A few basic questions about Power Amplifier theory - RF Cafe Forums
Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: A few basic questions about Power Amplifier theory Posted: Tue Apr 18, 2006 11:05 pm
Joined: Tue 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.
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
subject: RF PA DesignPosted: Wed Apr 19, 2006 7:06 pm
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 (saturated) power.
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.
book recommendation: What power levels are you concerned with?
Post subject: Posted: Wed Apr 19, 2006 7:47 pm
Joined: 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 systems. thanks
Post subject: Posted: Fri Apr 21, 2006 8:46 am
Joined: Mon Jun 27, 2005
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?
answer 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.
Post 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.
Hope it helps.