[AMPS] IMD Question

[Steve Thompson]

-----Original Message-----
From: Peter Chadwick <Peter_Chadwick@mitel.com>
>Has anyone checked the validity of the equations given in Pappenfus, Bruene
>and Schoenike (Single Sideband principles and Circuits, McGraw Hill, 1964)
>for deriving linearity from from the input/output curve of an amplifier?
>
>For 3rd order, it runs as follows:
>
>If the output voltage at full input voltage is V2, and the output voltage
at
>half that input voltage is V1, then the signal to distortion ratio is
>
>
>                            20Log [(8V1 - V2)/(2V1 - V2)]dB
>
>
>If you substitute in the values for 1dB compression (i.e. V2 is 5dB greater
>than V1) then I get the s/d to be 29dB. However, it is generally considered
>that the 1 db gain compression is between 7 and 15dB (usually about 10dB)
>below the third order intercept point, although the above would suggest it
>is actually nearer 15 than 10.
>
>So has anyone done any measurements to confirm or otherwise? Yes, I know I
>could do it myself, but only on 'hollow state'.......I'd be interested to
>know if solid state amps give different answers, and I don't have a solid
>state HF power amp.


I've never tried to arrive at the formula mathematically, but I did do some
empirical tests a long time ago.

In my experience, the formula holds at 0.5-2dB gain compression, but only if
the amplifier if truely linear up to when it starts to compress. If the
formula is mathematically derived, it has to assume linearity up to V/2,
then look at compression at full output. It can't differentiaite between
hard and soft compression either (although the effect in practice wasn't
that great in my tests).

It is constructive to draw a graph of gain vs output power - ideal amps have
a constant line until you approach full output, when saturation starts and
the gain falls. The formula seems to work well for such amps. Unfortunately,
very few seem to be like this. Many power mosfets have gain which drops
continuously with power (Po/Pin is banana shaped) latest LDMOS has an 'S'
shaped Po/Pi characteristic at most bias levels. Most bipolar amps are
dreadful because the particular transistor isn't suited to linear operation
and/or the biassing arrangements are rubbish.

Tubes aren't exempt - poor bias and/or anode voltage regulation, or wrong
loading can mess things up a treat.

Where amplifiers show low IP3 compared with 1dB compression, there's often
something to be seen in the gain/power graph, or there's more than one
factor contributing to the compression (e.g. multiple gain stages
compressing at the same time, or device saturation and bias failure both
occuring together).

>There is a very practical reason for this question......it's only likely to
>concern G's who buy (as opposed to build) amps in the future, though.


So what new nonsense is coming now?

Steve



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