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Output Power

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electroRF

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Hi,

I simulated a power amplifier design - inserted a constant input power and received the following output power:
View attachment 67942

I was wondering, why I get two different peak levels?
one peak reaches 2.1W and second peak reaches 1.8W.

Thank you.
 
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where would you apply decoupling?

On the incoming supply rails - the output stage in particular will require more substantial attention as it has to draw considerably current.

However, simulations aren't real life, and an RF design like this would be pretty critical on layout as well.
 
If I understand your simulator graphs correctly, this is running at 3.75GHz. *

The fluctuation in power between cycles is some effect occuring at half the drive frequency. *
I have seen this sort of thing happen in lower frequency amplifiers (50 - 150MHz) and seems to be caused by impedance matching and drive level problems.

JimB

On edit:
*Some bad assumptions and interpretations on my part here.
See my later comments in post #10 of this thread.
 
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Hi,

The input power is derived from a sine wave voltage source of 1.8GHz with 50Ω internal resistance, connected to the input port of the PA.
 
My earlier comments about the frequencies involved were based on a mis-understanding of what the graphs were trying to show.
The "Power" trace is showing the instantaneous power through the individual cycles, which to me is quite a strange concept.

If you examine those VL and IL graphs you will see that the phase varies through the range, something which again looks a bit odd.

In reality you would not use an oscilloscope on a circuit working at these frequencies, you would use a spectrum analyser.

Does your computer game (err sorry I mean simulator) have a spectrum analyser type display?
If it does, use it and then the output power may make more sense.

JimB
 
Hi Jim, Thanks!
You really help me understand things better.

The simulator I use is ADS (Advanced Design System) - It has the ability to simulate everything in the frequency plane.
I'll do that.


I checked in which class the PA is working.
As the transistor is always ON and since both Collector Current and Voltage (Ic, Vc) are full sine wave, would it be correct to say that this is a Class-A opreation?

Ic: View attachment 67967
Vc: View attachment 67969
 
I checked in which class the PA is working.
As the transistor is always ON and since both Collector Current and Voltage (Ic, Vc) are full sine wave, would it be correct to say that this is a Class-A opreation?
Yes, that is a reasonable assumption.

You may want to consider the collector circuit decoupling capacitors, 1pF sounds a bit small, even at 1.8GHz.

JimB
 
Thank you Jim :) again!

I have a "general" question.

Why are Power Amplifiers needed in telecommunication mobile devices?

I mean, say that a mobile device needs to transmit a certain modulated signal, why is it better to generate it with low power, and then amplify its power by a PA before transmission,
then to directly generate the modulated signal with the required power and transmit it as is?

Thanks!
 
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Thank you Jim :) again!

I have a "general" question.

Why are Power Amplifiers needed in telecommunication mobile devices?

I mean, say that a mobile device needs to transmit a certain modulated signal, why is it better to generate it with low power, and then amplify its power by a PA before transmission,
then to directly generate the modulated signal with the required power and transmit it as is?

It makes no difference - it's just a matter of the location of the PA, either external to the main transmitter, or built-in it.

If you're talking AM transmissions, where you need a linear PA, then you need a massive audio amplifier to high level modulate the transmitter - so doing the modulation at a lower level and using a linear PA makes a great deal of sense.

Incidentally, if you look at PMR type transmitters - you could often buy them with different powers, for reasons of expense and licence conditions. Such variants are usually identical, with a low power transmitter in the box, followed by a separate PA stage depending on what power is needed (usually mounted to the rear of the radio, so the heatsink can stick out the back).
 
Why are Power Amplifiers needed in telecommunication mobile devices?

I mean, say that a mobile device needs to transmit a certain modulated signal, why is it better to generate it with low power, and then amplify its power by a PA before transmission,
then to directly generate the modulated signal with the required power and transmit it as is?

Consider an FM transmitter (a real one not the two transistor kiddies noise toys which are so often debated here on ETO)
The frequency determining circuits will either be a simple crystal oscillator, or, more likely a frequency synthesiser of some kind which is locked to a crystal.
A crystal is inherently a low power device, a thin sliver of quartz. Too much current through the crystal and it will shatter.:(
A synthesiser will have a large number of divider stages, usually implemented in a small integrated circuit, do you want to build this circuit using power transistors rated at say 20 watts?(size and cost!:(), no, definitly not.

As Nigel mentioned, there could be several versions of a radio.
Once the frequency generator and modulator are built, you can use that same circuit for 1W, 5W, 25W, 100W versions.
Just the design of the power amplifier varies.

To summarise, it is easier and cheaper to build the complicated stuff at low power and then amplify as required.

JimB
 
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