[Amps] AN762 Bias Question

[Steve Thompson]

I suspect the use of 'bias' in two different contexts might be muddying 
the waters.

The 100mA per device is the standing or quiescent current the 
transistors should draw from the 12V supply in order to give linear 
operation to low powers. Quiescent current is often referred to as bias 
current, which can be confusing.

The 900mA bias current you calculated is the current the base bias 
supply needs to be able to deliver into the bases of the transistors 
when they are running at full tilt. The current drawn by the bases will 
vary according to the collector current, so will vary with power level.

Steve

> The question I have is; in the AN762 Application Note, it claims on the
> first page the device should be biased at approximately 100mA per device for
> a total of 200mA (Minimum).  I then reviewed the third page where discussion
> takes place with regard to the BIAS Voltage Source. Within that paragraph
> and on the following page it discusses the equation:

>
> BIAS = IC/hFE
>
> For the systems I am implementing this amplifier into, I get the following:
>
> 360W/13.2Vdc = 27.27 Amps
>
> 27.27 Amps/30hFE = 901mA Bias for Class AB.
>