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WTD: Small signal NPN, HFE >= 200 at Ic < 2uA...

G

Greg Neff

....surface mount, Vceo >= 30V. Low frequency.

That Ic is not a typo. I have to operate with an Ic in the range of
0.01 to 2 microamps. Not my idea, but I have been tasked to determine
the feasibility. Considering NXP PMBT6429 and Fairchild KST5089, but
my Ic is still way off the left side of the DC current gain curves.
Any recommendations?


================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]
 
J

John Larkin

...surface mount, Vceo >= 30V. Low frequency.

That Ic is not a typo. I have to operate with an Ic in the range of
0.01 to 2 microamps. Not my idea, but I have been tasked to determine
the feasibility. Considering NXP PMBT6429 and Fairchild KST5089, but
my Ic is still way off the left side of the DC current gain curves.
Any recommendations?


================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]
Click to expand...

BCX70K? Or a fet!

John
 
J

John Larkin

Hey, thanks. This very minute I was looking for a lower noise transistor
to drive a laser, has to be really low in noise below a few hundred
Hertz. The old 3904 doesn't really cut it but it looks like the BCX70
does. And it comes in SOT-23. Nice.
Click to expand...

The BCX70 is very well behaved. High beta, not too fast (tends to not
oscillate on its own), beta banded, low leakage, cheap.

John
 
J

Joerg

John said:
BCX70K? Or a fet!
Click to expand...

Hey, thanks. This very minute I was looking for a lower noise transistor
to drive a laser, has to be really low in noise below a few hundred
Hertz. The old 3904 doesn't really cut it but it looks like the BCX70
does. And it comes in SOT-23. Nice.
 
S

Spehro Pefhany

...surface mount, Vceo >= 30V. Low frequency.

That Ic is not a typo. I have to operate with an Ic in the range of
0.01 to 2 microamps. Not my idea, but I have been tasked to determine
the feasibility. Considering NXP PMBT6429 and Fairchild KST5089, but
my Ic is still way off the left side of the DC current gain curves.
Any recommendations?
Click to expand...

ON BC847C/848C?
 
M

[email protected]

I just wish there was more data on those kinds of gems. In my case I
need the low 1/f at around 10mA, they only give us data for 0.2mA.

Still mulling the BF862, a really nice JFET. en is only given for 100kHz
but sure looks nice, 0.8nv/rtHz (typ). Also, Digikey has about 40,000 of
these in stock and there has got to be a reason. Unfortunately not
pin-compatible with BJTs. Why did they...? Arrgh.
Click to expand...

BF862 is used in car radios. The IDSS is tighter than most JFETS. I
bought 50 on ebay from this vendor:
<http://cgi.ebay.com/BF862-SMT-J-FET-Junction-
Transistor-25mA-20V-50_W0QQitemZ170096508822QQihZ007QQcategoryZ4666QQssPageNameZWDVWQQrdZ1QQcmdZViewItem>
 
J

Joerg

John said:
The BCX70 is very well behaved. High beta, not too fast (tends to not
oscillate on its own), beta banded, low leakage, cheap.
Click to expand...

I just wish there was more data on those kinds of gems. In my case I
need the low 1/f at around 10mA, they only give us data for 0.2mA.

Still mulling the BF862, a really nice JFET. en is only given for 100kHz
but sure looks nice, 0.8nv/rtHz (typ). Also, Digikey has about 40,000 of
these in stock and there has got to be a reason. Unfortunately not
pin-compatible with BJTs. Why did they...? Arrgh.
 
W

[email protected]

Greg said:
...surface mount, Vceo >= 30V. Low frequency.

That Ic is not a typo. I have to operate with an Ic in the
range of 0.01 to 2 microamps. Not my idea, but I have been
tasked to determine the feasibility. Considering NXP PMBT6429
and Fairchild KST5089, but my Ic is still way off the left side
of the DC current gain curves.
Any recommendations?
Click to expand...

Not to worry, Greg. Many, if not most, small-signal transistors
will work very will in the 10 to 2000nA region. Their beta will
be somewhat lower, and their f_T will be substantially lower at
a given current (by sqrt(1/Ic), but most of the other parameters
will work fine. For some BJTs, this holds down to the pA region.
Vbe drops by 58mV/decade at room temp, but you already knew that.
 
E

Eeyore

Joerg said:
Hey, thanks. This very minute I was looking for a lower noise transistor
to drive a laser, has to be really low in noise below a few hundred
Hertz. The old 3904 doesn't really cut it but it looks like the BCX70
does. And it comes in SOT-23. Nice.
Click to expand...

You still use the 3904 ?

Aren't more modern parts like the BC549/559 somewhat better (notably higher beta
IIRC) yet the same price ?

SMT versions available as BC849/859.

Graham
 
E

Eeyore

Greg said:
...surface mount, Vceo >= 30V. Low frequency.

That Ic is not a typo. I have to operate with an Ic in the range of
0.01 to 2 microamps. Not my idea, but I have been tasked to determine
the feasibility. Considering NXP PMBT6429 and Fairchild KST5089, but
my Ic is still way off the left side of the DC current gain curves.
Any recommendations?
Click to expand...

BC849C ?
http://www.nxp.com/pip/BC849B.html

hFE typ 450 @ Ic = 10uA.

Graham
 
M

[email protected]

Not to worry, Greg. Many, if not most, small-signal transistors
will work very will in the 10 to 2000nA region. Their beta will
be somewhat lower, and their f_T will be substantially lower at
a given current (by sqrt(1/Ic), but most of the other parameters
will work fine. For some BJTs, this holds down to the pA region.
Vbe drops by 58mV/decade at room temp, but you already knew that.
Click to expand...

If you look at low Iq LDOs, the bandgap references are using current
in that neighborhood. Do the math. Figure 10ua for the bandgap, or 5ua
each side, the say 8:1 for the transistor ratio. So you have ic=5/8uA,
with transistors that are not really exceptional. Note such circuits
have replica devices out the wazoo to control (i.e. replicate) leakage
due to the low beta. The poster never indicated the function of the
transistor, so it is not known if such tricks are required.
 
G

Greg Neff

If you look at low Iq LDOs, the bandgap references are using current
in that neighborhood. Do the math. Figure 10ua for the bandgap, or 5ua
each side, the say 8:1 for the transistor ratio. So you have ic=5/8uA,
with transistors that are not really exceptional. Note such circuits
have replica devices out the wazoo to control (i.e. replicate) leakage
due to the low beta. The poster never indicated the function of the
transistor, so it is not known if such tricks are required.
Click to expand...

I am not at liberty to discuss details of the circuit, other than I
need low frequency (DC - 100Hz) linear operation, with Vce around 10V.

================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]
 
G

Greg Neff

BCX70K? Or a fet!

John
Click to expand...

A FET would be great, but I don't have enough headroom to drive the
gate. I can barely accommodate Vbe.

================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]
 
J

Joerg

Eeyore said:
Joerg wrote:




You still use the 3904 ?

Aren't more modern parts like the BC549/559 somewhat better (notably higher beta
IIRC) yet the same price ?

SMT versions available as BC849/859.
Click to expand...

Yes, but the beta is often not an important parameter in my designs.
Cost differences of fractional cents often are.
 
J

Joerg

Greg said:
Looks like a good candidate, thanks.

It would be nice if the web sites were better designed to make it
easier to track down these parts on parametrics, but that is an
entirely different topic.
Click to expand...

Amen to that! Mfgs should stop hiring script kiddies and talk to actual
web site users beforehand.
 
F

Fred Bartoli

Joerg a écrit :
Hey, thanks. This very minute I was looking for a lower noise transistor
to drive a laser, has to be really low in noise below a few hundred
Hertz. The old 3904 doesn't really cut it but it looks like the BCX70
does. And it comes in SOT-23. Nice.
Click to expand...


Yep, the 3904 isn't so great, noise-wise.

Have a look at Zetex (their low Vcesat eline, like FFMT618/718...)
They're not noise speced but I did measure them at about 0.5nV/rtHz and
1/f corner under 100Hz.
Lots of choice in this family.

Ohooo, listen Graham... they're much better than the 4400.
 
J

Joerg

Fred said:
Joerg a écrit :



Yep, the 3904 isn't so great, noise-wise.

Have a look at Zetex (their low Vcesat eline, like FFMT618/718...)
They're not noise speced but I did measure them at about 0.5nV/rtHz and
1/f corner under 100Hz.
Lots of choice in this family.
Click to expand...

Wow, that would be cool. A bit pricey but if they really hold the line
under 1nv/rtHz that would be ok. Do you remember at which current you
had measured them?

In case others need such a transistor, it's not FFMT but FMMT618. Data
sheet:
http://www.zetex.com/3.0/pdf/FMMT619.pdf
 
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