I need to design a few low-cost, linear, single stage, 20dBm power amplifiers for UHF radio consumer use. I am familiar with designing such PAs for saturated FM, but not for AM modulations. With linear PAs, how can I design them so that each PA will have pretty much the same output power from amp to amp (without tuning each one)?
Thank you,
-Nick
AGC?
Modules with fixed gain are pretty cheap these days, but if you need to use discrete transistors, the gain consistency will depend mostly upon biasing and temperature compensation. What gain consistency do you need?
For better linearity and efficiency: Google feed forward amplifier
20dBm linear is quite easy using a MMIC (RFIC). Actually 20dBm is about as much as they get without going silly on current. By 20dBm, I mean at 1dB compression, probably around 34dBm 3O-OIP See
Higher powers (watts) tend to use the traditional Class C deisgns, but with nasty, difficult to maintain forward biasing to bring it into "linear" operation. Jim
Thanks guys -- I guess MMIC is the way to go nowadays for a linear PA!
-Nick
I find class C and linear a contradiction in terms.
Could be ... depends on the linearity you need. How about a TOI or IM3 spec? You can also use a distributed amplifier or balanced amplifier. Are you using it for WiMax? The standard Sirenza / RFMD IC won't work at the power level you expect.
Frank
Hi Nick,
Some more information might help people give you better information. For example -- "UHF" is very broad: 300 to 3000 MHz. I have a feeling your need is for a small range of frequencies -- but which ones?
What is your load impedance? 50 ohms or what?
Is your modulation merely simple AM or is it something more complex like 16QAM?
Is this "20 dBm" the power of your unmodulated carrier or is it the Peak Envelope Power?
What level of distortion do you require? Harmonics of the carrier Distortion of the envelope
Are you going to create this modulation upstream and then hope to run the output stages open loop? Or are you going to have a detector at the ouput and then sum in the modulating signal with a level = reference into an ALC loop?
In any event, if 20 dBm is your peak envelope power, then your amp will need to provide (if my math is correct and assuming 50 ohms)=20
+/- 63 mA peak current and +/- 3.16 V pk into the load at your desired level of distortion.--=20 Regards, Howard snipped-for-privacy@ix.netcom.com
He is building a small pirate tv transmitter and doesn't want us to know about it. It is obvious.
Mark
snipped-for-privacy@q27g2000prf.googlegroups.com...
Thanks again guys, but I think I will just stick with MMICs for linear amplification, and design discrete amps for saturated only use. There are just too many "gotchas" that can be very problematic with linear PAs!
Best regards,
-Nick
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