PA DC Bias - RF Cafe Forums
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Post subject: PA DC Bias Posted: Tue Feb 13, 2007 4:04 am
Joined: Tue May 02, 2006 4:59 am
I am looking for information about DC Bias for class
In LNA i know to design the Q point for best NF.
the rules for PA.
let say i am using BJT, what i need to do to
design a proper bias.
Post subject: Posted: Tue Feb 13, 2007 5:19 pm
Joined: Fri Feb 02, 2007 5:22 pm
I believe "technically" for class A you would bias for
ID=IDSS/2, but no one really does that at RF since the bias point shifts
around with the load and you may not get better linearity compared to
a more efficient class AB bias.
As opposed to an LNA, the goal
for a PA is to present transistor with a resistive load that allows
the maximum possible voltage and current swings (the optimum load line).
Of course you'll also need reactance to cancel the output capacitance
of the device.
Take a look in Steve Cripps' book "RF Power Amplifiers
for Wireless Communications", or "High-Power GaAs FET Amplifiers" edited
by John Walker, from which I pulled the following equation for Class
Vdgb= gate to drain breakdown
voltage (or collector to base for your case)
Vp = pinch off voltage
Vk = knee voltage
Posted: Thu Feb 15, 2007 2:42 am
Tue Mar 15, 2005 11:43 pm
I agree with the previous post by madengr. Just added a few
considerations in designing bias circuit for PA in particular. (Taking
GaAs MESFET, in this explanation).
Generally, for Rg (gate resistance),
the considerations for minimum and maximum values are as follow.
1. For Rg minimum value :
Ensure that no negative resistance will
occur--> For low frequency stability consideration. It requires Rg
to have a sufficient value and the connection to the ground to be short
for low frequencies. Connecting Rg close to the gate reduces the connection
length to the ground.
1. For Rg maximum value limit:
While the biasing length
for drain and gate is lambda\4, with DC blocking capacitor. To ensure
that at low frequencies, the DC blocking capacitor is an open circuit,
the decoupling capacitors are short circuits and the quarter-wavelength
line is a very short electrical length.
The details can be found
in the following application notes :
I hope it helps.