[Amps] AN762 Bias Question

[Jim Garland]

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> Today's Topics:
> 
>    1. AN762 Bias Question (Rich Miller)
>    2. Re: AN762 Bias Question (Steve Thompson)
>    3. Jennings UCS puller interchangeability ? (Ronald Brown)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Mon, 2 May 2011 08:29:04 -0400
> From: "Rich Miller" <av8torrich at yahoo.com>
> Subject: [Amps] AN762 Bias Question
> To: <amps at contesting.com>
> Message-ID: <034301cc08c4$867cf490$9376ddb0$@yahoo.com>
> Content-Type: text/plain;	charset="US-ASCII"
> 
> 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
> 
Rich, I believe the 100mA per device is the quiescent collector current, not
the quiescent base current (which is 100mA/hfe=30 ~ 3mA per device). The
27Amps is the total collector current at 360W power input, not the quiescent
current. Admittedly, the word "bias" can be interpreted two ways, but
operationally one normally biases a transistor to get a specified collector
current; the base current required depends on hfe and is device-dependent,
so one doesn't usually try to specify it.
73,
Jim W8ZR