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A few basic questions about Power Amplifier theory - RF Cafe Forums

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jabb
Post subject: A few basic questions about Power Amplifier theory Posted: Tue Apr 18, 2006 11:05 pm

Captain

Joined: Tue Apr 18, 2006 10:40 pm
Posts: 9
hi
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


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fred47
Post subject: RF PA DesignPosted: Wed Apr 19, 2006 7:06 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!
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.

Regarding the book recommendation: What power levels are you concerned with?

Good Luck!
Fred


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jabb
Post subject: Posted: Wed Apr 19, 2006 7:47 pm

Captain

Joined: Tue Apr 18, 2006 10:40 pm
Posts: 9
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


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IR
Post subject: Posted: Fri Apr 21, 2006 8:46 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Quote:
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?


To 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.

_________________
Best regards,

- IR


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Jeanalmira
Post subject: Posted: Tue Jun 13, 2006 10:11 pm

General


Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
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.

Regards,
Jean







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