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Feedback resistor rating - RF Cafe Forums
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SDO
Post subject: Feedback resistor rating Posted: Tue Oct 13, 2009
4:26 am
Captain
Joined: Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Hello all,
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 resistor?
Thanks.
Top
IR
Post subject: Re: Feedback resistor ratingPosted: Wed
Oct 14, 2009 4:11 pm
Site Admin
Joined: Mon Jun 27,
2005 2:02 pm
Posts: 373
Location: Germany
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.
_________________
Best regards,
-
IR
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SDO
Post subject: Re: Feedback resistor
ratingPosted: Thu Oct 15, 2009 4:29 am
Captain
Joined:
Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Hi,
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?
Thanks.
Top
IR
Post subject: Re: Feedback resistor
ratingPosted: Thu Oct 15, 2009 2:48 pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
In the 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.
_________________
Best regards,
- IR
Posted
11/12/2012
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