Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Biasing circuit for Power Amplifier

Status
Not open for further replies.

haitaoz

Member level 3
Joined
Oct 11, 2004
Messages
61
Helped
13
Reputation
26
Reaction score
4
Trophy points
1,288
Activity points
372
It is known that biasing circuit is critical for power amplifier performance.Can somebody talk about the trade-off between them?Such as from linearity,efficiency,temperature stablity,biasing stability points of view?
Thanks!
 

electronics_kumar

Advanced Member level 2
Joined
Nov 29, 2004
Messages
659
Helped
34
Reputation
68
Reaction score
9
Trophy points
1,298
Location
Tamilnadu
Activity points
5,552
i)once if you are travelling from classA,B,C . the efficiency increases from 50%,78,100%.
ii)also its linearity decreases from class A to C.ie class A is more linear than B which is more linear than Class C
iii) CLASS A wll have operating point at Qpoint but for class B it goes near to cutoff.class C goes into deeper cutoff..
iv)class B is having crossover distortion to mitigate Class AB was proposed.
v)cLASS A will have zero signal high power dissipation ie no signal at its input high power dissipation...whereas class B will power dissipation only when it is having signal in its input


still to be explored
 

haitaoz

Member level 3
Joined
Oct 11, 2004
Messages
61
Helped
13
Reputation
26
Reaction score
4
Trophy points
1,288
Activity points
372
Thanks for your answering!
For the typical handset PA,most of them are biasing around class AB from linearity and efficiency point of view.However,there are some biasing circuits for each input port(like base of the transistors).I wonder what is the typical schematics of them and the trade-off between them.

Thanks again!
 

boy

Advanced Member level 1
Joined
Sep 30, 2002
Messages
481
Helped
96
Reputation
192
Reaction score
55
Trophy points
1,308
Location
Thailand
Activity points
3,963
Most PAs use active bias due to its temperatur stability. There are some active bias chips from infineon.
 

electronics_kumar

Advanced Member level 2
Joined
Nov 29, 2004
Messages
659
Helped
34
Reputation
68
Reaction score
9
Trophy points
1,298
Location
Tamilnadu
Activity points
5,552
class A,B are used as a audio frequency amplifier whereas class C are used as radio frequency amplifier
 

boy

Advanced Member level 1
Joined
Sep 30, 2002
Messages
481
Helped
96
Reputation
192
Reaction score
55
Trophy points
1,308
Location
Thailand
Activity points
3,963
A very linear amplifier for digital modulation uses class A. Also class AB is very popular for linearity and efficiency.
 

haitaoz

Member level 3
Joined
Oct 11, 2004
Messages
61
Helped
13
Reputation
26
Reaction score
4
Trophy points
1,288
Activity points
372
Is there any typical biasing circuit for CDMA or GAM PA?It should not be just voltage biasing,am I right?Hope you can share your valuable experience.thanks
 

vfone

Advanced Member level 5
Joined
Oct 10, 2001
Messages
5,267
Helped
1,547
Reputation
3,095
Reaction score
1,153
Trophy points
1,393
Activity points
33,358
Biasing a PA is a challenge especially when is using digital modulation.
In GSM handset the bias of the PA it depend for what topology is used for Power Amplifier: Base/Gate voltage control (usually for closed loop approach) and Collector/Drain voltage control (open loop approach).
In CDMA/WCDMA things are different. The PA should be very linear, having in the same time a good efficiency. Here one approach is to use Dynamic Bias (different bias for different output powers).
I have to underline one mistake that is made by people very often today. They believe that in GSM/GMSK because is constant envelope can be used a pure Class C amplifier, and in CDMA/WCDMA because is nonconstant envelope the PA class shall be a pure Class A. In reality for both cases the designers use Class AB. If pure Class C is used in GSM/GMSK the Spectrum due to Switching it will go crazy, and for CDMA/WCDMA is never used a pure Class A because the PA efficiency will be less than 25%.
Main challenges of a PA bias circuit are:
-Voltage/Current thermal runaway
-Stability vs Frequency vs Output Power vs Temperature vs Voltage
-Thermal Noise
Here are some articles about PA biasing:
http://xanadu.ece.ucsb.edu/~long/ece145a/Bias_and_Implementation_f04.pdf
http://amsacta.cib.unibo.it/archive/00000573/01/GM_03_03.pdf
http://europe.nokia.com/downloads/aboutnokia/research/library/electronics/ELE5.pdf
 

haitaoz

Member level 3
Joined
Oct 11, 2004
Messages
61
Helped
13
Reputation
26
Reaction score
4
Trophy points
1,288
Activity points
372
vfone,
Thanks for your reply and it is really helpful
 

lynnheflex

Newbie level 2
Joined
Dec 26, 2005
Messages
2
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,294
Does the biasing circuit influence the matching circuit?
 

vfone

Advanced Member level 5
Joined
Oct 10, 2001
Messages
5,267
Helped
1,547
Reputation
3,095
Reaction score
1,153
Trophy points
1,393
Activity points
33,358
lynnheflex said:
Does the biasing circuit influence the matching circuit?
Yes. Changing the bias it change the input impedance of the active device. For this reason the bias of the first stage should be carefully designed.
The output of the PA shall be load-pulled for different bias/Ic/Id, to find the right output match.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top