Amplifiers have more Pout with single tone than with multipl

B

billcalley

Guest
Hi All,
Will an RF amplifier put out more power with a single tone than
with multiple tones? For instance, if a (wideband) amplifier is able to
have a Pout of +10dBm with a single CW (or modulated) input signal,
will this amplifier's output power go down (per tone) if more tones
are placed at its input (assuming all tones have the exact same
amplitudes but slightly different frequencies). In other words, will
the +10dBm output power of the single tone stay at the same level even
if we insert 100 more tones into this amplifier at the same time but at
different frequencies?

Thanks!

-Bill
 
billcalley wrote:
Hi All,
Will an RF amplifier put out more power with a single tone than
with multiple tones? For instance, if a (wideband) amplifier is able to
have a Pout of +10dBm with a single CW (or modulated) input signal,
will this amplifier's output power go down (per tone) if more tones
are placed at its input (assuming all tones have the exact same
amplitudes but slightly different frequencies). In other words, will
the +10dBm output power of the single tone stay at the same level even
if we insert 100 more tones into this amplifier at the same time but at
different frequencies?

Thanks!

-Bill

Usually giving an amplifier more tones to contend with will make it more
Click to expand...
likely to pout.

RF amplifiers are remarkably like AF amplifiers, only faster. So an RF
amplifier will have an upper limit to the voltage swing (it's usually a
voltage limit) that it can deliver without clipping. If it can deliver
1Vp-p into 50 ohms with a single-tone input, then it can deliver 1Vp-p
into 50 ohms with a 10-tone input. Since the 10-tone input will tend to
have a much higher peak to average power ratio the total power that the
amplifier can handle will go down accordingly.

This is all complicated by the fact that one can generally tolerate a
much greater amount of distortion in an RF amplifier than one can
tolerate in an AF amplifier (cable TV amps are a notable exception).
This being so you can allow some clipping to go on, as long as your
filtered output signal doesn't raise any objections from users or
regulatory bodies.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Andy replies:

Damn good explanation, Tim.....

I tried to add something, but as I re-read your post, you seem to
have it all covered...

Andy
 
In other words, will the +10dBm output power of the single tone stay at the same level even if we insert 100 more tones into this amplifier at the same time but at different frequencies?
Click to expand...
Hi Bill. Defining ''the same'' as close to but not exact (95% or
better), the short answer is YES. The reason it's not exact is that
each tone has a loading effect on the power source.. so one approach is
to measure the output power of each signal seperately, then add up the
wattages for a total. If you find the total is less than the capacity
of the amplifier, conclude that you can make it this way.

The caveats are; When you say different frequencies (tones) , you need
to consider how the whole system responds to each frequency, then add
the power drain in response to each one.

(now I'm thinking .. gee, when a guy doesn't really know electronics --
he almost always has a Engineering PhD or Physics - and that is no joke
my friend) So assuming you haven't gone the PhD route and can still
understand anything beyond you ego;

One needs to consider too the output power of a modulated signal and
what frequencies and levels would contribute to the power usage. This
could readily become more math than you hoped for.

It sounds to me like this is really a ''what if'' sort of question - to
which you have an answer, but I'm certain there are people here (and
perhaps a few doughnut shops) with more hands-on RF experience than I
have.. so listen to them too.

summary;
So .. say an xxx MHz tone causes your output circuit to use 1 unit of
power. An adjacent 'tone' of equal amplitude would cause your circuit
to use 2 units of power.

Agent X
 
If the amplifier is operating in its linear range, the power
contributed by the 1st tone will not be influenced by the power
contributed by the 2nd tone. If the amplifier is not operating in its
linear range (an extreme example would be clipping) then the power
contributed by the 1st tone will be reduced if a 2nd tone is added.
Regards,
Jon
 
On 2 Aug 2005 09:44:24 -0700, "billcalley" <billcalley@yahoo.com>
wrote:

Hi All,
Will an RF amplifier put out more power with a single tone than
with multiple tones? For instance, if a (wideband) amplifier is able to
have a Pout of +10dBm with a single CW (or modulated) input signal,
will this amplifier's output power go down (per tone) if more tones
are placed at its input (assuming all tones have the exact same
amplitudes but slightly different frequencies). In other words, will
the +10dBm output power of the single tone stay at the same level even
if we insert 100 more tones into this amplifier at the same time but at
different frequencies?

Thanks!

-Bill
Click to expand...
The total output power of the amplifier will be +10dBm. This is a
limitation of the efficiency and power dissipation of that amplifier
and of the power supply to the amplifier itself. (hence all the
references to "clipping"). The voltages of the output signals will be
inversely proportional to the number of the signals and directly
proportional to the relative amplitudes of the input signals. If the
voltages must decrease and Zout remains unchanged, then the output
current of each signal into the load will decrease as well.

Is it not intuitively obvious that if you put one signal in and get 10
mW out that if you put 100 signals in you will still get 10 mW out? If
the input voltages are all the same amplitudes and the output
impedance is unchanged throughout the "frequency band of interest"
(meaning the input frequencies must all be within the designed
frequency band of the amplifier), then the total RMS power of the
signals must be limited by the capability of the amplifier and
consequently the currents of the output signals must decrease. The sum
of the voltages times the currents must equal the power available
minus losses. V(f1)*I(f1) + V(f2)*I(f2) ... = P(out). Linear
amplifier, linear equations. Since no realistic voltage sources are
pure and the 100 voltage sources you might attempt to connect to the
input of this amplifier must be isolated from each other the pads you
will need to keep the sources isolated would also have the effect of
reducing the driving voltages as seen by the amplifier.

In the modulated case the side frequencies (sidebands) generated add
to the total power of the spectrum and the carrier power must be
reduced (derated) so that the total peak power remains within your
10dBm limitation in order for that amplifier to remain in its linear
performance region. In the case of an AM carrier the peak to average
power must be limited or you will let the magic smoke out of the
amplifier. The carrier power must be maintained at a reduced level so
the power gain of the amplifier has enough reserve to accommodate the
sideband power and still remain within its designed power dissipation.
 
Really great stuff guys -- it will take me a bit to sort it all out!

Thanks Again!

-Bill
 
Jon wrote:
If the amplifier is operating in its linear range, the power
contributed by the 1st tone will not be influenced by the power
contributed by the 2nd tone. If the amplifier is not operating in its
linear range (an extreme example would be clipping) then the power
contributed by the 1st tone will be reduced if a 2nd tone is added.
Click to expand...
Actually, the phase and frequency of the 2nd tone are important.
Consider the elementary description of the DFT of the square wave in
terms of 1st, 3rd, 5th, ... harmonics.

John Perry
 
John,

You are right as far as distortion of the composite waveform is
concerned. However, if the amplifier is operating in the linear range,
then any distortion of the composite waveform will be due to phase
non-linearity. For example, if the composite consists of a fundamental
and a third harmonic, the composite will be delayed, but not distorted,
if the phase shift of the third harmonic is exactly 3 times the phase
shift of the fundamental. However, the addition of the third harmonic
will not affect the contribution of the fundamental. If the phase is
non-linear, the composite waveform will be distorted, but again, the
contribution of the fundamental will not be affected by the addition of
the third harmonic, as long as the amplifier is operating in its linear
range.
Regards,
Jon
 
when you double the number of equal power input tones, the AVERAGE
output power will be 2x, the PEAK ouput power will be 4x.

This assumes the amplifier is capable of creating these increased
levels. If it isn't, it will distort.

Mark
 
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