4
\$\begingroup\$

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

\$\endgroup\$
8
  • 1
    \$\begingroup\$ What do you want to achieve? The term "operation point" is used for DC bias without being affected by any feedback. \$\endgroup\$ Commented Jan 12, 2021 at 20:14
  • \$\begingroup\$ can you make a drawing to show what you mean? \$\endgroup\$
    – Aaron
    Commented Jan 12, 2021 at 20:28
  • \$\begingroup\$ 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. \$\endgroup\$ Commented Jan 12, 2021 at 20:32
  • \$\begingroup\$ @Aaron unfortunately this is a plain question from our professor without any context \$\endgroup\$ Commented Jan 12, 2021 at 20:35
  • \$\begingroup\$ 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. \$\endgroup\$
    – Aaron
    Commented Jan 12, 2021 at 20:56

3 Answers 3

1
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\begingroup\$ I don't think the OP only meant voltage and current feedback by the same network. I wonder though if this concept of power feedback has ever been employed... \$\endgroup\$ Commented Jan 12, 2021 at 21:54
  • \$\begingroup\$ It has in certain applications, it's just not suitable for general amplification \$\endgroup\$
    – Voltage Spike
    Commented Jan 12, 2021 at 22:05
1
\$\begingroup\$

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.

\$\endgroup\$
1
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.