How much Class A in a class ab amplifier

[madtecchy]

Hi.

The supply rails are approx + - 80 VDC. The bias = 400ma.
How many class a watt's can be expected?.

Kind Regards Ian

[woody]

About 2.5 W into an 8 ohm load. More into a higher load impedience less into a lower
load impedience.

[AndrewT]

+-80Vdc indicates a maximum ClassA output current of double the output stage bias current.
Therefore expect the maximum ClassA output current to be ~800mApk.

If you have to convert that to Watts then use the standard Power formulae

P = Vac^2 / Rload = Iac^2 * Rload = Vac * Iac when using rms values for sinewave signals.
P = Vpk^2 ? Rload / 2 = Ipk^2 * Rload / 2 = Vpk * Ipk when using peak values for a sinewave signal.

[jez]

And 64 Watts of heat so that's going to need a substantial heatsink!

[madtecchy]

Hi
Thankyou all for the replys.

@Jez i make that 128 watts of heat as i have two channels. It keeps the chill off my room

[jez]

Yes, per channel obviously! That's what I meant....

[LineSource]

"How much Class A in a class ab amplifier?"


For bipolar output stages, setting a ClassA bias that is optimal for the transistion to ClassAB operation is often the main design strategy. From early work by Barney Oliver at HP setting the drop across the output emitter resistor to ~23mv - 26mv has proven to generate the lowest distortion through the transistion to ClassAB and beyond.

To achieve minimum distortion for almost any ClassA power goal, or any maximum ClassAB power goal, the designer can change the value of the emitter resistor or increase the number of output transistors.

1) select the maximum ClassAB power goal to determine the number of output transistors required

2) select the emitter resistor value to provide the desired ClassA power from the 23-26mv drop over N-transistors.