[Amps] AN762 Bias Question

[Rich Miller]

Hello All:

 

I recently completed 180 Watt solid state amplifier based on the AN762
application note by Motorola. I am using a pair of the MRF421MP NPN
transistors. 

 

I re-reviewed the application note and the specification for the MRF421MP.
In addition, I have reviewed Practical RF Power Design Techniques by Irving
Gottlieb. Mr. Gottlieb's publication is light on the topic of BIAS, thus
this was not too much help. 

 

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. 

 

 

This seems a bit high to me, thus I wanted to check here regarding my
understanding of this document. Also, if anyone can point me in the
direction of some literature, which would enlighten me on the topic, it
would be most appreciated.

 

73

 

Rich, AJ3G