Why we need to add input buffer for RF amplifier?

rf amplifier input output match

Why we need to add input buffer for amplifier?

Is that for impedance matching and level shifting? I want to make sure that I am not missing anything important.

Can any one introduce some good document for me on that topic?
Thanks!

never herd of this one

You should describe the situation you know about. I have never heard of such a need. You specify the source and load impedance and design from that.

buffer? what buffer? you mean an op-amp?
no buffer is needed at RF frequencies.. you just match as flatulent said for your input and output impedances.

sorry for the confusion

It is my fault to describe the problem so briefly.

The case is that when designing limiting amplifier for SONET, may designs used input buffer and output buffer.

Output buffer is used to drive the offchip 50ohm resistor load, what's the usage of input buffer? Some papers said it is used for impedance matching. But I do not agree, any people familiar with that?

title only

I think that it is not a real gain of one buffer, it is just a convenient name to put on the block diagram. What I expect they mean is an amplifier stage whose input impedance is high enough that to the outside world it does not load down the transmission line and the load on the line can be set with a resistor across it.

I once designed a three stage power amplifier. I suppose that in this case your buffer is an impedance matching state with low noise (LNA) and also amplififyes the input power to the desired level for the next stage.

I hope this helps,

Best regards,

mimoto

Buffer amplifiers are used to avoid " frequency offset " phenomena in such a tranceivers. Because , input impedance of an power amplifier is changed by signal amplitude and this may be reason of pulling effect at the output of VCO ( or simply oscillator ). When impedance is changed by the signal , oscillator frequency is also changed.
To provide sufficient isolation between power amplifer and Oscillator ( or VCO) , a ' buffer amplifier" is put, hence the impedance that is seen by oscillator rests constant and pulling effect is minimized.

This is true also mixers ( especially diode mixers ) . When you drive into mixer from a VCO , you should use a buffer amplifier to avoid "pulling effect"

Rgrds

BigBoss said:

Buffer amplifiers are used to avoid " frequency offset " phenomena in such a tranceivers. Because , input impedance of an power amplifier is changed by signal amplitude and this may be reason of pulling effect at the output of VCO ( or simply oscillator ). When impedance is changed by the signal , oscillator frequency is also changed.
To provide sufficient isolation between power amplifer and Oscillator ( or VCO) , a ' buffer amplifier" is put, hence the impedance that is seen by oscillator rests constant and pulling effect is minimized.

This is true also mixers ( especially diode mixers ) . When you drive into mixer from a VCO , you should use a buffer amplifier to avoid "pulling effect"

Rgrds

Click to expand...

I would use an isolator to make impedance indepdent of next stage behavior..

anyway.. what does this 'buffer' consists of? is it just a simple interstage amplifier with input/output match? is this called 'buffer'? Never heard of such terminology at RF frequencies before (but very common at low frequnecies)..

Cheers...

Gemini1706 said:

I would use an isolator to make impedance indepdent of next stage behavior..

anyway.. what does this 'buffer' consists of? is it just a simple interstage amplifier with input/output match? is this called 'buffer'? Never heard of such terminology at RF frequencies before (but very common at low frequnecies)..

Cheers...

Click to expand...

A simple buffer amplifer may be defined with its own S12 parameter. If Reverse transmission reflection ( or isolation) parameter is high enough , we say that this a buffer amplifier.
The main duty of these amplifiers are NOT to amplifiy the small signals. The main purpose to use these kind of amplifiers to intend the isolation between the stages which will be on a chain of a tranceiver.

For instance , consider an amplifier with Mag[S12] <<1 ( let's say 0.01)
in this case isolation will be 40dB what means that output signal will infuluence to INPUT by 40 dB difference.

An example for buffer amplifier.. RF2301 ( RF Micro Devices )
It performs 50dB !! Reverse isolation

https://www.rfmd.com/DataBooks/db97/2301.pdf

For discrete example , it may be consist of a Differential Input Stage and then Emitter Follower for output stage. Therefore , input impedance will be relatively high regarding to Output impedance.

For more information , see " Bipolar And MOS Analog Integrated Circuit Design , Alan B. Grebene , Joh-Wiley Interscience , 1984 "

Rgds