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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?
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.
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.
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.