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Design >> RF Design >> DC Bias for RF amplifier
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Message started by yippee on Apr 25^th, 2007, 1:27pm

Title: DC Bias for RF amplifier
Post by yippee on Apr 25^th, 2007, 1:27pm

Hi All
i'm going to design a simple one stage RF amplifier with
input and output impedance matching. the output will be connected to
a 50 ohm antenna.
i should use a self bias circuit for DC bias.
my problem is the value of Rl(load resistor) for ac load line.
i want to know Rl to design for maximum sweeping.
please refer to the following picture ...
can u help me?!

Title: Re: DC Bias for RF amplifier
Post by mg777 on Apr 26^th, 2007, 6:13am

It depends on what 'RF' is with respect to your transistor's f_max. In most RF amplifiers the transistor reactances dominate the impedance matching, plus the matching network is usually narrowband with a Q < 10. So the load line won't be a pure resistance - it will swing around on the Smith chart.

Is this amplifier inside an IC, or is it external? If external you will be better off with a 25 cent part from one of the major vendors.

M.G.Rajan

Title: Re: DC Bias for RF amplifier
Post by yippee on Apr 28^th, 2007, 1:58pm

no, amplifier is not inside an IC i want to design it with a transistor(2SC2570) from NEC
the input of amplifier will be connected to antenna of a transmitter with less than 0.8 mW power
and in 433 Mhz.

and yes, thats right the load line is not a pure resistance but just consider a simple design.
in above circuit i just want determine the value for Rc & Re so the output wont be cut off.

Title: Re: DC Bias for RF amplifier
Post by stewa203 on Jun 8^th, 2007, 6:38am

I got this equation out of a book by Schubert and Kim.

Q = [(mdc/mac)*Vcc+C]/[1+mdc/mac]

mdc = slope of DC load line
mac = slope of AC load line
C = saturation voltage of the transistor

mdc = 1/[Rc +Re(beta+1)/beta]
mac = 1/Rc

Q then will give the 'best' Vce for low frequency approximations

If you don't want to do the math, a good rule of thumb says you should drop about a third in voltage between Re, Vce, and Rc each.

A couple of pointers on this application. You will probably need a high DC current in the transistor. This will help with linearity at higher powers (800mW = 29 dBm) which is a lot of power. I personally have not designed any power amps, which is what you are trying to do. Also, this amplifier configuration has a high output impedance compared to your low antenna impedance. You will need some sort of tuning network to match the amplifier to the antenna, otherwise you will have a horrible VSWR problem.

Title: Re: DC Bias for RF amplifier
Post by stewa203 on Jun 8^th, 2007, 6:40am

My mistake, I thought I read .8W and not .8mw. You may not need as high as quiescent current as I originally thought. Everything else still applies though about the matching network though.