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Can I use current negative feedback and voltage negative feedback together to set the operating point of an amplifier?

JRE
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Mete Takehara
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    What do you want to achieve? The term "operation point" is used for DC bias without being affected by any feedback. – Marko Buršič Jan 12 '21 at 20:14
  • can you make a drawing to show what you mean? – Aaron Jan 12 '21 at 20:28
  • So u are saying an "operation point" is never affected by any feedback? I tried to translate it to english but i think i failed somehow. It is called "Stromgegenkopplung" and "Spannungsgegenkopplung" in german, but i wrote "current negative feedback" which is not quite true, i suppose. "It" is meant to prevent current and voltage feedback. – Mete Takehara Jan 12 '21 at 20:32
  • @Aaron unfortunately this is a plain question from our professor without any context – Mete Takehara Jan 12 '21 at 20:35
  • So try and draw it up...see what you can get. You'll learn a lot more from that than just asking us a yes/no question. – Aaron Jan 12 '21 at 20:56
  • I see, you can read this: https://en.wikipedia.org/wiki/Bipolar_transistor_biasing It explains both two versions, pros, cons and finally the most used "Stromgegenkopplung" with a AC bypass capacitor parallel to emitter resistor and fixed base bias resistors. – Marko Buršič Jan 12 '21 at 21:28
  • "*...at the same time?*" Hmm, maybe in something like a [foldback current limiter](https://www.eeeguide.com/wp-content/uploads/2016/09/Foldback-Current-Limiting-1.jpg), (when the limiter is active)? – Pete W Jan 13 '21 at 03:40
  • I must admit that your question is not quite clear to me. According to your understanding - in case of "voltage feedback" is voltage the controlling or the controlled quantity ? I ask this because both versions are in use. Therefore, I prefer the detailed description "Voltage-controlled current feedback" or "current-controlled voltage feedback" or....or... – LvW Jan 13 '21 at 09:09

3 Answers3

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What you describe is something like power feedback where the power is feedback to control the power input, but this type of power amplification typically isn't done in conventional electronics.

The main problem is most commercial amplifiers can be configured to either measure voltage or measure current but not both at the same time.

The way to get around this would be to use an amplifier that measures current, and an amplifier that measures voltage and then use a multiplier to calculate what the power is, then use that value in negative feed back. I wanted to make one in this question: Constant controlled power source

The problem is power measurements are also inherently noisy as the noise from the voltage and current is multiplied.

It's much easier to stick to only one measurement.

Voltage Spike
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Yes you can with constraints. Current feedback compensates for load importance changes and raises output impedance. While Voltage feedback lowers output impedance. Thus the constraints must be defined by the expected load variation and desired output for those changes.

Examples might be speaker impedance or battery capacity. The latter uses each in separate modes of CC and then CV until 5% CC threshold to cutoff for LiPo and Li Ion. Hence not simultaneously for driving yet simultaneously for sensing.

For motor speed control and torque control, you would use current feedback for torque control and voltage feedback for RPM control. Depending on your desired control system for acceleration and rotational velocity you may use both inputs for regulating current and voltage.

Tony Stewart EE75
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In a common-emitter stage do the following:

  1. Connect the input voltage source through a resistor Rb to the base; thus you will "soften" its voltage.
  2. Connect the collector through a resistor Rf to the base; thus you will introduce a voltage negative feedback.
  3. Insert a resistor Re in the emitter; thus you will introduce a current negative feedback.

Rb and Rf act as a voltage summing circuit that subtracts in a parallel (shunt) manner the output (collector) voltage from the circuit input voltage. Re "creates" a voltage drop proportional to the output (collector) current that is subtracted in a series manner from the transistor input (base) voltage.

Circuit fantasist
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