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Old 23rd November 2012, 04:16 PM   #2171
bcarso is offline bcarso  United States
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Quote:
Originally Posted by linuxguru View Post
Back off- or on-topic, depending on your inclination. The posted circuit uses a p-JFET, it's only simulation circuit and the JFET models may be unrealistic - but still, it sims with remarkably low THD20 of around -120 dbr at 4V amplitude into 600R. This is about the best I've ever managed with just 12 actives.
It would appear Idss for the output JFETs is a tad low for the desired output current swing? Or are you intentionally allowing for significant forward gate bias?
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Old 23rd November 2012, 04:54 PM   #2172
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Originally Posted by bcarso View Post
It would appear Idss for the output JFETs is a tad low for the desired output current swing?
J175 also seems to work fine, and the samples I've measured have Idss in the 12 to 24 mA range - I probably need to hand-select those with Idss > 20 mA or so.
I wonder if there's any other high-Idss pair that will work here. J310 comes to mind, but its Vds is a bit low.
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Old 23rd November 2012, 05:09 PM   #2173
bcarso is offline bcarso  United States
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Originally Posted by linuxguru View Post
J175 also seems to work fine, and the samples I've measured have Idss in the 12 to 24 mA range - I probably need to hand-select those with Idss > 20 mA or so.
I wonder if there's any other high-Idss pair that will work here. J310 comes to mind, but its Vds is a bit low.
In the old process 51 N channel family, the 2N4391, bracketed for high Idss, has the advantage of a metal case and a power rating of an astonishing 1.8W iirc when that case is held at 25C. The somewhat similar P part 2N5114 can be similarly heat-sunk when the gate is tied to the case.

It appears both parts are being made by Linear Integrated Systems, who however only gives an ambient rating of 500mW for the P part and evidently does not tie the gate to the case like the 4391. Older 5114s definitely had the gate connected to the case.
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Old 24th November 2012, 12:16 AM   #2174
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Originally Posted by bcarso View Post
In the old process 51 N channel family, the 2N4391... . The 2N5114 can be similarly heat-sunk...
Thanks - however, these TO18 JFETs seem to be moderately expensive and/or available only on EBay.

An alternate approach is to use a JFET-BJT Douglas Self Type-II follower at the output, at the cost of two additional BJTs, which can however be high-current, like the bc639/bc640 pair. This works in the sim, giving up only about 5 dB in THD20, which can perhaps be regained by carefully optimizing the output stage bias currents and/or using a premium Japanese pair like the 2sb649/d669. This would be ideal for the professional 6-pin API2520 footprint and /- 24V rails.

A random thought - the BJT output pair can be on a base PCB with the API2520 pinout, into which the rest of the circuit with the DIP8 pinout can be plugged, to create a professional, high-current version.

The more compact DIP8 portion can be used independently as a low-rail, lower current version for line-level upgrades, etc.

Edit: This may require a few jumpers on the DIP8 portion to reconfigure some of the pins, depending on whether it is to be used independently or plugged into the API2520 base with the high-current output pair.

Last edited by linuxguru; 24th November 2012 at 12:29 AM. Reason: addendum
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Old 24th November 2012, 12:28 AM   #2175
dimitri is offline dimitri  United States
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what about DMG102?
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Old 24th November 2012, 01:11 AM   #2176
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Quote:
Originally Posted by linuxguru View Post
Back off- or on-topic, depending on your inclination. The posted circuit uses a p-JFET, it's only simulation circuit and the JFET models may be unrealistic - but still, it sims with remarkably low THD20 of around -120 dbr at 4V amplitude into 600R. This is about the best I've ever managed with just 12 actives.
linux, which SPICE do you use? Obviously not LTspice.

Can you post your SPICE model?
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Old 24th November 2012, 01:41 AM   #2177
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Originally Posted by kgrlee View Post
linux, which SPICE do you use? Obviously not LTspice.

Can you post your SPICE model?
Actually, I'm using LTSpice III Version 2.21r. It's 5 years old, but works for me under Wine/Linux, and I don't want to break things silently by upgrading.

I posted the models I used earlier - just paste them into standard.jft and standard.bjt (renaming some models if necessary) and you should be good to go.

Try J111/J175 with 100R source degeneration each for the output stage - it seems to work well.

If you don't like the 2sk246 models for the J1/J3 cascodes, try the default LTSpice 2n4393 model, which gives results which are within a few dB.

If the 2sk170bl model looks too optimistic, please post a more realistic model or suggest alternative FETs. I'd prefer to use the 2sk170 in the real prototypes since I can get them in both BL and V grades for 40c unmatched and 80c matched off EBay.

Last edited by linuxguru; 24th November 2012 at 02:03 AM.
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Old 24th November 2012, 08:00 AM   #2178
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linux, what current are you running in the output stages?
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Old 24th November 2012, 11:23 AM   #2179
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For the output - J74 will parrallel - match IDSS within 10% of the K170s and each P or K set also select IDss for 250 mw dissipation max per device - use 2 or 3 or 4 devices in parallel. Yes this only works if you have a nice box of hundreds of P and K jfets to make up matched triplets or quads if you squirreled away enough 74/170's for future projects before they they were discontinued .......(Maybe Santa Nelson will donate the beast with a 1000 jfets for parts if you ask nicely - while we are at it lets ask for a pair of the Xs 300 - if he says yes to the first, the second is easy) if only we weren't such greedy little boys..............

Last edited by ticknpop; 24th November 2012 at 11:43 AM.
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Old 24th November 2012, 05:41 PM   #2180
RNMarsh is offline RNMarsh  United States
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The rules have been very loose but not THAT loose so as to accept designs with unobtainium transistors. No obsolete parts, rare and unusually hard to get parts nor super expensive parts, pls. Its a design for the masses... a universal opamp.
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