How to choose a RF amplifier?

[caoquyen164]

how to choose RF amplifier ?

I'm designing a 5W (37dBm) HF PA with -10dbm input power.
the PA should have 2 or 3 stages ? and how can I choose the right PA ? I mean which parameters of PA at each stage should I care to choose the right one ?
tks a lot.

 

[BigBoss]

Re: how to choose RF amplifier ?

47dB Gain @ which frequency ??

 

[SLK001]

Re: how to choose RF amplifier ?

Depending on the frequency, you'll need at least 4 stages for that much needed gain.

 

[caoquyen164]

Re: how to choose RF amplifier ?

freq is upto 100MHZ. 4 stages are pretty high. maybe 3 stages suitable ?

 

[SLK001]

Re: how to choose RF amplifier ?

freq is upto 100MHZ.

From what? Come on dude, if you want help, you're at least going to have to give us all the information needed to help you.

4 stages are pretty high. maybe 3 stages suitable ?

Let's say that you are a talented RF amplifier designer and can make a three stage amp work. The final device is probably capable of giving you 10dB of stable gain over your frequency range. 47 - 10 leaves 37dB of gain that you need from two stages. The next stage device is probably capable of 15dB of stable gain, leaving 22dB needed from the first stage. Now, since you are a talented RF PA designer, you can do a stage of 22dB (that's a gain of ~ 160) of stable gain over your frequency range, when no one else would even try.

So, "yes" you can do it in 3 stages, because you are the most talented RF designer out there.

 

[BigBoss]

Re: how to choose RF amplifier ?

freq is upto 100MHZ. 4 stages are pretty high. maybe 3 stages suitable ?

If we assume the band-with is between kHz-to-100MHz, it's clear that will be a pretty wide-band amplifier and in order to maintain this bandwidth, at least 4 stages should be used because there will be a negative feedback to get a certain flatness and the stability.Otherwise either it will oscillate or flatness will be too bad.

 

[caoquyen164]

Re: how to choose RF amplifier ?

From what? Come on dude, if you want help, you're at least going to have to give us all the information needed to help you.



Let's say that you are a talented RF amplifier designer and can make a three stage amp work. The final device is probably capable of giving you 10dB of stable gain over your frequency range. 47 - 10 leaves 37dB of gain that you need from two stages. The next stage device is probably capable of 15dB of stable gain, leaving 22dB needed from the first stage. Now, since you are a talented RF PA designer, you can do a stage of 22dB (that's a gain of ~ 160) of stable gain over your frequency range, when no one else would even try.

So, "yes" you can do it in 3 stages, because you are the most talented RF designer out there.

sr bro, my exactly freq is 1.5MHz to 90MHz.
like you said. the last stage almost has 10dB of gain. but the 1st and 2nd maybe 18 to 20 dB of gain. but I want some gain margin. which is enough ? if it's pretty high, it seems like that I have to do with 4 stages.
and can you answer my question "which parameters of PA at each stage should I care to choose the right one ? ". eg : the last stage should have Pout of 20W. the 1st should have P1dB > 10dBm or st..
tks alot.

 

[SLK001]

Re: how to choose RF amplifier ?

sr bro, my exactly freq is 1.5MHz to 90MHz.
like you said. the last stage almost has 10dB of gain. but the 1st and 2nd maybe 18 to 20 dB of gain. but I want some gain margin. which is enough ? if it's pretty high, it seems like that I have to do with 4 stages.

Ain't gonna happen, dude. You have a power amp with a 6 octave range to design. You haven't specified any of the operating conditions, but I assume that you're going to need 5 or 6 stages, at a minimum, to be able to build a stable amp over that range. Since you seem to lack the knowledge of the pitfalls that you will encounter, I would say that you should instead contract the design out.

If you decide to trudge on and design it yourself, I suggest that you design the first stage with a gain of 10-12dB and test it over all your conditions to see if you have a stable design.

 

[mtwieg]

Re: how to choose RF amplifier ?

Rather than focusing on the number of stages, I would focus more on that final 5W, 10dB stage. That's likely going to be tougher than the preceding stages by a wide margin.

 

[caoquyen164]

Re: how to choose RF amplifier ?

Rather than focusing on the number of stages, I would focus more on that final 5W, 10dB stage. That's likely going to be tougher than the preceding stages by a wide margin.

I would use an attenuator at the before 1st stage to control the gain. so it maybe not the problem.

Ain't gonna happen, dude. You have a power amp with a 6 octave range to design. You haven't specified any of the operating conditions, but I assume that you're going to need 5 or 6 stages, at a minimum, to be able to build a stable amp over that range. Since you seem to lack the knowledge of the pitfalls that you will encounter, I would say that you should instead contract the design out.

If you decide to trudge on and design it yourself, I suggest that you design the first stage with a gain of 10-12dB and test it over all your conditions to see if you have a stable design.

there's alot amplifier with wide bw (from DC to several GHz) with high gain (upto 20). why did you suggest 5-6 stages ?

I'm going to use model D2013UK in the final stage with bias voltage at the gate is 3.9 and 14.4v at the drain.
(**broken link removed**). there're only spice model instead of ADS model. and I get the error "not converge" when I modulate it in ADS. can someone try to check out it ?
tks alot.
the spice model: **broken link removed**