need help on power amplifier

[bor]

hai guy..! i am here to get to know some information on power amplifier type AB...ehmm is there possible to design power amplifier type AB on RFID application at freq 868 Mhz..?? need opinion...

 

[batdin]

I doubt if you really need so powerful an output.Class A amplifier will deliver more than 100mW if properly designed.
If you need some more,use Class C amplifier (for RF class AB is hardly ever used).

 

[jupiter]

Basically, class-A amplifier is a linear amplifier with a low efficiency; while class -AB is also a linear amplifier with a higher efficiency. Class-C is highly nonlinear but with a even higher efficiency. In systems such as satellite systems or mobile stationhs which are power limited, class C is often. For systems such as the base stations of CDMA systems which need highly linear ampliification, then class A have to be used. In RFID systems which transmitted only CW, there is no need to use linear ampliifer because CW has a constant envelope or constant input power., so class C is good enough.

For the design if class C amplifiers, any communcaitions textbooks will have good decription about it.

 

[yeechyan]

Is the amplifier operating on fixed frequency as stated? (868MHz) Since it's meant for RFID, I'd suggest using a Class C amplifier as stated by Jupiter. Even it has a poor linearity, the LC tank circuit will generate a sine wave due to resonance.

 

[bor]

mr jupiter..sorry what is CW means tq

 

[Puppet1]

send the specs for RFID

frequency
output power
modulation scheme
bandwidth

 

[bor]

mr puppet1..sory to say..about the requirement...not all i have decided yet..basically the freq 868Mhz..and no idea for the rest..
truely story iam a new people doing this design..
ehmm maybe you can give me some example about design..for my guide line..to decided the requirement...after all thanx..buddy

 

[jetset]

AB is not for Hi freq.

 

[jupiter]

mr jupiter..sorry what is CW means tq

"CW" means continuous wave which is a single tone and so has a constant envelope. As a result, no distortion occures even with a nonlinear amplifier. For RFID system, the reader will transmit a CW to the tag. Upon receive the CW signal with enough energy, the circuit will start working, i.e. back scattering the signal back to the reader.

 

[bor]

mr jetset can you elaborate for me..or give me some explanation why thi AB cannot function for high freq...or anyone could help?

 

[jupiter]

It is not quite true that class AB power amplifier cannot be used for high frequency. In fact, whether a power ampliifer can be used for high frquency, to a very large extend, depends on whether the transistors used are for high frequency. Any configuration can be used for high frequency if the transistors are for high frequency.

The major difference between the different classes of power ampliifers are the efficiency and the linearity. Class A is very good in linearity, but low efficiency. Class C has a high efficiency, but us very nonlinear.

 

[Gemini1706]

I doubt if you really need so powerful an output.Class A amplifier will deliver more than 100mW if properly designed.
If you need some more,use Class C amplifier (for RF class AB is hardly ever used).

This is completely wrong.

Class AB is the MOST WIDELY used class for power amplification at RF frequencies..

In fact class AB will consume less DC power from the battery (or supply) than class A for the same output power, since it is more efficient..


Do not put completely wrong facts here, it is better to say: I do not know, or I am not sure, so you do not confuse new engineers..


Cheers...

 

[jetset]

mr jetset can you elaborate for me..or give me some explanation why thi AB cannot function for high freq...or anyone could help?

Ok . For AB class amplifiers, when you analyze the equivalent circuit for High frequencies you get a capacitance connected between the input and output, that means that you get a very significant Miller effect and that reduces your bandwidth.
The amplifiers used for High frequencies applications are Common Base or common gate amplifiers, because they don't have feedback capacitance in their equivalent circuit for high frequency, and there is no Miller effect for this configuration, and you get a higher bandwidth.
Hope it could help you.

 

[Gemini1706]

AB is not for Hi freq.

Oh my god, how you can clame this?? This is completely not true.

I design (for living) RF power amplifiers, and the ONLY class I use is class AB (and all other compnanies also use the same thing).

Class AB is THE MOST popular class at RF freq (like this 800MHz in question)..

Please people, do not mislead others into those mythes.

Class AB is more efficient, and has the same linearity (in practice, not in textbooks) as a class A.


Go and read Steve Mass's Book "RF Power Amplifiers for Wireless communication (or something like that), it is gonna tell you all the story..

All what is said above about class AB not suitable is completely misleading.


Unbelievable all those false advices..

 

[bor]

guys thanx a lot...

 

[jupiter]

AB is not for Hi freq.

Please people, do not mislead others into those mythes.

I totally agree with Gemini1706, if someone is not sure or does not know anything, please don't try to be clever and pretend to know it at all!!! It will confuse others and distort the whole purpose of these forum! Please! Please! Don'r ever do it again!

 

[startboy]

my dear:

I am analog IC design enginner in RFID field, i suggest you to use

class E or class F power amplifier, because RFID is CW type transmitter,

if you use classical linear amplifier, your reader is very easy to burn and

broken, so you need to consider power efficient and DC power consumption

problem.

For example, in class E case, if your DC power is about 5V, you could

get maxmum power about 100mw ~ 400mw (ideal condition).


c.c.huang

 

[Kral]

bor,
Class A. Transistor conducts current for 100% of the cycle (360 deg). Least efficient, most linear. Can be used single ended or push pull for linear operation.
.
Class B Transistor conducts current for 180 deg of the cycle. more efficient than class A. Must be used push-pull for linear operation.
.
Class AB Compromise between class A and class B. Less efficient than class B. More linear than class B. Must be used push-pull for linear operation.
.
Class C Transistor conducts for less than 180 degrees. Most efficient. Can not be used as a linear amplifier. Think of it as a synchronous switch that dumps energy into an LC tuned circuit once per cycle. The LC circuit converts the pulse to sine wave output.
Regards,
Kral