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A few basic questions about Power Amplifier theory - RF Cafe Forums
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Post subject: A few basic questions about Power Amplifier theory
Posted: Tue Apr 18, 2006 11:05 pm
Apr 18, 2006 10:40 pm
i am starting a class A
PA design project and i had a few basic unanswered questions.if someone
can answer them it will be nice.
1)why does saturation occur?
i know there might be many reasons but is it also because that the transistor
transconductance,gm, starts to reduce and then reaches some steady level?
if so, why does gm decreases and then reaches some certain level? but
if what i said is true then even at high power, as far as i understand,
the bias circuit is still providing the same base current and thus same
Ic so why would gm be changing since gm=Ic/Vt?
2)this could well
be explained by above answer--i biased for class A and this bias is
for certain output power that i want and this requires some certain
intput power, Pin1. what happens when the input power is less than Pin1
as far as dc biasing in concerned? what happens when input power is
at a high level? what i am really interested is in how the bias voltage
Vbe and current Ic change? if they increase (i don't know why they would)
then gm should also increase and if that's truly the case then the saturation
is not occuring because of gm (question 1)?
3)what is the best
book for power amplifier theory/design. thanks
Post subject: RF PA DesignPosted: Wed Apr 19, 2006 7:06
Joined: Wed Feb 22, 2006 3:51 pm
Saturation in an RF amplifier occurs because the collector
or drain voltage swing is limited - it can't go negative. (In fact,
it can't go below 0.7V for a bipolar transistor if you don't want excess
charge in the base region - which causes all sorts of bad results).
So the downward swing is from Vsupply to about 0; that means that the
upward swing can be no more than 2 x Vsupply. Limited voltage = limited
Please observe that this is not the same
as bipolar transistor saturation, which occurs when Vce<Vbe.
In Class A service, the transistor (BJT or MOSFET, doesn't matter)
never has zero collector/drain current and never has zero collector/drain
voltage. Likewise, it never exceeds 2 x Ic center or 2 x Vcc.
Regarding the book recommendation: What power levels are you concerned
subject: Posted: Wed Apr 19, 2006 7:47 pm
Tue Apr 18, 2006 10:40 pm
hi fred...thanks for the reply.
it makes sense, you are saying that as the input power is increased
the output voltage swing can't increase accordingly because of clipping,
meaning that after optimum power output levels the voltage swing is
stuck at the level but input keeps increasing so the saturation occurs.
But, another question: if I keep increasing the input power won't the
Vbe become large and evetually Vce<Vbe and that the transistor is
in saturation and the gain drops to ~unity, right?
as far as
book is concerned i am looking for a good book on overall PA design...power
level doesn't matter. but if you want to know i am looking at moderate
power levels not high levels. Levels needed in wireless communication
Post subject: Posted:
Fri Apr 21, 2006 8:46 am
Joined: Mon Jun
27, 2005 2:02 pm
biased for class A and this bias is for certain output power that i
want and this requires some certain intput power, Pin1. what happens
when the input power is less than Pin1 as far as dc biasing in concerned?
what happens when input power is at a high level?
your question: If the input power is less than Pin1 then your amplifier
will have lower efficiency. Class A by definition is the most inefficient
Class of operation, yet the most linear one. When the input power is
at a higher level the efficiency will improve.
subject: Posted: Tue Jun 13, 2006 10:11 pm
Joined: Tue Mar 15, 2005 11:43 pm
Hi Jabb :
Regarding the good textbooks for power amplifier
design, you may want to try the followings :
1. Gonzalez Guilermo,
Microwave Transistor Amplifiers Analysis and Design, New Jersey: Prentice-Hall
Inc, 1997, 2nd edition.
2. S.Cripps, RF Power Amplifiers for Wireless
Communications, Artech House, 1999.
3. S.Cripps, Advanced Techniques
in RF Power Amplifier Design, London: Artech House, 2002.