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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.


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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.


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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



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