# Why only common collector method is used in class AB amplifiers?

I have recently started to study about class AB amplifiers online, as a newbie, i have found every schematic of the class AB amplifier is in common collector circuit just like this

Schematic i find everywhere in internet...

simulate this circuit – Schematic created using CircuitLab

But i eager whether the circuit can be used in common emitter mode, so i have drawn my own circuit below , in which, everything is same but the connection of the transistors. I dont know whether my made circuit will work or not, but i don't want any harms to happen while i am testing the circuit so , i considered that it would be better if i ask this topic on stack exchange before implementing.

My drawn circuit below...

simulate this circuit

So here are my questions ...

1. Why is class AB amplifier used in common collector method?

2. Can class AB amplifier be implemented according to my drawn schematic in which transistors are in common emitter ? If my drawn circuit cant be applied, then please explain why .

3. Why is common collector chosen best for class AB amplifiers?

Thank you so much !

• It can be arranged either way. But the 2nd case won't work as shown. You'll need some different way of driving the power output pair of BJTs. A Sziklai pair comes immediately to mind. But that's not the only approach. – jonk Apr 23 at 8:10
• Not an expert, but it has to do with cross conduction issue. The sequence NPNPNP is more beneficial than PNPNPN.. – Marko Buršič Apr 23 at 8:12

1. Another name for that config is emitter follower. It provides current drive ability with voltage gain of (almost) 1. It basically just buffers the signal.

2. Nope. The semiconductors would blow up. Assume that each semiconductor junction (diodes and transistor B-E) has a voltage drop of 0.7V when forward biased. Try and simulate it.

3. It may not be the best, but most simplest and practical two transistor push pull configuration that is good enough for whatever the puropose is - such as learning the basics from textbooks. If you need better performance for another purpose then you can easily find better but more complex configurations that will fullfill the requirements of those other purposes.

The premise is false.

You can make class B or Class AB amplifiers without emitter followers; indeed that was the norm for at least the first thirty to forty years.

Here's a counter-example.

simulate this circuit – Schematic created using CircuitLab

This is only a representative, with bias details and overall negative feedback omitted, but it is a classic example of a Class AB or Class B amplifier, depending on the voltage applied at Vbias.

Older engineers may laugh to see it drawn with transistors, but (a) Circuitlab, inexcusably, doesn't offer valves, and (b) this is actually the way early transistor amplifiers were built (usually with PNP transistors, since NPN Germanium transistors were rare beasts) when transistors were new and this topology was familiar. So you will find it in 1960s transistor radios, identifiable by the output transformer.

Q1 and Q2 are the output devices, both in common emitter configuration. (It would be common practice to add small emitter resistors for better current control, bypassed with large capacitors. This matches "automatic bias" in valve circuits, where Vbias would then be 0V). With Vbias up to 0.6V this is Class B, with some element of crossover distortion : above 0.7V it's into AB, and above 0.7V plus (Vin peak * Q4 gain) pure class A.

Q3 is a phase splitter : Q1 is driven by its emitter follower output (gain = 1) while Q2 is driven by its common emitter output with gain = -R1/R = -1. (Approximately; I did say I was omitting details). Another common phase splitter used a long tailed pair input stage as seen in most opamps, providing both true and inverted outputs.

Q4 is the main voltage gain stage though the output pair also provide voltage gain.

The reason this configuration fell into disuse is obvious : the output transformer. But feel free to invent transformerless configurations (perhaps based around full bridges) if you like; just don't blame us fi it turns out to be more difficult than at first glance.

The basic circuit configuration lives on where transformers are unavoidable; typically in cheap 12V to mains voltage inverters, with MOSFETs, and either square wave drive, or PWM drive to produce a "pure" sinewave.

I have recently started to study about class AB amplifiers online, as a newbie, i have found every schematic of the class AB amplifier is in common collector circuit

If you have studied class AB amplifiers then you should already have studied class A and class B amplifiers. So you could already have the answers.

A Class A amplifier can provide high voltage gain along with low distortion, but it has very high output impedance. And thus it's not suitable to drive a low-impedance load such as a speaker. Still a class A amplifier can drive a low-impedance load but requires a transformer for impedance conversion (Google "transformer coupled class A amplifier").

A class B amplifier can provide high current gain along with unity voltage gain and very low output impedance thanks to the emitter-follower configuration, but it has poor distortion performance because of the unbiased transistors. It's ideal to drive low-impedance loads but still requires the input voltage's peak to be greater than VBE. That's why class B amplifiers have poor distortion performance.

The idea behind a class AB amplifier is to provide a class A amplifier's low distortion and a class B amplifier's low output impedance (i.e. high drive capability). That's why a class AB amplifier has a common collector (a.k.a. emitter follower) output configuration (answer 1 and answer 3). However, it's possible to build a common-emitter push-pull stage but we may not name it a class AB amplifier. Google "common emitter push-pull amplifier". Your configuration will not work, or even the transistors can break down (answer 2).

• A Class A amplifier...And thus it's not suitable to drive a low-impedance load such as a speaker. I'm not sure I agree with that, I can take a class AB with common collectors (1st schematic in question) and then change it such that the output transistors never have Ic = 0. In essence, I increase their bias current by a lot. Then I can call this amplifier "class A" since the output transistors never switch off (Ic = 0). – Bimpelrekkie Apr 23 at 8:48
• @Bimpelrekkie Then I can call this amplifier "class A" since the output transistors never switch off (Ic = 0). True. The "Class A amplifier" mentioned in my answer is the basic one with only one switch that the output is taken from its collector/drain/anode, of course. Even with that one, it's possible to drive a low-impedance load, but it'll be way more impractical and inefficient. And we're talking about the practical/applicable models. – Rohat Kılıç Apr 23 at 9:34