RF Amplifiers related jargon..

[xitronics]

Hello,

i find a confusion in calculating the input power delivered by a signal to an FET amplifier..

Normally at LF, FETs offer very high reverse resistance and reactance at the ip side, consuming micro or nano level power from the ip ac signal. At HF, VHF or UHF, the reverse jn capacitive reactance reduces the overall ip impd to add extra loading at the ip side.. This action is dependent on Ft of the device.

However, the power consumed at the ip will be significantly reactive and not true.. The true power must still be very small..

However i find this concept contradictory to what the designers of RF(VHF) amplifiers say. It is said that the RF amplifiers consume larger true power(particularly at VHF) due to which the power gain is limited to a value(generally around 10 - 50) decided by Ft.

Can anyone provide a satisfactory explaination of the beaviour of VHF ampllifiers or some helpful web links on the same.. I find this same confusiion in BJTs too.

Regards,
Prasanna.
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[darkfenriz]

Input impedance of these amps is not high, most often it is matched to characteristic impedance of cable like 50 or 75 ohm for example (truly resistive ohms). Matching impedances is used to diminish reflections in cable, whose length is comparable or higher than electric wavelength.

 

[xitronics]

Okay.. But howz the calculation of input signal power to the VHF amp. done.. Is it the true power consumed by the input section or reactive power consumed by the parasites..?

For eg. If say that 25mW of RF input power is required to drive the VHF ampli for generating 200mW of output power.. Here which element comsumes 25mW power..? What are the governing relations..?

Waiting for a reply..

Thanks in advance,
With Regards,
Prasanna.
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[darkfenriz]

Just assume there is a 50ohm resistor from signal input directly to ground. Real resistance and ral power across this resistor.
power=voltageRMS/50R
regards
Adam