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| Current Sink with CFBOA |
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| stcoso:
Hi everyone! I'm trying to build a litttle amp to drive (fast) some power LEDs from a function generator with square wave. I've read a little about using VFB op amps to build current sink for this purpose and their compensation for capacitive loads. I haven't found much about using CFB op amps to do this. I've attached a schematic. The circuit attached works good enough but i've noticed that varying the supply voltage for the LEDs (in the schematic i've only reresented one but there are 5 of them in series with a total Vf=17V and maximum current of 600mA) from 20 to 24 volts may induce some oscillation in the output waveform. I imagine that this is caused by the variation of capacitance in the FET that i'm using. In VFB would be "easy" to compensate for the FET capacitance... but in CFB? I'm using a ths3491 and a pd55015-e. With this circuit i'm obtaining 25ns rise/fall time (measuring the current at the shunt resistance)... can this be improved in your opinion? Thanks in advance for every comment. |
| capt bullshot:
According to this article: https://www.analog.com/en/analog-dialogue/articles/compensating-current-feedback-amplifiers.html compensation should work similar to VFB amps. Edit: forget it, that's something else - and me posting too fast ;) Anyway, 25ns already is pretty fast, don't see an obvious point to improve that, but didn't go into details ... |
| stcoso:
Actually i've read that page some days ago ;D. Interesting insights on how this amplifier works... |
| capt bullshot:
Compensation should work by varying the series resistance in the "-" input (R2). |
| wraper:
--- Quote from: stcoso on August 27, 2019, 08:37:48 pm ---With this circuit i'm obtaining 25ns rise/fall time (measuring the current at the shunt resistance)... can this be improved in your opinion? --- End quote --- Are you trying to make radio transmitter? |
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