Vbe Lowest? In lownoise bipolar transistors - diyAudio
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Old 20th July 2008, 03:21 PM   #1
lineup is offline lineup  Sweden
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Smile Vbe Lowest? In lownoise bipolar transistors

.

I am working with designing a Very Low Voltage discrete op-amp.

It should be able to operate well at 1.20 - 1.65 VDC = One alkaline AA Battery
Click the image to open in full size.

I have already created/simulated one circuit that can work at 1.10 Volt ( +- 0.55 Volt )
I have discovered that the lownoise transistors I use, have different level of Vbe.
For example it appears 2SC2240/2SA970 has lower base-emitter voltage than BC550/560 and 2N5551/5401.

I want to know which audio transistors has got lowest VBE
Under low current working conditions and very low voltage.
Say 0.2-2.5 mA and Collector-Emitter voltages of 0.1 - 1.0 Volt.


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Old 20th July 2008, 06:19 PM   #2
Mooly is online now Mooly  United Kingdom
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Hi Lineup,
That's a tall order If it's any use I have circuit for a 3 volt moving coil head amp -- draws 1.5 ma. It's one of JLH's designs and uses a power transistor in common base mode for the input stage. Gives lower noise apparently due to the larger die size in this application.
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Old 20th July 2008, 07:17 PM   #3
lineup is offline lineup  Sweden
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3 Volt and 1.5 mA supply current.
Perfect for battery!

I would say making a discrete amplifier for 2 x 1.20-1.65 Volt (2 Batteries) is more attractive and useful.
This way we can use DC-output stage (without capacitor).

Of course, if my op-amp can work at 1.10 Volt (1 Battery)
it is no problem to make it work for 2.00 - 3.30 Volt.

When working at as low voltage as around 1 Volt,
with one input differential pair,
we are almost restricted to only use Inverting Amplification.
With non-inverted input tied to half supply voltage.
This Gives almost constant VCE operation and allows some input voltage span.

Is no coincidence so many Headphone Amplifier Chips, for lower voltages,
shows the typical appplication is INVERTED Amp.


There are a few very low voltage chips, for lower voltage than 2.7 V.
The many with voltage 2.7 is clearly the target value for use in 2 x 1.5 V Battery devices.
... like mp3 players etc.
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Old 21st July 2008, 07:34 AM   #4
Mooly is online now Mooly  United Kingdom
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Hi Lineup,
If you want copy send me an E-Mail. It's not an OpAmp configuration though, it's a a 3 transistor low input impedance design for use with a MC cartridge.
I think Maxim ? may have produced some OpAmps for very low supply voltages in the past.
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Old 21st July 2008, 09:00 AM   #5
AndrewT is offline AndrewT  Scotland
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try Zetex for low Vbe and low Vcesat
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Old 21st July 2008, 11:06 AM   #6
sandyK is offline sandyK  Australia
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Default Vbe Lowest? In lownoise bipolar transistors

lineup

Have a look at the LM194/LM394. I understand that they are actually multiple transistors in parallel in both halves. They have a lower than average VBE, and VCE sat. is quite low at 1mA collector current.

Collector to Emitter IC e 1 mA, IB e 10 uA 0.2 typical
Saturation Voltage IC e 1 mA, IB e 100 uA 0.1 typical

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Old 21st July 2008, 12:15 PM   #7
acid_k2 is offline acid_k2  Italy
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As I remember, old germanium transistors have low Vbe.
But they have narrow bandwidth, great instability and create a lot of noise... keep them away from audio circuits...
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Old 21st July 2008, 07:15 PM   #8
lineup is offline lineup  Sweden
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Default Re: Vbe Lowest? In lownoise bipolar transistors

Quote:
Originally posted by Mooly
Hi Lineup,
If you want copy send me an E-Mail. It's not an OpAmp configuration though, it's a a 3 transistor low input impedance design for use with a MC cartridge.
Thanks, I wait till you publish it, Mooly

Quote:
Originally posted by AndrewT
try Zetex for low Vbe and low Vcesat
Quote:
Originally posted by sandyK
lineup
Have a look at the LM194/LM394. I understand that they are actually multiple transistors in parallel in both halves. They have a lower than average VBE, and VCE sat. is quite low at 1mA collector current.
Collector to Emitter IC e 1 mA, IB e 10 uA 0.2 typical
Saturation Voltage IC e 1 mA, IB e 100 uA 0.1 typical
SandyK. Thanks for info.
But as I think, the LM194/LM394 are very hard to find transistors.
Maybe even obsolete ...
But they are no doubt high class. Super Matched Lownoise Pair .. as I recall.

AndrewT.
Thanks. I will try some of my ZETEX transistor spice models.
And see if they compare lower VBE.
What are the Lownoise or General small signal transistor devices from ZETEX?
TO-92 standard/higher gain.
Suitable for low level audio circuits?
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Old 21st July 2008, 07:34 PM   #9
lineup is offline lineup  Sweden
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I see in this
ZETEX Application note from 1995
they use ZTX650 npn and ZTX750 pnp

Low Noise Audio Input Stage
http://www.zetex.com/3.0/appnotes/design/dn11.pdf
Attached Images
File Type: png zetex_low-noise_input.png (7.8 KB, 385 views)
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Old 22nd July 2008, 02:02 AM   #10
gain is offline gain  United States
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Default Re: Vbe Lowest? In lownoise bipolar transistors

Quote:
Originally posted by lineup
.

I want to know which audio transistors has got lowest VBE

Lineup
short answer is it (mostly) depends on the batch(s) of trannys you have. measure with diode check on VOM. run 'em nice and very hot and you should get very low Vbe ... kidding !!!
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