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| Voltage boosting an opamp |
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| John Heath:
--- Quote from: Yansi on December 29, 2016, 09:25:27 pm ---Hello! Just got me thinking... For some project I might need a precision DC voltage source controlled via a DAC. The problem is I need 4 quadrant operation - so opamp is the obvious solution. The voltage we are talking about might be say few +5V downto -50V, output current 1mA is more than enough, BW not critical, 1kHz very well sufficient. The problem is, for +5-50V output I'd need an opamp designed for at least 60V supply operation. I know that some high voltage opamps do exist, but mostly expensive. I'd like to try to stick with jellybean parts for this, not afraid the component count will be a little higher. Only if that fails, I'll buy a high voltage opamp for this :-) Do you have any suggestions, how to voltage boost the output of an opamp? One of my first ideas was to ground the output of the opamp through a rather small impedance and then to use a pair of current mirrors in the opamp supply rails, to control the output voltage. I have drawn my idea, please see below. Does it stand a chance to work? I think it just might, but I may well be mistaken. Thank you for ideas as always! Yansi //EDIT: Just found this voltage booster (the second image below). Almost exact schematic! Thats pleasing I have reinvented the wheel and it might work just fine. Also interesting to note the local feedback through R1 C1. Still I do not get the purpose of matching/pairing the trannies in here. What will the assymetry cause? I can't figure out. --- End quote --- I built a sub woofer for my computer based on your same design. However in my case the op-amp rails went to two darlington NPN / PNP outputs through a resister to base then resister to emitter with both collectors joined for output to speaker sub woofer. It ended up with + and - 30 volts and an average of 10 volts across the op-amp. For thermal protection you can place the op-amp close to the output heat-sink. This way bias temperature corrected current built into the op-amp will temperature correct the output darlingtons. It worker for a few years. I think I still have it around somewhere. In summary I had good luck with your design and found it could be simplified by resistively going from op-amp rails to darlington bases. An added benefit is it looks like it can not possibly work on paper with op-amp output going to ground and + input as negative feedback due to output stage inversion. Lots of fun showing off the diagram to onlookers scratching their heads , ha. |
| Zero999:
--- Quote from: John Heath on January 04, 2017, 02:00:01 pm ---I built a sub woofer for my computer based on your same design. However in my case the op-amp rails went to two darlington NPN / PNP outputs through a resister to base then resister to emitter with both collectors joined for output to speaker sub woofer. It ended up with + and - 30 volts and an average of 10 volts across the op-amp. For thermal protection you can place the op-amp close to the output heat-sink. This way bias temperature corrected current built into the op-amp will temperature correct the output darlingtons. It worker for a few years. I think I still have it around somewhere. In summary I had good luck with your design and found it could be simplified by resistively going from op-amp rails to darlington bases. An added benefit is it looks like it can not possibly work on paper with op-amp output going to ground and + input as negative feedback due to output stage inversion. Lots of fun showing off the diagram to onlookers scratching their heads , ha. --- End quote --- It becomes fairly obvious how it works, once it's pictured, showing the internal schematic of the op-amp. It's been discussed before: https://www.eevblog.com/forum/beginners/non-standard-op-amp-configuration/ |
| basinstreetdesign:
I have a suggestion: You can boost the output range of a jelly-bean op-amp like the 741 by sliding the operating voltage of the amp up or down within a higher voltage than it is rated for. See the first circuit below. The output of the 741 determines what voltage is applied to its own power pins. The resistor dividers connected to the transistors bases set the fraction of the total supply range is applied. It can be shown mathematically that the voltage between the + and - pins of the op-amp never changes regardless of what the output voltage is. The operating power for the amp just slides up and down between the rails as needed by the signal. I have used this a couple of times and it works well for not-so-large bandwidths and reasonably low loads. Don't push the drive specs of the op-amp. The only worry is that the op-amp becomes slightly less stable at higher frequencies than normal and may oscillate if driving capacitive loads or long lines. In that case stick a few hundred ohms at the output pin of the 741 and all is well. The first attachment below shows this arrangement. But if the combined voltage tolerance of the op-amp and two transistor is not enough then the transistors can be stacked to give even more range to the amp. The second attachment shows this with a 741 operating with close to 115V output range. One thing you cannot do with this arrangement is use it to get large gain at the same time as that might require the inputs to be a large voltage away from the output. |
| Yansi:
I think I have already mentioned that and this supply boosting topology has some serious disadvantages, against any other topologies posted here. Please see this paper. http://joebrown.org.uk/images/DualPSU/BootstrappingOpAmps.pdf (Also note you have only a buffer, I need voltage amplifier.) |
| David Hess:
--- 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. |
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