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Design >> RF Design >> How to calculate PAE
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Message started by .matteo on Apr 14^th, 2012, 1:13am

Title: How to calculate PAE
Post by .matteo on Apr 14^th, 2012, 1:13am

Hi everybody,

I'd like to ask a question about a system made of a VCO connected to the gate of a nMOS transistor that acts as a PA and delivers a certain amount of power to a 50Ohm load.

Power Added Efficiency is calculated as:
PAE = (PoutRF - PinRF)/PDC

I'm kind of confused. Let's say that the swing at the input of the nMOS gate is 3.3Vpp, what is the input power in such a high impedance system?

1 Does it really make sense talking about input power in this case? In my opinion input power makes sense when there is a source with some impedance and to apply a voltage you need to spend some current, but here since its a high impedance system and there is no current-voltage dependance it does not really make sense to me.
2 I would say that I should only take into account the efficiency of the whole system delivering the amount of power that I desire.
2 Or should I disconnect the PA from the VCO, match it to 50Ohm and calculate PAE?

Any help understanding this topic is welcome ;)

Title: Re: How to calculate PAE
Post by aaron_do on Apr 14^th, 2012, 7:06pm

Whichever method is the most logical is probably the right one. Ultimately you care about the entire system's efficiency. PAE is helpful as a FOM if your PA doesn't have a huge amount of gain. Likewise, if you are only delivering a small amount of power, then I think I would be more concerned about the entire system's PE than the PAE of the PA.

If you absolutely must calculate PAE, then another thing you could do is consider the VCO tank as part of the PA. Then you have an input power. However, in your case, option 2 is probably the best.

Title: Re: How to calculate PAE
Post by .matteo on Apr 15^th, 2012, 12:10am

@aaron_do
I definitely agree with you. But in the VCO+PA system, what is the input power? How do you "calculate" it?

Title: Re: How to calculate PAE
Post by aaron_do on Apr 15^th, 2012, 6:00am

First of all, I think you know, but just to make sure, the PAE only applies to the PA itself, not the PA + VCO. In theory there is some input power to the PA (probably very small), so you could calculate PAE if you wanted to (just probe the power going from the VCO to the PA to find Pin). But if the power gain is really very high, then your PAE will be the same as the drain efficiency. I'm not sure if I'm missing something here...you seem to already know the answer to your question.

Aaron

Title: Re: How to calculate PAE
Post by RFICDUDE on Apr 15^th, 2012, 3:49pm

Power added efficiency is a figure of merit that considers the power gain of the amplifier. Because, after all, what good is a power amplifier that doesn't provide much more power at the output than is delivered to the input?

On the other hand, PAE for a VCO buffer amplifier PAE may not be the most important parameter because the VCO may not be capable of driving a load without the buffer, so the amplifier is required for the circuit to function with a given load. PAE may be appropriate for power amplifiers after the buffer amplifier stage since there the purpose is to increase the output power compared to the input.

Title: Re: How to calculate PAE
Post by .matteo on Apr 16^th, 2012, 5:54am

I think that all the considerations you made are right and I totally agree with them.

Also, I thought that in this particular case the PAE may not have an important meaning also because if you take the PA alone, without input matching network, the true (or actual) input power (that is the input power you have to use in the PAE formula) is almost zero. All the power is reactive power (yes, I know there is a series resistor on the gate but its small...). Hence, i such a system the PAE is the same as the efficiency. :(