Post subject: Feedback resistor rating Posted: Tue Oct 13,
2009 4:26 am
Joined: Tue Aug 26, 2008
I am designing
a class AB Power Amplifier and Have to use negative feedback. Power
of the amplifier is 20W (Vs=24v) and my question is what should
be rating of the feedback resistor? some people say that peak RF
signal can go as high as twice the drain voltage which is 24v in
my case. So do we follow that rule and consider 48V across feedback
Re: Feedback resistor ratingPosted: Wed Oct 14, 2009 4:11 pm
Joined: Mon Jun 27, 2005 2:02 pm
This depends between which nodes you
connect the feedback resistor?
But it is better in PA design
to be on the safe design, and to assume that the voltage can reach
twice the supply voltage (Vdd) att eh positive part of the input
signal, and the voltage drop between Vds and Vdd is on the RF Choke.
You can also see that the brekadown voltage of most RF transistors
is at least 2*Vdd and that the capacitors connected (In the RF transistor
referene design schematics) to the supply voltage should be able
to withstand 2*Vdd.
Post subject: Re: Feedback
resistor ratingPosted: Thu Oct 15, 2009 4:29 am
Joined: Tue Aug 26, 2008 5:18 am
Thanks for your reply.
Parallel feedback resistor
is connected from drain to gate using a dc block capacitor. So the
conclusion is that, resistor rating should be calculated based on
drain voltage which determines the peak RF signal (can go twice
the drain voltage and it the ac voltage).
Next question is, does
it matter if it is a 20w or 40w RF amplifier? from the discussion
it sounds like all that matter is the darin voltage?
Post subject: Re: Feedback resistor ratingPosted:
Thu Oct 15, 2009 2:48 pm
Jun 27, 2005 2:02 pm
case of increased power, the peak voltage will remain 2*Vdd for
a given Vdd.
The current through the feedback resistor will
increase according to the increase in the output power.