what's the difference between Class A and class AB amplifiers?

Question

I'm trying to determine which sound I want in a guitar amplifier, the class A or the Class AB, I know how they work, I'm just trying to figure out the difference in tone.

Answer 1

Guitar amplifiers usually use tubes, cheap ones use transistors.
Transistor amplifiers are very linear, and try to reproduce the amplified voltage. The problem is that the power supply positive, and negative limit how high the voltage can go. When you overdrive a transistor amplifier, the output voltage rises to the power supply limit and hits the upper limit. The signal that is produced is no longer a sine wave, but more like a sine wave with the upper and lower peaks cut off. This produces broad harmonic distortion creating high frequency distortion caused by the sharp edges of the voltage cutoff. This is called " Clipping Distortion" because the top of the wave is "clipped" off.
Vaccuum tube amplifiers dont work quite that way. Tube amplifiers cant ever quite get to the peak power supply voltage.
As the input signal peaks towards the upper or lower supply, the tube amplifiers output becomes mushy, and cant make it to the peak. It is much smoother than the ridgid cutoff of a transistor amplifier.
Tube amplifiers that are overdriven sound good, but transistor amplifiers overdriven sound bad.
Class A tube is the best ( although I dont play the guitar).
Class AB tube would be ok.
I would avoid transistor based A or AB, but they are much cheaper.

In "Class A" amplifiers, there is usually one output transistor that is turned on all the time. The power supply is made of two supplies in series. For example +80V, and -80V.
The transistor in a class A, is set to produce 0V, when there is no input signal. The transistor is always "ON", and dissipates the maximum amount of power when it is sitting idle like this.
The speaker coil having 0V, dissipates 0 Watts.
For this reason ,Class A amplifiers dissipate the most power, because they are always ON.

In Class "B" There are usually two transistors that drive the output. Bipolar NPN can drive the output with positive voltage, and a PNP transistor handles the negative voltage. But when the music signal cross zero volts, the one upper transistor shuts off, and the lower transistor begins to turn on. The base emitter drop of 0.6V or so, causes a dead band ( +0.6V to -0.6V)right as the voltage crosses over, and leads to cross over distortion. This occurs when the base of the two transistors are connected together, and causes the dead band.
This is Class B operation.

In Class "AB", The bases are not connected directly together. They are connected together using three diodes in series. The upper diode (Base) is connected to +V with a resistor, and the lower diode in the string ( lower base connection) is connected to -V through a bias resistor. This diode arrangment holds the two bases apart by 2.1V, and when the music voltage crosses 0V, both transistors are held in the ON state slightly by the bias network. this prevents cross over distortion. This is Class AB operation, where two transistors are used, but are biased on to prevent cross over distortion.
In actual practice, the three diodes are selected to have the same characteristics as the output pass transistors. The three diodes are usually in a metal package that resembles a regulator, and is bolted to the same heat sink as the output pass transistors. This prevents thermal runaway.
When the output transistors get hotter with use, the base emitter voltage decreases, and this makes the transistors turn on harder without applying any additional voltage bias. This continues, and the transistors gets hotter, turns on more, gets hotter, and turns on harder, and can overheat and burn out. Hence "Thermal Runaway".
The three diodes are bolted to the same heatsink, and when the base emitter diode of the output transistors heats up and decreases in voltage drop, so do the three diodes in the bias network, and tends to turn off the transistors by reducing the bias, preventing thermal run away.

MOSFET transistors output current decreases when they get hotter, so they dont suffer from the thermal runaway proplem.

Austin Semiconductor