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| Voltage boosting an opamp |
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| Zero999:
--- Quote from: David Hess on January 05, 2017, 02:18:06 am --- --- Quote from: Hero999 on January 04, 2017, 04:51:26 pm ---It becomes fairly obvious how it works, once it's pictured, showing the internal schematic of the op-amp. --- End quote --- It works even better if the output of the operational amplifier is used as a current feedback input to stabilize the voltage gain of the output stage. --- End quote --- Like this? |
| David Hess:
--- Quote from: Hero999 on January 05, 2017, 07:44:36 pm --- --- Quote from: David Hess on January 05, 2017, 02:18:06 am ---It works even better if the output of the operational amplifier is used as a current feedback input to stabilize the voltage gain of the output stage. --- End quote --- Like this? --- End quote --- Yes, exactly like that. Some implementations just use AC coupling (capacitor in series with R12) back into the output. Even if you do not use feedback, the series resistor on the output (R13) should be used to trim the output stage frequency response. The output stage feedback is especially important when the load impedance is not well defined making the voltage gain at the collectors of Q11 and Q12 vary. A high voltage implementation adds either zener diodes or cascodes in series with the operational amplifier's supply pins. |
| Yansi:
After loooong loong time, I finally got back to work a bit on this project. So this is what I came up with for a prototype to test: The idea was to have at least a bit of bias flowing through the power stage - in this case 1mA. That equals to about 0.7V across R3 (R4), requiring about 850uA of current through T1, T2. Diodes D1 and D2 shall do just that, as about 1.2V is left across R14+R15 (resulting in about 850uA. Of course, it all depends on the exact voltage drops across all junctions, but I think as a ballpark figure it should work. Resistor R7 was calculated such that under any circumstances it can produce a maximum of 2mA through T1 (T2), resulting in about 10mA maximum output current. T1 and T2 are purposely biased from 0 and 5V, which is the same voltage as the opamp has, so that with a zero output voltage, the opamp output should sit just in the middle around 2.5V. What do you think about that? Will it work or do you see there any major flaws? Thank you for any hints. If all goes well, I will test the circuit tomorrow. |
| dom0:
Looks fine on first glance. Since the MCP6001 is easily outpaced by a melting glacier, you might even get away without C2. Circuits that put gain or extra poles into the feedback loop of an op amp will often need at least some tweaking in practice, outside the simulator. The faster the OP, the more tweaking. |
| Yansi:
Haha, melting glacier... funny. ;D Fortunately in this case, I don't need more bandwidth than say 100Hz, so 6001 opamp will be I think fine in this application. So let's have some fun doing a test PCB for it. |
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