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Old 4th October 2012, 03:51 AM   #1511
fas42 is offline fas42  Australia
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Had an idle moment, so here's another toy to play with for LTspice people ...

Frank
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Old 4th October 2012, 06:20 AM   #1512
dadod is offline dadod  Croatia
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Originally Posted by scott wurcer View Post
Here's a first shot at a FET input JE990 for 600 Ohm type loads. This was easier than I thought, almost the same values worked with minor changes and the performance was virtually identicle to the original. Most importantly the noise is just slightly higher, but still well below 2nV.

Notes

Input FET's are BF862, 2SK170, LSK389, etc. cascoded by J111 or J305 depending on voltage range. Personally I think it is easy to select for +-18V or +-24V from actual FET's but +-15V will always work. Even a 100k input R is OK now.

I used 2SA970 and 2SC2240 in all locations, later I will try to do a high current version.

C1 is the comp cap, this can be reduced to suit the closed loop gain, a big advantage in having access to the whole circuit.

R10 (5k pot) selects the standing current in the output stage, I find this essential because trying to match diodes to transistor Vbe's is silly.

The first thing I would change is to use a current source to generate bias and provide for an offst trim in the current mirror.

I think this would do well up against an OPA627/37.

EDIT - No inductors needed.
I suggest to use deplation mode mosfets from Supertex for J3, J4, DN2540 could be good for it, more voltage for input fets.
dado
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Old 4th October 2012, 10:41 AM   #1513
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Originally Posted by dadod View Post
I suggest to use deplation mode mosfets from Supertex for J3, J4, DN2540 could be good for it, more voltage for input fets
dadod, I downloaded your LTSpice script but it won't run. Keeps asking for supertex.txt

Can you post a copy or give a link to this please.
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Old 4th October 2012, 11:04 AM   #1514
dadod is offline dadod  Croatia
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Originally Posted by kgrlee View Post
dadod, I downloaded your LTSpice script but it won't run. Keeps asking for supertex.txt

Can you post a copy or give a link to this please.
Just remove the line .include supertex.txt, you don't need it as in the simulation no supertex fet was used.
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Old 4th October 2012, 11:07 AM   #1515
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dadod, i think it is J1, J3 where you would put the depletion mosfets.
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Old 4th October 2012, 11:27 AM   #1516
dadod is offline dadod  Croatia
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Originally Posted by Joachim Gerhard View Post
dadod, i think it is J1, J3 where you would put the depletion mosfets.
Depends what schematic you are looking at, Wurcer (J3, J4) or fas42 (J1, J3), of course upper cascode fets.
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Old 4th October 2012, 11:40 AM   #1517
dadod is offline dadod  Croatia
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Hello Scott, just one question.
Did you put the Q3(in your schematic), connected as a diode, intentionally to get higher secon harmonic distortion(as benign one)? My simulation shows that distortion was more than halved if that diode was not connected parallel to the compensation RC, and at that only second harmonic.
Damir
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Old 4th October 2012, 02:30 PM   #1518
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Quote:
Originally Posted by dadod View Post
Hello Scott, just one question.
Did you put the Q3(in your schematic), connected as a diode, intentionally to get higher secon harmonic distortion(as benign one)? My simulation shows that distortion was more than halved if that diode was not connected parallel to the compensation RC, and at that only second harmonic.
Damir
JE990 had that as a clamp? just Muntz it.

As before the diode connected transistors could just be diodes, just my habit because we usually don't have diodes per se in an IC. A very low C discrete diode here probably would not have the problem as much (in case the clamp is really needed). The worst latch-up tests are usually power on with signal already applied, this always violates the data sheet since with power off and any input signal you are exceeding the supply on the input.

Also I left the "fiddly" compensation components as is, with different devices you may want to revisit them.
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Last edited by scott wurcer; 4th October 2012 at 02:40 PM.
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Old 4th October 2012, 03:50 PM   #1519
bcarso is offline bcarso  United States
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Originally Posted by dadod View Post
I suggest to use deplation mode mosfets from Supertex for J3, J4, DN2540 could be good for it, more voltage for input fets.
dado
Or possibly DN1509, a 90V part with lower capacitances, about the same threshold voltages. It seems somewhat less characterized than the older parts as the datasheet lacks curves.
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Old 4th October 2012, 05:13 PM   #1520
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Originally Posted by bcarso View Post
Or possibly DN1509, a 90V part with lower capacitances, about the same threshold voltages. It seems somewhat less characterized than the older parts as the datasheet lacks curves.

How about CPC3730 from C P Clare? With that and some 140V transistors this could be the front end of a 100W amp.
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Last edited by scott wurcer; 4th October 2012 at 05:29 PM.
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