Single Op Amp as a voltage regulator?

  • Thread starter nand_1
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  • #1
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Hello all.

I was just wondering if it is possible to use an inverting opamp configuration with feedback to regulate an output voltage V0 without the use of BJT's?

For example, the following sketch:
http://img411.imageshack.us/img411/2389/opamp.png [Broken]

V0 = Vs * (R1-R2) /2R1
Mathematically, V0 seems to be independent of load resistance Rl (not shown in diagram) so in effect V0 is regulated (provided we are within the limits of op amp).

My problem is that in every regulator example i have seen, a minimum of an Op Amp and a bjt is used. This gets me thinking that there must be something so wrong/bad with this single opamp regulator that it is never used.

Any comments appreciated.
 
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Answers and Replies

  • #2
245
13
In general, signals and power should be treated seperately. Signals are typically low currents that carry information and power is something with a high current that keeps your device running. Op amps are typically used to control signals. A cheap run-of-the-mill op amp cannot push too much current. That makes it a less than ideal voltage source. A BJT can be chosen to handle a large power throughput.

So, you could use an op amp as a source but it wouldn't be able to output much power. Instead, you can use the op amp to control a BJT which can provide a huge power.

If you want to pay big bucks you can get an old fashioned "brick" that can serve the purpose of an op amp and supply big currents too.
 
Last edited:
  • #3
1,762
59
I'm guessing that in the examples you've seen the BJT is used to increase the current sourcing capability. As long as you don't exceed the current sourcing capability of the opamp, that circuit looks reasonable. Don't forget that the output voltage will still depend upon the stability of the voltage source and the tolerance of the resistors.
 
  • #4
4,662
5
Hello all.

I was just wondering if it is possible to use an inverting opamp configuration with feedback to regulate an output voltage V0 without the use of BJT's?

For example, the following sketch:
http://img411.imageshack.us/img411/2389/opamp.png [Broken]

V0 = Vs * (R1-R2) /2R1
Mathematically, V0 seems to be independent of load resistance Rl (not shown in diagram) so in effect V0 is regulated (provided we are within the limits of op amp).
This circuit in the URL is not a real regulator but a voltage divider with a low output impedance. The circuit depends on the stability of the 10 volt battery to provide 1 volt output. A real regulator has a voltage reference independent of the input voltage.

Bob S
 
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  • #5
16
0
Hi Guys,

Thanks for the great explanations, i really appreciate it. It all made sense.
 
  • #6
1,762
59
A few questions,

What is your voltage source? Is it a regulated power supply or a battery? If it's a battery, what type?

What is the purpose of R8 833K?

What is the purpose of R10 0.1k?

What is your load?
 

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