Here, I am referring to voltage-feedback amplifier (VFA) and current-feedback amplifier (CFA). A basic schematic is given below for each.


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I am curious about the position of compensating capacitor, when both circuits are compared. In VFA, compensating capacitor is connected in parallel between input and output of voltage amplifier (between base and collector of BJT). But in case of CFA, the compensating capacitor is connected in parallel between output (collector) and ground of voltage amplifier. Why is it so? Why not the same in both configurations? How does compensating capacitor has any influence on CFA's frequency plot?

Obviously, in both circuits, frequency compensation technique differs. In VFA, compensating capacitor compensates Miller Capacitance of a BJT. Why isn't the same technique used in case of CFA? On what principle is frequency compensation of CFA through compensating capacitor (between voltage amplifier's output and ground) based on?

  • \$\begingroup\$ You are showing the amplifier designs, but not the capacitor connections or the value of the capacitors. As a general rule a comp capacitor goes from the output pin to the negative input pin, but a CFA needs an RF response to its limit so the caps value tends to be much less than a VFA. \$\endgroup\$ – Sparky256 Oct 4 '18 at 22:34

Cs is effectively in the same location.
Q13+Q22 is equivalent to Q4+Q6 below.

The CFA has the +1 buffer shown while ,
the VFA also has a +1 buffer but is handled in bipolar form with a DC bias offset then reduced to a single-ended output with the offset reduced to null by (Q14+Q20).

The cap is differential could have been two single-ended caps to gnd from each collector current amplifier.


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