Single Op Amp as a voltage regulator?

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Discussion Overview

The discussion revolves around the feasibility of using a single operational amplifier (op amp) in an inverting configuration to regulate an output voltage without incorporating bipolar junction transistors (BJTs). Participants explore the mathematical implications of such a configuration and its practical limitations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the possibility of using an inverting op amp configuration to regulate output voltage V0 without BJTs, suggesting that V0 appears mathematically independent of load resistance.
  • Another participant emphasizes that op amps are typically used for low current signals and are not ideal for high power applications, suggesting that BJTs are necessary for handling larger currents.
  • A different participant notes that while the proposed circuit could work within the op amp's current limits, the output voltage's stability would depend on the voltage source and resistor tolerances.
  • One participant critiques the proposed circuit as a voltage divider rather than a true voltage regulator, highlighting the need for a stable voltage reference independent of the input voltage.
  • Several participants express curiosity about the specifics of the voltage source and load, indicating a desire for further clarification on circuit components.

Areas of Agreement / Disagreement

Participants express differing views on the viability of the single op amp configuration as a voltage regulator. While some acknowledge the theoretical possibility, others highlight practical limitations, indicating that the discussion remains unresolved.

Contextual Notes

Participants mention the importance of the stability of the voltage source and the tolerance of resistors, as well as the distinction between signal and power applications, which may affect the circuit's performance.

Who May Find This Useful

Individuals interested in circuit design, particularly those exploring op amp applications in voltage regulation, may find this discussion relevant.

nand_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

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.
 
Last edited by a moderator:
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In general, signals and power should be treated separately. 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:
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.
 
nand_1 said:
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

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
 
Last edited by a moderator:
Hi Guys,

Thanks for the great explanations, i really appreciate it. It all made sense.
 
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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