TLDR - a lot about setting the bias of a MOSFET amplifier by examining the 3rd order intermodulation distortion products.? If not interested move along quietly :)
The prior posts left me many questions that were difficult for me to answer.? I dont have an electrical engineering background so much of this was opaque to me, betginning with the definition of a class AB amplifier.? It sounds simple - class A conducts for all 360 degrees of the waveform and class B conducts for exactly 180 degrees of the waveform.? Class AB is in between - but where in between?? ?That means anwhere from conducting nearly class B to nearly class A 181 degrees to 359 degrees.? ?Using an osicilloscope to view the input and output waveforms it looks like this:
The blue trace is the input to the RD006 and the yellow trace is the output.? And if you increase the bias you can see the shape of the output waveform move closer and closer to class A.? Anywhere in between would be considered class AB.? ?But this view tells you nothing about IMD.? To do that you need to measure IMD using a spectrum analyzer.? And the gurus tell you to "minimize IMD"? but that was confusig to me as well because there are two variables you control, bias and drive.? At any particular bias as drive increases the third order intermodulation products increase by 3 dB.? So then question is "at what drive level should you measure IMD?"
I asked the question of multiple folks I know including some very experienced EEs and none could give me a satisfying answer.? The EMRFD bible didn't provide the answer either nor did any of the various QST and QEX articles I poured over.? ?I finally found a good reference, a 1993 ?NXP / Motorola Engineering Bulletin that explains how to properly measurer IMD -
The technique I used is Method 2 on page 3 of the engineering bullentin.? The steps are essentially these:
1. Chosose an initial bias using parameters from the data sheet
2. Vary the drive to find the 1dB compression point of the amplifier
3. For the two tone test set the drive level 6db below the 1dB compression point
4. I used the TinySA Ultra to measure the 3rd order intermodulation distortion product
5. Vary the bias until the 3rd order products are 25-30dB below the fundamentals
That turned out to be at around 250mA.? ?You can see what it loooks like on the TinySA Ultra in post above.? For those looking closely - just look at the delta between the fundamental signals and the distortion products and ignore the OIP3 number - I did not tell the TinySA how much attenuation I was using so the calculation is off.
That still begs the question of my earlier post - why do the examples on the RD06 data sheet show a bias of 500mA and claim 10 watts out at 30MHz?? The best answer I have is "marketing" - the test circuit on the data sheet is not broadband - it is a tuned circuit for exactly 30MHz using microstrip and very specific components to acheive the claimed output.? A broadband amplifier using toroidal transformers on a Manhattan board will never see that kind of performance.
I hope this is helpful to some as a reference.? And if I have made any incorrect statements I hope someone more knowledgeable than I can correct and amend as necessary.
73,
Dean
KK4DAS
