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Old 25th October 2012, 07:45 PM   #1901
bcarso is offline bcarso  United States
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Quote:
Originally Posted by grhughes View Post
Been there! Done that! I'm interested in people who have DONE IT and interested in avoiding the pittfalls and interested in the circuit parameters of the SWOPA that I should be looking for; what optimum voltage, what optimum Idss of the BF862. They are probably all over the map out of a sample of 100. Thanks, Ray
I posted some statistics a while back in this thread, with the mean and standard deviation of Idss and Vgs for 100uA Id for a batch of BF862 from the same date code. They are fairly wide-distribution, compared to reports I've gotten privately for the Toshiba 2SK117. Note as well that NXP does not offer an Idss sort. None that I tested were out of spec, but one came close on the low end for Idss.
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Old 25th October 2012, 08:04 PM   #1902
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Thanks! Very helpful. Yes, I noted that. Some matching circuits I see on the web are all over the place complexity wise.
Digital JFET Test Rig
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Old 25th October 2012, 10:32 PM   #1903
fas42 is offline fas42  Australia
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Originally Posted by scott wurcer View Post
This is looking better, but there also still seems to be a kink at the transition. The problem for me was simulating the null of seconds for a simple phase splitter. It always walks down to the kink, where a real device has a sweet spot with a very deep null.

The level 3 model has an ALPHASP parameter that fits the entire curve smoothly with a tanh function which has a continuous derivative.

What did you end up using for lambda and B, and did B start doing something?

You might ask why something as fundamental as lambda is so far off in the extracted model.
Curve looks pretty smooth to me, here's a zoom in of that area:

BF862-gm06.jpg

Currently I'm using a Lambda=6, B=0.37. B has always done something, but depending upon the other parameters it can be very subtle. These are easily massaged in various directions without major hiccups, alter the curvature in the transition area.

Biggest problem is that Ids vs. Vgs curve is now almost linear, which is probably the reason for the published model being such. As mentioned before, you can get around this by adding a Vgs dependent voltage source at the gate; may not be correct procedure, but if the model flies ...

Frank
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Old 25th October 2012, 10:50 PM   #1904
fas42 is offline fas42  Australia
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Originally Posted by RNMarsh View Post
IMO curve fitting as used here is often a sub for an incomplete model. but often necessary and useful.
what model is being used and is it complete?
Exactly. Still need to add a voltage source at the gate, and to try some more tests; if anyone has other test rigs for JFETs it could be very handy ...

Frank
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Old 26th October 2012, 04:23 AM   #1905
RNMarsh is offline RNMarsh  United States
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If one needs to know what the transistor characteristics "really" are for the part you will be using in a circuit.... get a characteristic curve tracer.... used TEK (576). Or, you can get a new one from Chinese company which will display two devices at the same time on the crt screen (yes, crt). Good for matching. Cost new including shipping is $800. Or play with a couple PC based curve tracers developed by Elecktor or other here in a forum. measure the transistor and put all the values you measure plus some from data sheet, perhaps, into sim or just build with them from your measured information. -Thx RNM

Last edited by RNMarsh; 26th October 2012 at 04:28 AM. Reason: Transistor testing equipment
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Old 26th October 2012, 11:55 PM   #1906
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Default RB & rbb'

I'm looking at Noise performance for this & other circuits.

For good LN transistors, noise can be modelled simply with rbb'

From what I can see, RB = rbb' in SPICE

I can get a good estimate of rbb' from noise contours to replace the default RB = 10R which is far too optimistic for audio noise.

What are the other implications and are they small, eg VHF & UHF response, if I just change RB?

I note the default LTspice BC547s have superb RB=1R . Wish I knew where they bought their stuff.

But even Cordell's stuff is suspect. His npn 2n4401C has RB=13 while the pnp 2n4403C has RB=38 which is more believable and closer to Bcarso and my own Jurassic measurements. I've always found npn to have higher rbb'

I get rbb' from the high current 1kHz noise contours and that's been pretty good for me.

I know rbb' at 100MHz may be different but if I just change RB, to get closer to real life audio noise, will I be mucking up my ability to see 100MHz oscillation?
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Old 29th October 2012, 12:36 AM   #1907
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Default CMR ?

What is the CMR of the SW-OPA? If -100DB for thd is a good number to shoot for, I think same (-100) would be good for CMR.
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Old 29th October 2012, 02:31 AM   #1908
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Originally Posted by RNMarsh View Post
What is the CMR of the SW-OPA? If -100DB for thd is a good number to shoot for, I think same (-100) would be good for CMR.
Sim'ers --- use worse case conditions: Input source Z of 1K-10K and C/L gain as low as is stable design (X2-3?).

Some generalities about CMR that I am focusing in upon - THD is proportional to CM voltage squared for voltage changes --- at low C/L gains; and maybe this relates to PIM??? --> Phase Modulation from b-c or g-d capacitance being changed by CM signal. When and which topologies are affected most and how much. How much can PM be reduced by cascoding? References? Thx - RNM

Last edited by RNMarsh; 29th October 2012 at 02:36 AM.
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Old 29th October 2012, 04:36 AM   #1909
RNMarsh is offline RNMarsh  United States
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CM induced thd. Asym waveforms have a CM component. Need to have best CM performance for music appls. Thx-RNM
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Old 29th October 2012, 10:19 AM   #1910
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Any pcb made yet?
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