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| StillTrying:
Well I'm glad the anti-phase optos seem to work in keeping the linearity without any feedback, because AFAIK that was untested in practice. But, (there's always a but :)) have you seen my photo diode amp step response at the same 500ns/Div speed. LEDphotoTRopamp.gif "the more BW the better!" Yep, it's quite possible to get it much higher than that, I'm setting the target at 1.5M+. :) I simulated the 5 transistor long tailed pair arrangement in the end. The problem is the high 2k5 impedance at the bases, it's much too high, together with the PD capacitance it limits the BW at that point to ~600kHz. Everybody will know more about that 5 TR arrangement than me, so is it possible to get the base impedance down to a few 100 R. After about an hour of trying I gave up and drew this 1 transistor instead. I've got the impedance at the base ~160R, and the BW down to 3MHz, because I think that might be about the best the opto can do. The input current variation is 7mA, -3.5mA to +3.5mA, and the collector OUTput 138mV pk-pk, I've doubled the frequency of my test waves to 200kHz, and they still look good! A problem with that simple 1 transistor is that the output ~2V DC level is not very well defined and varies a bit with temp. I'll add the LT.asc and hopefully someone will suggest simple improvements. :) |
| David Hess:
The 4N35 and similar transistor output optocouplers which provide access to the base connection are a lot faster if the base connection is used. I suspect for a linear application it might be best to treat the output transistor as a photodiode driving a transimpedance amplifier. |
| BrianHG:
--- Quote from: StillTrying on September 12, 2018, 08:22:45 pm ---Well I'm glad the anti-phase optos seem to work in keeping the linearity without any feedback, because AFAIK that was untested in practice. But, (there's always a but :)) have you seen my photo diode amp step response at the same 500ns/Div speed. LEDphotoTRopamp.gif "the more BW the better!" Yep, it's quite possible to get it much higher than that, I'm setting the target at 1.5M+. :) I simulated the 5 transistor long tailed pair arrangement in the end. The problem is the high 2k5 impedance at the bases, it's much too high, together with the PD capacitance it limits the BW at that point to ~600kHz. Everybody will know more about that 5 TR arrangement than me, so is it possible to get the base impedance down to a few 100 R. After about an hour of trying I gave up and drew this 1 transistor instead. I've got the impedance at the base ~160R, and the BW down to 3MHz, because I think that might be about the best the opto can do. The input current variation is 7mA, -3.5mA to +3.5mA, and the collector OUTput 138mV pk-pk, I've doubled the frequency of my test waves to 200kHz, and they still look good! A problem with that simple 1 transistor is that the output ~2V DC level is not very well defined and varies a bit with temp. I'll add the LT.asc and hopefully someone will suggest simple improvements. :) --- End quote --- Place compensation caps across R1 and R2, maybe around 100pf. You will improve bandwidth further. Change that BC546 to a MPSH10 or MMBTH10LT1. Also, Toshiba's optocoupler TLP2719 is around 10x faster than the 4N25. |
| JS:
Well, I got -3dB at 550kHz but with some ringing.. +0.8dB at 440kHz. I can observe the AC component on the ringing in the virtual earth node, so I think here is the NE5532 not being able to keep up, I could try an NE5534 which is a bit faster but I'm not sure if it's going to be stable in this config without external compensation. Compensating with 200pF still doesn't solve the ringing, 270pF completly eliminates it but with a BW limit of 280kHz. Using a 220pF seems to be the sweet spot with very little ringing and the -3dB point is at 350kHz. Here is the response with the 220pF cap with some series resistance, -3dB point a bit over 350kHz. JS |
| BrianHG:
2x J113, or J112 jfet in a push pull source follower/constant current configuration replacing the TLO81 opamp and a bias adjustment on R1 and R2's values could improve bandwidth and get rid of some ring. |
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