Need low power high beta transistors - please drop me a few part numbers... - diyAudio
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Old 26th October 2006, 09:09 AM   #1
peufeu is offline peufeu  France
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Default Need low power high beta transistors - please drop me a few part numbers...

This is for a DAC I/V, using transistors and cascodes in the jocko style. Since it is open loop the main source of error is transistor base current.

I have spent a few hours hunting transistors and have so far found the perfect one, except noone sells it.

So I'd be very grateful if you could share your finds. Here are the prerequisites :

- Vce max : 30V, maybe 20, but I don't need a high voltage part unless it has other advantages.
- Maximum current 100 mA
- Power dissipation 0.2 W (will fry SOT323 chips)
- PNP and NPN, complimentary part would be nice

Now before someone says "BC547C" :

- high hFe (>2-300)
- constant hFe between 5 and 30 mA with the curves to prove it
- LOW CAPACITANCE

See these graphs :

- BC547C gives up long before 10 mA
- BC317/337 would be good but it oscillates in Spice
- ZTX851 starts its linear region AFTER 10 mA
- MMBT489 is perfect but too low power dissipation and high capacitance

Thanks to anyone who can help !

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Old 26th October 2006, 09:39 AM   #2
EC8010 is offline EC8010  United Kingdom
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BC557C. Easily obtainable. I just picked one at random and obtained this - hfe = 400 @ 10mA.
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File Type: gif bc557c 10ma.gif (13.2 KB, 592 views)
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Old 26th October 2006, 09:57 AM   #3
peufeu is offline peufeu  France
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That was a quick reply, but unless the datasheet curve is wrong, this transistor isn't suited for the application... look more closely at the hFe versus Ic curve that I posted ; it has a knee at about 10 mA.

Using BCP56 or BC317-40 which has a flatter curve yields about 4x linearity improvement, but 317 oscillates in the simulator. BCP56 might be suitable but it has large capacitance.

Can your gear trace hFe vs Ic (this was Ic vs Vce for various Ib, right ?)
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Old 26th October 2006, 10:03 AM   #4
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Default 2sa970 and 2sc2240

You could consider jocko's favs.... (If I remember correctly)
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Old 26th October 2006, 10:06 AM   #5
Elvee is offline Elvee  Belgium
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Hello Peufeu

Have you looked for superbeta transistors?
Some might be suitable; maybe it will be difficult to find one with your voltage requirements.
You could also opt for a composite: it is easier to achieve good performance in every respect than with a single device: see the discussion here:
http://www.diyaudio.com/forums/showt...15#post1023315
LV
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Old 26th October 2006, 10:35 AM   #6
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by peufeu
That was a quick reply, but unless the datasheet curve is wrong, this transistor isn't suited for the application... look more closely at the hFe versus Ic curve that I posted ; it has a knee at about 10 mA.

Can your gear trace hFe vs Ic (this was Ic vs Vce for various Ib, right ?)
No, I can't plot hfe against Ic, but I can measure hfe at spot currents. For the transistor I measured at Vce = 2.76V, hfe = 400 @ 10mA, falling to 363 @ 18.2mA. If you increase Vce, the fall becomes less noticeable due to Early effect, so when I increased Vce to 6V, hfe = 432 @ 13mA and 442 at 11mA. Yes, I saw the claimed fall in hfe in the curves you posted. Me, I believe what my own measurements say under my own conditions.
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Old 26th October 2006, 10:54 AM   #7
peufeu is offline peufeu  France
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Check this :
http://audio.peufeu.com/node/57

Superbeta. Interesting. Do you have references of ones you like ?

I'm simulating BF861C JFET but it's academic only as noone sells it. I'm gonna try obtainable JFETs first
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Old 26th October 2006, 11:12 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
here's some suggestions but you'll still need to check they meet your criteria.

2sb716/d756
2sa1085/c2547
2sa872/c1775
2sa1350
2sa1815/c4432
2sa1845
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Old 26th October 2006, 11:34 AM   #9
Eva is offline Eva  Spain
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If some transistor produces oscillation in the simulator, don't discard it, blame the model instead.

Also, simulation becomes completely inaccurate when trying to predict exact gains and capacitances of bipolar transistors. Models are not that good.

And last but not least important, given the same part number, gain magnitude and linearity and capacitances change from one manufacturer to another and from batch to batch, so a curve tracer is strongly recommended instead of simulations.
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Old 26th October 2006, 11:42 AM   #10
peufeu is offline peufeu  France
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I looked at the datasheets and 2SA1845 might have potential.

Eva : I know. Some models are quite suspicious, especially BC317/327/337. I will test them in circuit when I make the test boards and we'll see.

So far JFET simulations fail to bring improved linearity ; it's difficult to find the right PJFET, too.
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