# Examples of current-controlled negative feedback?

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?

• 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. Commented Oct 19, 2020 at 6:58
• Are you looking for a transimpedance amplifier? Commented Oct 19, 2020 at 13:25
• OP asked for CC FB Commented Oct 19, 2020 at 13:58
• @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) Commented Oct 20, 2020 at 0:42
• @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! Commented Oct 20, 2020 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:

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.

• 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) Commented Oct 19, 2020 at 13:55
• 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? Commented Oct 19, 2020 at 14:07
• 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).
– LvW
Commented Oct 19, 2020 at 14:56
• A resistor between C and B provides voltage-controlled current feedback . The feeedback loop can be seen.
– LvW
Commented Oct 19, 2020 at 14:58