# BJT AC Analysis - feedback pair equivalent circuit

#### jorje

May 5, 2022
2
At "BJT AC analysis" chapter of "Electronics devices and circuit theory" of R Boylestad and L Nashelsky there is such scheme (picture attached) showing feedback pair configuration. For AC analysis they made equivalent circuit - what I do not understand WHY in equivalent circuit the current source [Beta1*Current_base1] (transistor Q1) is UP going against Vo(output)? in equivalent this current also go to Rb resistor, the only current to Rb from original scheme is from Q1 base, but as far as I understand [Beta1*Current_base1] means collector-emitter current of Q1 which goes to Q2 base, not current from Q1 emitter to Q1 base . please explain

#### kellys_eye

Jun 25, 2010
5,138
Q1 is a PNP transistor so you drive its base negative with respect to the emitter to cause it to conduct, this 'negative' drive shown as opposing flow hence the arrow direction.

#### CircutScoper

Mar 29, 2022
300
At "BJT AC analysis" chapter of "Electronics devices and circuit theory" of R Boylestad and L Nashelsky there is such scheme (picture attached) showing feedback pair configuration. For AC analysis they made equivalent circuit - what I do not understand WHY in equivalent circuit the current source [Beta1*Current_base1] (transistor Q1) is UP going against Vo(output)? in equivalent this current also go to Rb resistor, the only current to Rb from original scheme is from Q1 base, but as far as I understand [Beta1*Current_base1] means collector-emitter current of Q1 which goes to Q2 base, not current from Q1 emitter to Q1 baseView attachment 55088 . please explain

If Ic1 = beta1 x Ib1, then Ie1 = (beta1 + 1) x Ib1. Perhaps the +1 term was omitted because beta1 >> 1, making Ie = Ic a reasonably accurate approximation. Meanwhile the direction of Ie is up because the Ie vs Ib relationship is non-inverting.

#### jorje

May 5, 2022
2
Q1 is a PNP transistor so you drive its base negative with respect to the emitter to cause it to conduct, this 'negative' drive shown as opposing flow hence the arrow direction.

Thank you for your answer, I have to admit that this is still intuitively not clear for me entirely.
I think also I confused two things DC biasing and AC signal
I will try to analyze the functioning of both feedback pair and darlington in simulator and ask you kindly if still will have questions. thank you for your help

#### Audioguru

Sep 24, 2016
3,650
Also look in Google for Sziklai Pair.

#### CircutScoper

Mar 29, 2022
300
Is it just me, or are other folks also looking at that goofy bias arrangement and going EEE-YUCK!

If built with real transistors with real-world beta variation, you wouldn't be able to predict quiescent Vc2 within an order of magnitude!

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