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Old 22nd July 2008, 01:09 AM   #11
sandyK is offline sandyK  Australia
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Default Vbe Lowest? In lownoise bipolar transistors

lineup

The dual PNP SSM2220 may be worth a look at too. May not be so easy to come by these days, though.

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Old 22nd July 2008, 02:36 AM   #12
lineup is offline lineup  Sweden
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SandyK.
I was wrong about LM394.
This super matched low noise pair is still in production.
Strange we do not see these excellent bjt in more hi-fi amplifiers input stages.

AndrewT.
You were absolutely right about ZETEX small signal transistors.
For example when running low voltage simulation,
ZTX689B has got only VBE = 0.505 Volt at 0.5 mA
... this is low base-emitter voltage!
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Old 22nd July 2008, 11:01 AM   #13
Mooly is online now Mooly  United Kingdom
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Is it for any particular application you need this or is it just a bit of fun/research ?
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Old 22nd July 2008, 11:24 AM   #14
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Quote:
Originally posted by lineup
SandyK.
I was wrong about LM394.
This super matched low noise pair is still in production.
Strange we do not see these excellent bjt in more hi-fi amplifiers input stages.

AndrewT.
You were absolutely right about ZETEX small signal transistors.
For example when running low voltage simulation,
ZTX689B has got only VBE = 0.505 Volt at 0.5 mA
... this is low base-emitter voltage!

Maybe your model is wrong, as the datasheet shows 0.59V at 0.5mA.
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Old 22nd July 2008, 11:52 AM   #15
Elvee is offline Elvee  Belgium
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If you want a really low Vbe from your LM394 or any dual transistor, you simply have to use half of the pair as a heather.
With a metal can, you can probably reach 200C reasonably safely, which will halve the Vbe.
As a bonus, this might confer to your design tube-like qualities, such as "warmth" in the sound (and inside the box too).
LV
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Old 22nd July 2008, 11:56 AM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Andy L. Francis
Maybe your model is wrong, as the datasheet shows 0.59V at 0.5mA.
my copy of the 689B datasheet (may'94) goes down to 1mA. Tc=25degC &1mA Vbe~=0.5Vbe
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Old 22nd July 2008, 01:49 PM   #17
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I would either go FET or use a DC-DC converter.
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Old 22nd July 2008, 05:14 PM   #18
Mooly is online now Mooly  United Kingdom
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Quote:
Originally posted by Elvee
If you want a really low Vbe from your LM394 or any dual transistor, you simply have to use half of the pair as a heather.
With a metal can, you can probably reach 200C reasonably safely, which will halve the Vbe.
As a bonus, this might confer to your design tube-like qualities, such as "warmth" in the sound (and inside the box too).
LV
On a single "AA" cell
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Old 22nd July 2008, 05:36 PM   #19
lineup is offline lineup  Sweden
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Quote:
Originally posted by Elvee
If you want a really low Vbe from your LM394 or any dual transistor, you simply have to use half of the pair as a heather.
With a metal can, you can probably reach 200C reasonably safely, which will halve the Vbe.
As a bonus, this might confer to your design tube-like qualities, such as "warmth" in the sound (and inside the box too).
LV
Elvee.
Interesting idea. I remember that one.
Now for BATTERY, one or two, 1.20-1.50 V / 2.40-3.00 V,
we must consider current consumption.
We want our batteries to last as long as possible.


Quote:
Originally posted by AndrewT
my copy of the 689B datasheet (may'94) goes down to 1mA. Tc=25degC &1mA Vbe~=0.5Vbe
AndrewT.
ZTX689B is the device I have put into my low voltage opamp.
It now has been simulated to work rather well at +- 0.500 Volt supply.
The VBE drop is like you say 0.490 - 0.505 Volt.

At least in the spice models I have.
Of course I run my transistors at low currents:
150 uA and to max 500-700uA for the output stage


It is a convential basic opamp =

1. Input differential NPN LTP, two ZTX689B, fead by CCS
---
2. Current mirror on top: Two ZTX718
---
3. PNP VAS=OUTPUT .. right now ZTX718 .. fead by CCS number 2.
Of course use Output from Collectors.
To achieve output like +- 400 mV peak at PSU 1.0 V Battery.
---
4. Both current sources uses ZTX689B with that low VBE drop
and low Saturation Voltage



Notice that if you use rechargable batteries, NiMH 1.2 Volts nominal,
they often have like 1.0 Volt , when empty and need to be charged again.

What is left for me, is to see if anyone of ZETEX NPN transistors
will work better than ZTX689B. ( Less THD % )
For PNP ZTX718 seems to be the best
FARNELL, UK, sells:
http://uk.farnell.com/9525602/semico...=zetex-ztx689b
http://uk.farnell.com/9526536/semico...u=zetex-ztx718
DATASHEETS: ZTX689B and ZTX718
http://www.farnell.com/datasheets/62411.pdf
http://www.farnell.com/datasheets/58736.pdf


Regards
Lineup .. Sweden July 2008
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Old 22nd July 2008, 06:40 PM   #20
Nordic is offline Nordic  South Africa
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If current consumption is your main worry, why not look at J201... you'll need more voltage I guess, but you can bias them pretty low...
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