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?
Usually giving an amplifier more tones to contend with will make it more 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.
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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
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.
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.
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
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