He's got negative feedback. Walk the U2/Q1 loop and note polarities: if U2 output goes positive, Q1 conducts less and its collector goes negative. Q1's collector (and the + input of U2) goes negative, and U2's output goes back negative. Ergo, negative feedback.You want negative feedback, not positive feedback.
It only looks like positive feedback. The transistor adds inversion. The transistor has much more bandwidth than the op-amp so that, by itself, should not be the problem.You want negative feedback, not positive feedback.
I have tried a few values with no joy. The best result is with I have found so far is 100 ohms an 10nF (!). The result is not ideal. For one thing it leaves a nasty 1/2 microsecond overshoot and an even nastier glitch at the other edge of the input that is over 10 times the amplitude of the stable voltage.What I would suggest for stabilizing this thing is to insert a resistor (≈1-10 kΩ or so) in between the input voltage and the (-) input of U2, and a small capacitor (≈100 pF or so) from U2's (-) input and its output. It may take some fiddling with the RC values to get adequate stability while not slowing down the response too much, but it should do the trick.
Dangit. I just HATE it when that happens...My gut feel from what I am seeing is that your idea, although it seems reasonable, does not work. I will look at more combinations and get back to you.
This is part of a circuit to charge a timing cap in about 5ns. It is used to make the triangle in my 100 MHz function generator project.What exactly are you trying to achieve with this circuit?
Are you trying to do some type of constant-current circuit?
The idea seemed so promising. I thought I had done something similar in another circuit but did not think to try it here.Dangit. I just HATE it when that happens...
Hi,I am at a loss finding away to make this circuit stable. The output current has to be adjustable using a pot connected to ground.
I don't mind some reduction in bandwidth. I will just use a faster op-amp and transistors if I have to.
Thanks for any help.
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No, I have done the .AC analysis.Have you done a .AC analysis on this circuit yet? If so, did it give any clues?
Also, have you built this circuit yet, and if so, did it oscillate like in the LTSpice simulation? I ask because I've had a couple of times over the years when Spice indicated a serious problem with a circuit, but when built it worked just fine. Not all Spice component models are good; some are screwed up.
I am hoping that the current source bandwidth will be at least 10 MHz. I will settle for 1 MHz.I couldn't help but start playing with this thing. What does the bandwidth of this circuit need to be to get your 100MHz clock? 1GHz? Man that's some crazy fast stuff.
That is a really bad idea. Solderless breadboards are about the worst thing you could use for any fast circuit. Ideal would be dead-bug style wiring on a solid copper plane, or at least a prototype board with square pads.I keep meaning to test the circuit on a solderless breadboard but other things always seem to come up.
Thanks a lot for the information. This is not something I do lot of so anything helps. I will take some study on my part.Unwanted feedback can cause problems also, so it's a good idea to keep element impedances as low as possible in any circuit you decide to use.