JFet matching and measured distortion - Part 2: LTP

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...or the death of jfet-matching?

2nd part that looks at matching jfets and distortion performance in simple circuits (1st part was JFet matching and measured distortion - Part1 ).


It is my pleasure to bring you something more interesting, the differential pair (aka LTP).

And yes there's is benefit. Distortion is reduced by using matched jfet-pairs.

And no it's not much.

The details:

(See schematic below) A simple LTP consisting of 2x Toshiba 2SK369GR jfets (same as last time), no source degeneration, a simple tail current source using a ZTX450, setup for 1mA. Load resistors of 100 Ohm were chosen to keep Drain-Source voltage variation low as the used rails (0-13V) do not permit the use of a cascode.

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THD+N plot

For starters the THD+N plot.

Jfets A+B represent the worst case where 2 randomly chosen jfets have largely different IDSS; in this case THD+N is constantly higher by about 1.5% absolute.

Jfets B+C is so-so matched (to 0.3 mA) and is nearly indistinguishable from the best matched pair in the comparison, jfets C+D (0.068mA difference in IDSS). If you look closely, you see that this closest pair actually performs worse for larger input signals than the mediocre matched B+C pair (look at 180-200mVrms).

To summarize the plot, avoid largely unmatched jfets for LTP-use, but even mediocre matched pairs can work better than tightly matched ones. So to reliably get better performance by matching, one need to use a distortion analyzer for verification.

One does not get automatically better performance.

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Distortion spectra

As there's often the discussion of distortion spectra, I might contribute some data as well.

Here the overlay of the mediocre matched pair B+C (blue) and the tightly matched pair C+D (red). No notable differences. Small differences for 4th and 6th order; odd orders largely unchanged.


Finally the spectra of the worst case pair: large 2nd order distortion compenent,distortion cancellation no longer effective.

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Part 3 would be the complementary case, however I do not have an appropriate power supply in the moment. I can build one, but as I will be busy the next weeks this has to wait a bit.

Summarizing the results, distortion is easily held under control by using even only medicre matched pairs (within about 0.5mA, just like Borbely sells them). Closer matching is not guaranteed to give better results. Of course that only applies to distortion. How important that is to 'good sound' is to everybody's taste.

Of course discussion is welcome

Again a big thank you to Mr G, who made the fun possible.

Have fun, Hannes

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Ups, sorry spectra for the worst case were missing.

So here they are:

Finally the spectra of the worst case pair: large 2nd order distortion component, distortion cancellation no longer effective.

[PMA]

Your circuit in
http://www.diyaudio.com/forums/showt...41#post1761341
is a perfect example how NOT to design with JFETs.

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Of course not.

AC-coupling is no fun, higher rails, cascode would be nice etc. - however the comparison is still valid as all pairs are equally affected.

And the comparison was the whole point in doing this

Have fun, Hannes

[AndrewT]

Hi,
you have compared similar and dissimilar Idss.
What about transconductance comparison?

I have found it very easy and very quick to match Idss to any tolerance.

When comparing well matched pairs at any current different from Idss, the Idss matching turned out to be nearly useless. It simply grouped the devices into batches.

Getting a loose match of transconductance AND Idss was difficult.
Getting a close match of transconductance over a range of Id is very laborious. I don't believe it can be done by comparing graphs/testpoints taken with individually measured devices.
It needs a pair matching jig and careful attention to matching currents and dissipations and temperatures.

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What about transconductance comparison?

Sure, it's doable and of course it would also be interesting. There are lots of things one could have a look at.

I'm however especially curious about the effect of simple things, these that often give most benefit. More sophisticated methods start often the diminishing returns.

Anyway, the data does not suggest that curve matching would significantly improve distortion performance.

I think that one can do that to enjoy the symmetry, but otherwise there seems nothing to be gained.

Have fun, Hannes

[AndrewT]

using similar Idss devices is not comparing the performance of matched devices.