Discrete Opamp Open Design

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#3,222
john curl said:
Remember guys that the voltage noise of a bipolar transistor is equal 1/2Gm+Rbb'. At 1ma this is 13+Rbb'; at 10ma is 1.3+Rbb' so usually we don't operate that high in current, except for very low Rbb' devices.
Of course, at 100uA the effective noise RESISTANCE is 130+Rbb', so now BETA becomes more important, because we usually have to figure in an I(noise) contribution that is proportional to beta with source resistances over 50 ohms or so. This is where the beta droop of the 2N2219 could really effect things, and another, higher beta (at 100ua) device might be better, even if the Rbb' is somewhat higher.
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Thanks John - that really gives good insight!

Jan
 
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#3,225
The 2N2222 has been mentioned as an early linear transistor still being used. I found in my garage some NOS 2N2222A and tested for noise:


10Hz 100Hz 1KHz 10KHz 100KHz

4.5nV 3.3nV .9nV 1nV .9nV

3mA/15v Beta = 133 @ 5mA

I'll compare with some new 2N2222A on order.

THx-RNMarsh
 
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#3,227
grhughes said:
The 2n2484/pn2484 was also listed as a low noise transistor. Jensen used this in the JE990 as THE CCSink transistor. GRH
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I have that one... I already tested it also but it isnt as quiet as many others. If you need the numbers for SIM models, I can give it to you but it isnt a good low noise transistor at 1-3mA. It may have other redeeming qualities.


THx-RNMarsh
 
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#3,231
See if you guys can guess where this dac's output stage/buffer is from. With these transistor it could drive horns with a couple of mods.


Cheers George
 

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#3,235
I guess adding a DC servo or offset trim was not stable enough. During warmup there will be a lot of DC shift with tubes. Anyone watching a Tek 547 warming up would be familiar with the problems. The NE5534 I/V converter is the real limitation. I presume its an older design.
 
#3,236
ammel68 said:
Not sure where it's from, but I'm not crazy about the 10uF caps on the outputs of the buffers.
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davada said:
Probably Luxman.
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It's the MF trivista dac,

Yes I think the caps can be eliminated by carefull adjustment of the dc offset, but it still had a small switch on bump.
I've shown how to do this on the same/ different model which has the same output stage, the MF A3-24 a few years back in another thread, with circuit diagram. But then this one has tube input so it may not be successful

Cheers George
 
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#3,237
Mr. Marsh,
If you have a 2N4250 in you stock, please show noise results. The reason I ask is because years ago I was looking for that transistor and found it in the Central Semiconductor catalog. Central Semiconductor told me the 2N4250 part number was obsolete and to use 2N5087 instead. I'm interested in a comparison.
 
#3,239
john curl said:
Now Richard, try the 2N2222 at 100uA and a 10K source. Then try a 2N5087, or 2N5089 and compare.
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2N5087 is PNP, the original complement was the 2N5089 but the 2N5210 became a better choice. So those would be the righ ones to compare to a 2N2222

The issue with most of the transistors under discussion was that I believe they are straight NPN or PNP construction so the beta vs current curve is not very flat. Of course when used with a current source that is not much of an issue.

Later transistor designs are more complex to prevent base spreading among other improvements.

I still keep all of these in stock and use them.

Richard is almost convincing me to get a Quantech, although I probably will try a few other techniques first. Maybe even build a modern DIY version. Of course I am overloaded with work for now.
 
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