On Tue, 28 Dec 2004 21:21:43 GMT, Lancer wrote in
53j3t0torc3illhomnmagddsi4un5noth8@2355323778:

On Mon, 27 Dec 2004 12:37:19 -0800, Frank Gilliland
wrote:


Depends on the amp. To be perfectly frank, if I used an amp I wouldn't
even mess with anything that runs from 12-14 volts. If solid-state I
would use 28 volt -minimum- for several reasons: power leads are
smaller and have less loss; transistors are more linear; efficiency is
higher because bias is a smaller percent of supply (CE bias can be as
high as 3-4 volts under heavy load, dropping nearly a third of your
supply voltage); voltage difference between battery and alternator is
smaller; etc, etc.


Ok Frank;


Ok Lancer (whichever one you are);


Please explain, (CE bias can be as high as 3-4 volts under heavy load.

Isn't that set by the operating point set by bias you put on the base?


A bipolar transistor requires both a BE bias -and- a CE bias.


Ex: Class A, 6 volts for a 12 volt system.


For a Class A amp the DC bias on the base is set so the output is
centered in the linear portion of the curve (which is what I think you
were trying to illustrate with your example).


Or are you refering to the losses in the transistor when its fully
turned on?


Not necessarily. CE bias increases with collector current regardless
of saturation (and RF bipolars don't saturate easily). But if the
supply voltage can be increased while maintaining the same collector
current (by changing the BE bias), the loss due to CE bias is not
changed, and that loss is therefore made to be a smaller percentage of
the output power. IOW, the transistor is more efficient with a higher
supply voltage.

On Tue, 28 Dec 2004 16:19:02 -0800, Frank Gilliland
wrote in
:

snip
.....But if the
supply voltage can be increased while maintaining the same collector
current (by changing the BE bias).....


Correction: that should be "by maintaining the same BE bias".

I mistakenly assumed that the base bias would need to be adjusted
after an increase in power supply voltage, disregarding the fact that
some amps have regulated bias supplies.