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Old 15th August 2014, 04:55 PM   #3111
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I did not recognize those parts, so I would default to the 2N4401/3 pair for general stuff, and the PE8050-8550 for MC low noise.
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Old 15th August 2014, 06:43 PM   #3112
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Thanks John, but what about a second stage complementary push pull cascode with + and - 90v rails in a power amp. You need good high voltage transistors and not many have been made since RCA/Motorola made the 2n3439 and the 2n5415. I know there must be some more modern transistors than mid 60s devices. Haven't they improved the process with less impurities? Ray
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Old 15th August 2014, 06:48 PM   #3113
jcx is offline jcx  United States
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most any current production parts that you buy today under old part numbers have gone through several redesigns for newer processes, die shrinks enabled by better thermal bonding/materials for the same power rating
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Old 15th August 2014, 07:20 PM   #3114
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The 2N3439/5415 are very, very slow in modern terms and Hfe is weakly defined. Today we have the 2SC4793/A1837 which have already been popular for many years. The Sanyos available through OnSemi may be even nicer. The MJE340/350 would also work, though they aren't as fast.

Is there something that makes these transistors not very good? It seems to me there are plenty of possibilities.
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Old 16th August 2014, 12:03 AM   #3115
RNMarsh is offline RNMarsh  United States
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Default Old one's are still good to use.

Oh.

Uh the current, IIRC was 1mA. Using the Quan-Tech as shown in action. Settings are on the meters and switches. Click on the picture and then magnify it. [No series R to the base.]

And, the 2N2219 is Not the other one I tested... it was a 2N2219A. The A is better and with new processing equipment of today, it is better than ever.

The 2N2219A and the 2N2905A are both still in the running for some uses. They are very linear on a curve tracer and in use.


THx-RNMarsh

Last edited by RNMarsh; 16th August 2014 at 12:05 AM.
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Old 16th August 2014, 06:57 AM   #3116
RNMarsh is offline RNMarsh  United States
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Note for only the 10Hz it is on the X3 multiplier.... a reading of 1nv = 3nV.

I doubled checked the 'low noise' transistor [BC560C] measures 1.6nV at and above 1Khz. At 100Hz = 2.5nV and at 10Hz is 3 nV

Seems like a good number. I'll go test some other low noise transistors.

THx-RNMarsh

Last edited by RNMarsh; 16th August 2014 at 07:00 AM.
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Old 16th August 2014, 07:22 AM   #3117
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What would it take to replicate these measurements in simulation so that BJT models can be improved?
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Old 17th August 2014, 10:06 AM   #3118
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Quote:
Originally Posted by john curl View Post
I did not recognize those parts, so I would default to the 2N4401/3 pair for general stuff, and the PE8050-8550 for MC low noise.
JC, is your recommendation of PE8050-8550 for MC low noise based on experience?

2n4401/3 has known low rbb' and you can sorta see this from the datasheet though it is not spec'd as a LN device .. eg Vce at high Ib

But PE8050-8550 doesn't seem to show this type of behaviour.

If it is from experience, what do you estimate rbb' to be on PE8050-8550?

Maybe I'm looking at the wrong maker's datasheet.

Last edited by kgrlee; 17th August 2014 at 10:14 AM.
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Old 17th August 2014, 10:37 AM   #3119
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Quote:
Originally Posted by RNMarsh View Post
I doubled checked the 'low noise' transistor [BC560C] measures 1.6nV at and above 1Khz. At 100Hz = 2.5nV and at 10Hz is 3 nV
Mr. Marsh, are these 1mA measurements?

As JC points out in #3100, a 'perfect' transistor at 1mA has Rnv = 13R and will have voltage noise of 0.47nV/rtHz

By looking at the real voltage noise at various currents (ie the Noise Contours) you can get a very good estimate of rbb' .. at least for predicting noise. This is a good model for noise at audio frequencies if there isn't much 1/f LF noise. ie letting rbb' represent how close the BJT is to 'perfection'

Quote:
Originally Posted by keantoken
What would it take to replicate these measurements in simulation so that BJT models can be improved?
The first step is to use the 'correct' value for rbb'.

This is RB in LTspice. I use the following to match datasheet noise plots and my Jurassic measurements.

2n4403 38
2n4401 40

2sc2240 39
2sa970 25

bc550c 167
bc560c 170

If RB is unspecified, RB defaults to 10 which is rather good. Pity you can't buy the default LTspice transistors from Mouser or Digikey

If there isn't much 1/f rise in noise, (which should be the case for a 'Low Noise' device) you are done.

You can also estimate rbb' by looking at Vbe at high Ib but doing it from noise measurements is more accurate for noise estimation .. surprise surprise.

A caveat is that rbb' for HF response is often different from that for audio noise.

I dunno how to model 1/f noise in SPICE
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Old 17th August 2014, 10:49 AM   #3120
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Thanks for those numbers. I wonder where the BC3x7 lie?

Noise isn't modeled in transient simulation in LTSpice. Only in noise analysis. You would have to manually add noise voltage sources which is possible, but difficult. However there are BJT model parameters which model the different noise sources that you would see in the noise simulation.

If there are H-parameter plots in the datasheet, that will allow you to model RF behavior including base resistance much more accurately. Transconductance at RF is one thing that usually can't be modeled accurately because even if datasheets show the Ft curve, they don't show you what happens to the transconductance. It falls with frequency, so you can make circuits stable in simulation that in real life are unstable, even impossible to stabilize.

THAT transistor models actually have two different versions, one for accurate noise simulation and another for accurate RF simulation. I don't know whether this is necessary or not, it could just be a limitation of the simulator the models were designed for, although the creators apparently payed a lot of attention to model quality.

Last edited by keantoken; 17th August 2014 at 10:54 AM.
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