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Strange phenomenal at CLASS AB PA

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Full Member level 5
Apr 6, 2006
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I saw in the lab some phenomenal that I saw in the past but I dont forget his name
If I increase signal at the IN of PA by 1 dB the output Power increase by more than 1dB

It Class AB PA.
Like I know it more close to C phenomenal
So: What name of this phenomenal and how u explain it?


Class A amplifiers are linear. Anything else is....non-linear. The amount of non-linearity varies, but it is always there.

I know that Class A is most linear AMP and it going and decreasing

BUT AMP that i am using is also pretty linear
what is strange that when i increase 1dB at the input output increase by 1.5dB...

important to mention that i am working at small signal and not close to 1dB compression ...

The name of the phenomenon is Gain Expansion, and is direct related to the class of the amplifier.
The carrier gain expansion is direct related to the harmonics nulls (especially of 3rd harmonic). At particular output power (just before compression), in-band distortion falls over the fundamental will add to the carrier, and so an expansion phenomenon will appear.
Adjusting the quiescent current of the amplifier can help reducing this effect.
It is caused by assisted drive biasing for class AB RF power amplifiers. As more input RF drive is applied, the PA stages DC biasing increases causing the stage gain to increase. The way the stage is idle biased and decoupled effects this phenomenon.

There is also a similar effect based on optimization of the interstage impedance match. The first stage will deliver more output power to final stage due to better match at higher power.
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There can be the most stable or unchanging bias as possible, and gain expansion it will appear at specific bias current.
As I mentioned, gain expansion is a phenomenon which is direct related to the in-band distortions, which add in power to the carrier, at particular current bias AND output power.

There is some thing called Gain expansion, This is due to bias condition, if you are under bias to decrease DC consumption, when input power reaches certain level then bias is driven by RF ( similar to Class C) so bias will change and cause gain to increase. Let me know if you not understood.
The gain expansion is mostly due to the non-linear transconductance(gm) of the transistor. Ideally, gm is constant, so class A and class B are linear amplifiers, while class AB doesn't. In small signal region, class AB operates as class A, full wave amplification, while in the very large signal region, the performance is close to class B. As we know, the difference of gain between class A and B is 6 dB. So the gain of class AB amplifier should decrease with an increasing input power in the ideal case. However in our real world, the gm is, more or less, a bell-shaped curve referring to the input power. So gain expansion appears. This effect can be compensated by adjusting the conduction angle, which is controlled by the bias point. In this way, sometimes, class AB might be a linear configuration.
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