may not affect the end application.
Using values of 1k and 9k to get a non inverting gain of 10 with a lone LM358 (no additional parts) and a 10v peak output, the max frequency is close to 8kHz before distortion sets in. With the current mirrors and the LM358 used as a voltage follower, the max frequency goes up to about 80kHz, which is a 10 fold increase in usable frequency and that's pretty significant.
This is still in theory, so there may be practical issues such as current mirror transistor matching and stuff like that, but it looks very promising.
I could show a simulation picture but it just looks like two sine waves, one is the real output of 10v peak at 50kHz and the other is the 1v peak input voltage synthetically amplified to 10v peak just for direct comparison of the input vs the output waveforms. They look identical except for the small output DC offset caused by what i think is the input offset of the LM358 itself.
I might try to take this further up in gain using the current mirrors.
One side issue is that the last stage of your current circuit is using an op amp at the output too, with a gain other than one. If we can change that gain to 1 we can see a higher bandwidth for this circuit provided we can make the gain of 10 elsewhere in the circuit.
The theoretical maximum frequency would be the unity gain frequency of the op amps, but at whatever gain we can get it up to before secondary factors set in, which means we would have effectively increased the gain bandwidth product.
 
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By changing voltage test from 1v to 0.1v I have 10v at the output but this happens when I change 10k at the output of both input buffers and also 1k resistors at the output of current mirrors. Actually I change 4 resistors value.
one of the reasons that I wanted to change the resistors at the output is getting a high cut-off frequency. As cut-off frequency depends on 1/RC. I tried to ONLY reduce resistors values at the output stage to have a better cut-off frequency but I was confronted with "Singular matrix" error. However I'm not totally sure the way that I thought is right. Maybe decreasing these resistors causes other problems.
Very soon I'm going to breadboard this circuit. hopefully with TLo81 op amps this time oscillation doesn't happen. Actually Occurence of oscillation reveals instability of this circuit. Untill now we can't see any oscillation in LTspice that shows us circuit is instable. I don't know why in real world this problem occurs.
 
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Hello again,

Ok, when you increase the gain past a certain point the output of the op amps (one or more of them) may saturate, which could cause problems. This means when you change the two 10k resistors on the output of the two input buffers to 1k then you should use an input test voltage of 0.1v instead of 1.0v. That means we should still get 10v peak on the output even with resistors 10k and 100k for the output stage resistor pairs.

BTW i did verify that a lone LM358 with the two current mirrors does in fact get it's apparent slew rate increased as per our previous discussion, where the voltage gain is increased because of the current mirrors and choice of output resistor and current mirror output resistor.
This means that i was able to pass a 50kHz signal through the amplifier with a voltage gain of 10. This would be unheard of for just an LM358 with no additional parts (other than gain resistors) as that would only go up to about 8kHz with 10v peak output, and anything higher in frequency would cause severe distortion.
So we see one benefit from using the current mirrors, the slew rate has increased 10 fold. I did not try any higher but i have a feeling we could go up more yet and still get acceptable operation.
Of course there is still the DC input offset but that's another issue which may or may not affect the end application.
Using values of 1k and 9k to get a non inverting gain of 10 with a lone LM358 (no additional parts) and a 10v peak output, the max frequency is close to 8kHz before distortion sets in. With the current mirrors and the LM358 used as a voltage follower, the max frequency goes up to about 80kHz, which is a 10 fold increase in usable frequency and that's pretty significant.
This is still in theory, so there may be practical issues such as current mirror transistor matching and stuff like that, but it looks very promising.
I could show a simulation picture but it just looks like two sine waves, one is the real output of 10v peak at 50kHz and the other is the 1v peak input voltage synthetically amplified to 10v peak just for direct comparison of the input vs the output waveforms. They look identical except for the small output DC offset caused by what i think is the input offset of the LM358 itself.
I might try to take this further up in gain using the current mirrors.
One side issue is that the last stage of your current circuit is using an op amp at the output too, with a gain other than one. If we can change that gain to 1 we can see a higher bandwidth for this circuit provided we can make the gain of 10 elsewhere in the circuit.
The theoretical maximum frequency would be the unity gain frequency of the op amps, but at whatever gain we can get it up to before secondary factors set in, which means we would have effectively increased the gain bandwidth product.

A few weeks ago I ONLY breadboarded upper section of this circuit without buffer that you added and I got oscillation with frequency of 2Mhz. I guess at high frequencies top and bottom transistors in current mirror couldn't keep track of each other that's why circuit started oscillating at 2Mhz. besides I think adding those buffers may help because you somehow separated current mirrors from next stage. I would be grateful if I have your thought and idea on this matter. I'm not sure about my statement it's just a guess.
 
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A breadboard has high capacitance between its rows of contacts and its many wires all over the place. The capacitance couples signals together which causes oscillation.
Use a breadboard for low current DC circuits. Use a pcb for AC circuits.
 
A few weeks ago I ONLY breadboarded upper section of this circuit without buffer that you added and I got oscillation with frequency of 2Mhz. I guess at high frequencies top and bottom transistors in current mirror couldn't keep track of each other that's why circuit started oscillating at 2Mhz. besides I think adding those buffers may help because you somehow separated current mirrors from next stage. I would be grateful if I have your thought and idea on this matter. I'm not sure about my statement it's just a guess.

Hi again,

It's difficult to say what would cause the oscillation. Power supply bypassing is the first thing to try.
Maybe the current mirrors fight for control over the output current. In this case maybe some added damping would help.
It's very hard to troubleshoot this kind of problem from a distance though and this is especially strange since there is no feedback. The only real feedback is though the power supply lines, and if that is causing the problem then there would be high frequency ripple on the power supply lines.
 
Before start assembling the entire circuit I have a few questions and prefer to ask first because later It may cause problems. First of all should I use 5 potentiometers for this circuit? I mean I should use one potentiometer for every single op amp that I have or not? Second of all for regulating offset voltage should I use only the last potentiometer which is for the last stage? Because when I assembled the upper section (without buffer) and wanted to get the DC offset voltage to zero the first opamp which is connected to current mirrors didn't have any influence on it so i tried to regulate offset from the second op amp in last stage then the oscillation occurred.
 
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Hi,

I dont think i would use potentiometers (please note the correct spelling of this word).
Also, since you had a problem with only one section then you should start by building just one section and making sure it works right first. You will just complicate the whole thing if you build up the whole circuit when even one section might be at fault.

Are you going to use a solderless plugboard or just solder it together on a regular breadboard?
 
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