UHF 20-dBm PA circuit design...

[[email protected]]

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

 

[[email protected]]

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?

 

[Frank Raffaeli]

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

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

 

[Jim]

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

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 www.sirenza.com and lots of similar RFIC companies.

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

 

[Nick]

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
Seewww.sirenza.comand lots of similar RFIC companies.

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

 

[JosephKK]

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 www.sirenza.com and lots of similar RFIC companies.

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

I find class C and linear a contradiction in terms.

 

[Frank Raffaeli]

Thanks guys -- I guess MMIC is the way to go nowadays for a linear PA!

-Nick

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

 

[TheM]

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?

He is building a small pirate tv transmitter and doesn't want us to know
about it. It is obvious.

Mark

 

[Nick]

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)
+/- 63 mA peak current and +/- 3.16 V pk into the load at your
desired level of distortion.

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