I'm learning negative feedback topologies and am stuck on current-series and current-shunt. I can't find examples of simple circuits to help me understand their function. All I can find is block diagrams but I'm looking for simple op-amp or BJT circuits that I can learn from (like when I used inverting and non-inverting opamps to learn voltage-controlled feedback). What is the simplest circuit using current-controlled negative feedback possible?

I read somewhere (apologies, can't find it now!) that a common emitter can show both these topologies. A resistor from collector to base can show current-shunt and the emitter resistor can show current-series. But how do these circuits match up with the block diagrams? What parts of the circuit get included in the amplifier and feedback network boxes in block diagrams?

  • \$\begingroup\$ What you need is a decent book, but you could get by with the search term: feedback amplifiers. The textbook example of a current-series amp is the a common-emitter amplifier without the emitter bypass cap. Read all of this: notesmilenge.files.wordpress.com/2014/09/…. Examples at the end. \$\endgroup\$ – Buck8pe Oct 19 '20 at 6:58
  • \$\begingroup\$ Are you looking for a transimpedance amplifier? \$\endgroup\$ – Se1fie Oct 19 '20 at 13:25
  • \$\begingroup\$ OP asked for CC FB \$\endgroup\$ – Tony Stewart EE75 Oct 19 '20 at 13:58
  • \$\begingroup\$ @Buck8pe, what is the author/source of the file you shared with me (thank you - it looks excellent and just the thing I was looking for) \$\endgroup\$ – nuggethead Oct 20 '20 at 0:42
  • \$\begingroup\$ @nuggethead the link I posted I found using the term "feedback amplifier" with additional terms to weed out sources with examples. The best introductory text I've seen is Analog And Digital Electronics by Bakshi & Godsay. It's an older book and not very common, but I'm sure you'll find similar in other text type books. Hope that helps! \$\endgroup\$ – Buck8pe Oct 20 '20 at 13:40

The classical example for current-controlled voltage feedback is the common-emiiter stage with "degeneration" (feedback) using an emitter resistor RE. But this is one of the few cases, where the feedback loop cannot be identified by simple visual inspection.

But the block diagram helps:

enter image description here

The diagram is simplified assuming Ie=Ic (Ib neglected). The given gain expression (transconductance gm=Ic/Vt) can be derived directly from the circuit diagram as well as from the shown block diagram.

LOOP GAIN: As can be seen from the diagram: The loop gain is Al=-gm*RE.

  • \$\begingroup\$ I agree, but show how to get gm for a PN2222 at various Ic values onsemi.com/pub/Collateral/…. Using eGen FB to Vbe(Icj) \$\endgroup\$ – Tony Stewart EE75 Oct 19 '20 at 13:55
  • \$\begingroup\$ If the feedback loop can't be identified by simple visual inspection, how can it be identified? Also, can anyone show how current-controlled current feedback would look in a CE amp with resistors added from collector to base? \$\endgroup\$ – nuggethead Oct 19 '20 at 14:07
  • \$\begingroup\$ My answer: The shown block diagram was found by simply transferring the known relations describung the transistor operation: gm=d(Ic)/d(Vbe) and d(Vbe)=d(b)-d(ve). Note that current and voltages are time-varying quantities (ac) - in the diagram shown with capital letters (rms values). \$\endgroup\$ – LvW Oct 19 '20 at 14:56
  • \$\begingroup\$ A resistor between C and B provides voltage-controlled current feedback . The feeedback loop can be seen. \$\endgroup\$ – LvW Oct 19 '20 at 14:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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