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Design >> RF Design >> Physical reasons for gain compression of amplifier?
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Message started by jianke on May 16^th, 2012, 3:08pm

Title: Physical reasons for gain compression of amplifier?
Post by jianke on May 16^th, 2012, 3:08pm

Hi everyone, as we know that amplifiers suffer from “gain compression” because of its nonlinearity. Assume the input of ampllifier is “vi=Vi*cos(wi*t)”, the amplitude of the output fundamental wave can be expressed by the following formula (see the following fig.).

When “k3>0”, it is called “gain expansion”; while “k3<0”, it is called “gain compression”. Well, we know that gain compression is the real case for amplifiers. But what are the physical reasons for gain compression of amplifiers? Or, anyone can physically explain why “gain expansion (k3>0)” cannot happen for amplifiers? Thanks!!!
[img][/img]

Title: Re: Physical reasons for gain compression of amplifier?
Post by RFICDUDE on May 16^th, 2012, 5:01pm

There are two good physical reasons for gain compression:

1. A transistor can run out of current (cutoff on negative swings).
2. The output swing reaches the supply limit and cannot increase further.

Once an amplifier waveform reaches either a current or voltage limit the output ceases to increase at the same rate the input increases.

Now it is physically possible for k3 > 0 (gain expansion) before the amplifier reaches a physical supply limit. This happens, by design, in class AB amplifiers where the DC current in the amplifier is a function of the input signal (DC current increases with increased input power), but eventually the supply limit takes over and the amplifier enters compression following the gain expansion.

Title: Re: Physical reasons for gain compression of amplifier?
Post by rfcooltools.com on May 18^th, 2012, 1:20am

jianke,
in a classical sense the total power in any compression is distributed among harmonics and distortion products. the reason the compression of desired tone is experienced is that power is distributed at non desired frequencies. This is another way of looking at it and RFICDUDE is correct in example of some cases where this occurs, but ultimately the compression is a result of other harmonics playing a bigger part of the output spectrum, where the gain is still achieved until clipping but the desired frequency gain is competing against the introduction of new tones competing for the output power. In other words total power gain deviating from desired frequency gain is what is observed in gain compression.

http://rfcooltools.com

Title: Re: Physical reasons for gain compression of amplifier?
Post by aaron_do on May 18^th, 2012, 3:06am

Quote:

in a classical sense the total power in any compression is distributed among harmonics and distortion products. the reason the compression of desired tone is experienced is that power is distributed at non desired frequencies. This is another way of looking at it and RFICDUDE is correct in example of some cases where this occurs, but ultimately the compression is a result of other harmonics playing a bigger part of the output spectrum, where the gain is still achieved until clipping but the desired frequency gain is competing against the introduction of new tones competing for the output power. In other words total power gain deviating from desired frequency gain is what is observed in gain compression.

that's a very interesting way of looking at it, but I think ultimately the simple explanation given by RFICDUDE is more correct. In the end, even if you add up the harmonic power, the total power will still compress due to limited headroom or current drive.

regards,
Aaron

Title: Re: Physical reasons for gain compression of amplifier?
Post by rfcooltools.com on May 18^th, 2012, 8:55pm

aaron_do,

I believe what you are referring is clipping which is well beyond compression.
Consider 1dB compression point extrapolated from IM3 why does it happen around 10dB from the intercept? Its not a coincidence that the power going into the IM3 products at 10dB backoff are equal to the power lost in the desired.

http://rfcooltools.com

Title: Re: Physical reasons for gain compression of amplifier?
Post by loose-electron on May 20^th, 2012, 10:16am

What is wrong with the simple explanation of "large signal effects" and be done with it?

Title: Re: Physical reasons for gain compression of amplifier?
Post by aaron_do on May 20^th, 2012, 5:47pm

Hi rfcooltools,

I understood your point, but I think voltage headroom and current limiting are the root cause of the compression. IMO, the harmonics and intermodulation components are just a consequence of the compression. Also, your explanation doesn't cover gain expansion. When the gain expands, you get more power in both the fundamental and the harmonics.

regards,
Aaron

Title: Re: Physical reasons for gain compression of amplifier?
Post by RFICDUDE on May 20^th, 2012, 6:10pm

I think we are all saying the same thing from equivalent points of view. A nonlinear circuit creates a nonlinear response that is dependent on the input signal and the underlying nonlinear function (gain compression/expansion, sampling, mixing, phase detection, etc...).

Creation of harmonics, fundamental, mixing products, aliases, etc. are all frequency domain measurements of the nonlinear response. The sum of all the frequency domain components is equivalent to the time-domain clipped waveform. So we can look at the clipped waveform and say wow the output is swinging rail-to-rail or we can look at the spectrum and say look at all those strong harmonics. For stringent linearity specs the distortion is too subtle to even see in a time domain waveform, so the frequency domain is really the only practical way to quantify the distortion. But for gross clipping distortion it can sometimes be easier to debug what is physically going on by inspecting the time domain waveforms.

Title: Re: Physical reasons for gain compression of amplifier?
Post by jianke on May 23^rd, 2012, 3:13pm

Hi, thanks sooo much for all your reply. I read the book <Microwave Engineering(2011_4th) David Pozar>.
It simply explain the physical reason for the "gain compression". But I still cannot understand it. Anyone can give me some comments?

Title: Re: Physical reasons for gain compression of amplifier?
Post by aaron_do on May 23^rd, 2012, 8:48pm

Its exactly what RFICDUDE said. It u dont understand, i recommend you trying simulating a simple common-source amplifier.

Regards,
aaron

Title: Re: Physical reasons for gain compression of amplifier?
Post by vivkr on May 24^th, 2012, 7:31am

At the risk of being heckled for my ignorance, let me ask a question which deviates a fair bit from the main topic of discussion.

Is gain compression a factor which limits performance of RF circuits critically? If so, is it at all possible/common practice to avoid the unpleasant effects of this by use of an on-chip 1:N transformer which would deliver significantly larger signal swings at the load side (Nx) while keeping swings low at amplifier output?

Or are on-chip transformers not yet efficient enough?

In some sense, the various load matching techniques used are doing something similar but while they all provide more signal at output (more power), they do not always avoid gain compression.

Vivek

Title: Re: Physical reasons for gain compression of amplifier?
Post by loose-electron on May 24^th, 2012, 12:36pm

compression cause linearity problems and maximum signal amplitude issues.

I really think people have to back away from the math and go back to the basics of whats happening in the transistor.

Title: Re: Physical reasons for gain compression of amplifier?
Post by aaron_do on May 24^th, 2012, 5:51pm

Hi all,

I agree with loose-electron (i.e. what's important is what's really happening), but on the other hand its often useful to be able to put some numbers in. For instance, using what rfcooltools said, if you're at the 1dB compression point, you should be able to roughly calculate your third harmonic (I think it needs to be a well behaved system).

Vivek,

its a tradeoff (as it always is :D). Maximizing the signal swing is necessary if you want good effiency. Also, reducing the load impedance (using your method or any other) reduces the power gain of the amplifier. On-chip transformers are used extensively in practice, but whether they are efficient enough depends on the application. Also, the method you described is exactly a load matching technique. Whether you choose LC matching or transformer matching depends on a few things, but transformation ratio is one factor. In theory, LC matching is better for small transformation ratios while transformers are better for large transformation ratios. However, in practice, it is difficult to realize large transformation ratios using on-chip transformers.

Correct me if I've misunderstood your question.

regards,
Aaron