A class-AB output from Huijsing's opamp theory and design

op amp theory

on page 155, figure 5.3.18 in Huijsing's opamp theory and design,

He said, for this general-amplification feedforward biased class-AB output stage,

1. "Q3, Q5, Q4, Q6 form a positive coupling loop with a current gain of slightly lower than1, which keeps this loop stable."

I think this is a current loop, but I don't how to explain it as a positive loop with current gain less than one. Any help?

2. "The currents which are lost in the diode connected biasing transistors Q3 and Q4 are collected by the emitters of Q5 and Q6 respectively, and are returned as driving current to the complementary side."

how to understand this?

3. "All input current which is not used to drive one output transistor is used to drive the other. So, the gain decrease due to the class-AB biasing circuit is eliminated. In fact, the positive loop-gain coupling is a way to implement a class-AB bias circuit with high input impedance."

And how to understand this also?

Appreciate your help.

class ab biasing

I believe many of you can give a thought on it. Thanks.

class ab output design

I hope this helps, but I am not certain it is correct.

1. "Q3, Q5, Q4, Q6 form a positive coupling loop with a current gain of slightly lower than1, which keeps this loop stable."

assume IB3 and IB4 are the same. If you broke the loop between Q4 and Q5 and injected a current into Q4, ICQ6 would drop by an equal amount of current as would ICQ3. ICQ5 would rise by the same amount of current as injected into ICQ4. If this was negative feedback loop you would expect ICQ5 to drop. The slightly less than one gain comes from supplying the base currents of Q6 and Q5, the current is not exactly equal all around the loop because of needing to supply the base currents. Despite being a positive feedback loop, the system stays stable because the gain cannot go above one.

2. "The currents which are lost in the diode connected biasing transistors Q3 and Q4 are collected by the emitters of Q5 and Q6 respectively, and are returned as driving current to the complementary side."

This just means that the currents are not wasted. Q4, Q6 could have resistors to VDD and Q3, Q5 could have resistors to ground but it would waste bias current.

3. "All input current which is not used to drive one output transistor is used to drive the other. So, the gain decrease due to the class-AB biasing circuit is eliminated. In fact, the positive loop-gain coupling is a way to implement a class-AB bias circuit with high input impedance."

You have to look at other Class AB output biasing methods. This circuit design describes the signal inside the amplifier as always a current. It is difficult to say exactly the author's meaning. Translating from differential input to single ended output of other Class AB designs, current gain would be lower but you usually are thinking about voltage gain.