Class AB buffer stage

--- No.

In a push-pull class A stage current is always present in both of the output transistors.

In a push-pull class B stage there is never a time when current is present in both transistors.

In a push-pull class AB stage a small current is allowed in both transistors when there is no signal input and during the short time when the transistors are making the transition from ON to OFF and vice versa.

The "AB" designation is used because sometimes there is current in both transistors ("A") and sometimes there isn't. ("B")

JF

They are defined as:

1. Class A - The operational characteristics is defined such that current is flowing at all times through the amplifing device(tube, transistor, etc).

1. Class AB - The operational characterics is defined such that current is flowing more than half but less then the full electrical cycle.

2. Class B - The operational characterics is defined such that current is only flowing when the input changes beyond is steady dc value.

1. Class C - The operational characteriscs is defined such that current is only flowing for less than half of each cycle.

They are in some sense a shifted version. Assume the device can only conduct one way. Then the distance then "dc" of the input from the input of no current defines the class.

i.e., if the averaged "dc" input is much greater than the 0-current state then it is class A. If it is close to but above then it is class AB. If it is the same as the 0-current state then it is class B, and if much lower then class C.

Mathematically, Suppose f(t) is the input signal with the "average dc" defined as [f(t)] and 0 meaning the input with 0 current(usually 0V DC ).

Class A - [f(t)] >> 0 Class AB - [f(t)] > 0 Class B - [f(t)] = 0 Class C - [f(t)] < 0

(note [f(t)] > and < are made more precise if one knows the max swing of f(t) but can also be defined as above by saying the current is not flowing for a minority, half, or majority of the cycle)

(also keep in mind it all has to do with the input that produces no current. Most of the time this is an input of 0 but not always.)

Another way to think about it is:

Class A - the input is "shifted up" so that the amplifier is always conducting current. (you must know the max signal swing for this to be completely valid because it's possible that a class A could then be a class B in some cirtumstances(but somewhat useless circumstances)).

Class AB - The input is "shifted up" only some so that part of the signal "clips" when it goes below 0(since it can't go below 0 (unless push pull of course which is different)). Remember, 0 is the no-current state so when the signal gets "clipped"(or limited) at 0 we have no current for some of the cycle. It shouldn't clip more than half a cycle else it is class B or C.

Class B - The input is not shifted and then only the "positive" part conducts(the negative part gets clipped).

Class C - The input is actually shifted down so the signal conducts only on part of the positive cycle.

(this assumes the input has no DC-bias... if it does then just remove it first)

Another way to look at it is in terms of power. Class A is conducting all the time and hence uses more power. Class AB uses less power because it is not conducting between 50% and 100% of the time. Class B is conducting only for 50% of the time and class C is < 50% of the time.

(although 49% would still be called class B)

Mathematically,

P(class C) < P(class B) < P(class AB) < P(class A)

All the discussion above only references single ended. For push-pull and other stuff it is different. (but usually related)