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Re: Emitter follower stage as a feedback amplifier, Part 2 (with corrections)

Posted by kurt s on July 13, 2003 at 16:29:24

In Reply to: Emitter follower stage as a feedback amplifier, Part 1 (with corrections) posted by kurt s on July 13, 2003 at 16:10:06:

The top of this page shows the diagram again for the emitter follower, or common collector amplifier in its midband small signal model. Equation 6 is repeated from Part 1.

I added a new form for equation 6, where Vout=Vin*(G/(1+G*Beta)). This is exactly in the form of a negative feedback equation with a forward gain G and feedback network Beta. There is a diagram above showing G, Beta, and summing node where the negative of the output is feeding back through Beta.

There is an added term here to analyze the feedback network, V1, after the summing node.

Equation 7 is what V1 equals in its most basic form. The positive term is Vin and the negative term is V1 times the cascaded networks G and Beta as it goes around the feedback loop.

Equation 8 combines V1 terms.

Equation 9 shows what V1 equals with respect to Vin, G, and Beta.

Equation 10 shows what Vout equals with respect to V1, and then its substituted terms.

Equation 11 is the final form of this feedback network, and it precisely equals the form shown in Equation 6 above. From this it can be inferred that Vout is a negative feedback network with a forward open loop gain G and a feedback path Beta. Notice that Beta equals 1, which is equivalent to saying all is fed back, or there is 100% negative feedback.

I think this is good enough proof that the emitter follower can be modeled as an amplifier stage with 100% negative feedback. This same modelling applies to all followers, since the form is the same with minor detail changes within the B, C, and E electrodes. It applies to Sziklai followers, Darlingtons, FETs, tubes, and any BJT type follower.

The extra complexity of the Sziklai and Darlington means there is embedded some more detail between the main 3 electrodes of this model, but in the end they all have negative feedback. The Sziklai has another feedback loop within this loop, in other words. The Darlington, by inspection, would also have a loop within a loop since Rpi will become an Rload equivalent from the second transistor follower.

All amplification devices, tube or solid state, can be modeled with a constant current source between the common electrode and the output when operated as a follower circuit. This will keep the form of the feedback equation valid even though the open loop gain variable, G, can change. The feedback factor, Beta, will remain a constant 1.

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