Determining feedback topology: series-series or series-shunt?

[minifhncc]

Series shunt or series series??

Hello,

I'm having trouble in general determining the feedback topologies...

My guess is that this is series-series since the source is in series with Rf and since setting io=0 for the output loop disables Vf... But I'm not sure... Could someone please tell me if I'm correct or not...

A good method in determining the topologies would be also appreciated.

Thanks in advanced

 

[sophiecentaur]

I would say that, as the fed-back signal changes the current in the emitter resistor and, hence, the voltage appearing at the emitter, the feedback would be, essentially, voltage feedback. The fed-back voltage appears in series, effectively, with the signal on the base. So my term would be series voltage feedback. Its effect would be, I think, to increase the input resistance of the circuit because more volts would need to be applied to produce a given input current. (It's just a 'stronger' form of emitter follower circuit)

Someone may well disagree with me. But it's only a name, remember.

 

[uart]

Someone may well disagree with me.

But not me.

It's been a long time since I've named feedback topologies like that, but I definitely agree with sophiecentaur here. One way to help distinguish between voltage feedback (aka series f/b) versus current feedback (aka shunt f/b) is by it's effect on input impedance. In the circuit given the output voltage is in phase with the input, so the voltage fed back to the 1st stage emitter is in phase with the input voltage. It therefore reduces the input stage vbe differential and so reduces the input current.

It is what I would call a "voltage sample" - "voltage feedback" topology. It reduces the output resistance and increases the input resistance, effectively making the amplifier closer to an ideal voltage amplifier (at the expense of some gain loss).

My guess is that this is series-series

You're correct about it being series (voltage) feedback but not about it being series (current) sample.