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Old 1st February 2010, 10:03 PM   #1
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Default matching jfets

I am designing a no feedback MC amplifier using low noise JFETs (2SJ74BL/2SK170BL). Simulations indicate that the circuits yield very low distortion (< 110 dB) if each device of a given polarity is Idss matched to within ~200 uA. My question is: about how many devices of each polarity would I need to purchase to get matched quads of each polarity.
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Old 1st February 2010, 10:27 PM   #2
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Default Is Idss the best FoM for matching JFETs

It occurred to me when simulating the Ids vs. Vgs curves that device matching involves more than just Idss. Vto is also involved with defining the curve. However, if one attempts to match both parameters, then the probability of getting matched devices drops quadratically, and matching may be unfeasible. Instead of matching Idss it would also be possible to match Vto. Does anyone have experience which parameter is the best for yielding minimum distortion is a zero feedback, complementary differential topology?
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Old 1st February 2010, 10:54 PM   #3
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Sure... you're not likely to get matches of IDSS at 200ua unless you buy a huge batch of jfets, so put that idea out of your head?

If you want to match dynamic curves andidss you've got a big problem since the curves are not likely to be identical between Pch and Nch devices.

Which is why simulations and real world circuits are frequently somewhat different.

But having said all that, there are a number of nice MC front ends (head amps) using jfets.

I'd suggest you look into John Curl designs and Erno Borbely designs to start with... there are a number of other worthy ones as well. Curl is a frequent contributor in this forum.
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Old 1st February 2010, 11:07 PM   #4
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A simple trim can be added to almost any circuit to yield the same results.
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Old 2nd February 2010, 12:06 AM   #5
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The next best thing to 2SK389 / 2SJ109 ?

The level of match that you want, in quads, and if you also want to match Yfs on top, I would advise that you buy at least 200 pieces of each type.

Even stock 2SK389s / 2SJ109s are not perfect matches. You would need at least 50 pieces of each to find 2 pairs with closely matched Idss and Yfs amongst all 4 devices.

I am not commenting though whether your application really requires that level of match. I presume you know better than I do.


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Old 2nd February 2010, 03:08 AM   #6
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Default Matching JFETs

The matching problem is not as bad is it may appear. For the topology I am considering (See my earlier posting on no feedback MC amplifier) the outputs of N and P channel stages are AC coupled together. This has several advantages. The first is that it effectively reduces noise since the signal of interest appears equally on both outputs, but the noise from the two outputs is uncorrelated. The second advantage is that the need for matching N and P devices is reduced. It is still necessary, however, to match devices of a given polarity.

A 200 uA Idss match is required to hold distortion below 110 dB for an input of 2 mVRMS, with a gain of 15. I chose the 2 mV number based on the nominal output of a typical MC cartridge (200 uVRMS) and then adding 20 dB of headroom. It is still possible to maintain distortion below 95 dB if the Idss mismatch is raised to 500 uA.

My comment on matched quads assumes that two devices are paralleled to further reduce noise. Simulations yield an noise level of 0.33 nV/sqrt (Hz), although measurements will still be the final word.
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Old 2nd February 2010, 06:05 AM   #7
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Here are some Idss figures for a batch of 12 2SK389 in the blue grade (5 - 10mA IDss IIRC). The three figures are the IDss for each half and the difference between them.

6.52 6.59 0.07
6.53 6.77 0.24
6.71 6.77 0.06
7.34 7.34 0
7.35 7.53 0.18
7.62 7.63 0.01
7.78 7.83 0.05
7.96 8.08 0.12
8.35 8.42 0.07
8.67 8.85 0.18
9.05 9.17 0.12
9.05 9.51 0.46


You can see that 10 of the 12 devices were matched within 200 uA.

Within these ten devices, the first six form three pairs where the average is matched within 200 uA, but in two cases the difference between the lowest and highest "half" is greater than 200 uA.

Assuming the same spread in 2SJ109s I think if you bought 12 of each you'd get fairly close to what you want.

If you go with 170s / 74s I think 20 of each will get you close.

Last edited by Mark Kelly; 2nd February 2010 at 06:09 AM.
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Old 2nd February 2010, 10:31 PM   #8
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Default Matching JFETS

A quick look at a JFET SPICE model shows that Ids depends on three device parameters: beta, lambda, and Vto according to the following equation:

Ids =beta(1 + lambda - Vds) - (Vgs - Vto)**2

To truely match devices would require that all 3 parameters be measured. I tried adjusting beta and Vto to yield the same Idss, but the results were not very good. I think that at least Vto and beta need to be matched. Vto is easy to measure, but beta is a bit more difficult. If lambda is small enough the above equation will simplify. I'll run some more sims to see if ignoring lambda is feasible.
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Old 3rd February 2010, 12:04 AM   #9
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Default Error in the Last Equation

The last equation was in error, since it failed to take into account the Rs term. Solving for Ids requires solving a quadratic equation in Ids. The expression can be simplified with the following substitutions:

A = vgs vto
B = 1 + lambda * vds
C = (2A/rs + 1/(B * beta * rs**2)

Then Ids can be expressed as follows:
Ids = (C sqrt(C**2 (4*A**2/(rs**2))))/2

When I plot the simulated Ids vs the simulated Ids the two overlay nearly perfectly. The next step is to determine which of the above parameters: beta, lambda, vto, or rs need to be matched. It seems that lambda is fairly small and can be ignored. That leaves only three parameters.
Attached Images
File Type: jpg Ids.jpg (40.7 KB, 553 views)
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Old 3rd February 2010, 12:44 AM   #10
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Quote:
Originally Posted by analog_guy View Post
The first is that it effectively reduces noise since the signal of interest appears equally on both outputs, but the noise from the two outputs is uncorrelated.
Nope. That's precisely why matching JFETs will do absolutely nothing for the noise.
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