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Author Message
Guest
Posted: Wed Apr 30, 2008 6:36 pm
Hi Group:
I normally match pairs using the Hfe method. But this would be a repair
situation only.
But soon I'll need to do matching of some TO126's on a small production
scale.
I also will be matching some TO92 current mirrors.It seems to me that Vbe
would
be the more important requirement. I can't see matching both Vbe and Hfe.
I don't think you'll find many pairs that match both parameters. It would
also be
very time consuming. What do you guys recommend. I'll be building a test
fixture for the test
and can build what I need for the project.

Any input appreciated.


RonL
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John Popelish
Posted: Wed Apr 30, 2008 6:50 pm
Guest
r.laury@comcast.net wrote:
Quote:
Hi Group:
I normally match pairs using the Hfe method. But this would be a repair
situation only.
But soon I'll need to do matching of some TO126's on a small production
scale.
I also will be matching some TO92 current mirrors.It seems to me that Vbe
would
be the more important requirement. I can't see matching both Vbe and Hfe.
I don't think you'll find many pairs that match both parameters. It would
also be
very time consuming. What do you guys recommend. I'll be building a test
fixture for the test
and can build what I need for the project.

Any input appreciated.

It all depends on what will be driving the matched pair-- a
common voltage or two equal (or a common) currents. For the
current mirror, it is almost certainly a common voltage.
That goes for differential pairs, also.

--
Regards,

John Popelish
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Phil Allison
Posted: Wed Apr 30, 2008 6:58 pm
Guest
"John Popelish"

Quote:

It all depends on what will be driving the matched pair-- a common voltage
or two equal (or a common) currents. For the current mirror, it is almost
certainly a common voltage.
That goes for differential pairs, also.


** Huh ??

A diff pair of BJTs needs to have matched gain ( Hfe) transistors as well as
( ideally ) matched Vbe.

With unmatched Hfe devices, input offset current can become very
significant resulting in large DC offsets at the output of the circuit that
could only be fixed with an offset trim control.



....... Phil
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mike
Posted: Wed Apr 30, 2008 9:39 pm
Guest
r.laury@comcast.net wrote:
Quote:
Hi Group:
I normally match pairs using the Hfe method. But this would be a repair
situation only.
But soon I'll need to do matching of some TO126's on a small production
scale.
I also will be matching some TO92 current mirrors.It seems to me that Vbe
would
be the more important requirement. I can't see matching both Vbe and Hfe.
I don't think you'll find many pairs that match both parameters. It would
also be
very time consuming. What do you guys recommend. I'll be building a test
fixture for the test
and can build what I need for the project.

Any input appreciated.


RonL


Do the math.

Calculate the cost of the fixture, training, redesign, repair...
+ the labor cost of the matching
+ the inventory/distribution/repair cost of managing another part
number of pairs
+ the cost of redesigning the circuit when you get tired of all the
matching issues.
Divide by the quantity.
Compare that to the incremental cost of buying a matched dual.
If your time is expensive and your production run is "small"
you've probably already wasted more $$ than it would have cost to
do it right the first time.

Horror story.
One company saved 20-cents by using matched fets and poor layout.
Somebody decided to improve system cooling.
The resultant changes in heat distribution shut down
the production line 'cuz the fets didn't track any more.

Sounds like you've got a temperature-sensitive situation.
Recipe for disaster.
It's easy to see the 20-cents extra cost. What's hard is the
cost of fixing all the problems it causes.

--
Return address is VALID!
Bunch-O-Stuff Forsale Here:
http://mike.liveline.de/sale.html
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Robert Baer
Posted: Thu May 01, 2008 1:38 am
Guest
r.laury@comcast.net wrote:

Quote:
Hi Group:
I normally match pairs using the Hfe method. But this would be a repair
situation only.
But soon I'll need to do matching of some TO126's on a small production
scale.
I also will be matching some TO92 current mirrors.It seems to me that Vbe
would
be the more important requirement. I can't see matching both Vbe and Hfe.
I don't think you'll find many pairs that match both parameters. It would
also be
very time consuming. What do you guys recommend. I'll be building a test
fixture for the test
and can build what I need for the project.

Any input appreciated.


RonL


If you are making current mirrors, Vbe is the only important parameter.

In fact, one can mismatch Vbe by a specific value to give a 2:1 or
3:2 or some other ratio.
In this manner, emitter "balancing" resistors are not needed, and the
current ratio is kept over a decently wide temperature range.
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Jim Thompson
Posted: Thu May 01, 2008 9:21 am
Guest
On Wed, 30 Apr 2008 23:38:25 -0700, Robert Baer
<robertbaer@localnet.com> wrote:

Quote:
r.laury@comcast.net wrote:

Hi Group:
I normally match pairs using the Hfe method. But this would be a repair
situation only.
But soon I'll need to do matching of some TO126's on a small production
scale.
I also will be matching some TO92 current mirrors.It seems to me that Vbe
would
be the more important requirement. I can't see matching both Vbe and Hfe.
I don't think you'll find many pairs that match both parameters. It would
also be
very time consuming. What do you guys recommend. I'll be building a test
fixture for the test
and can build what I need for the project.

Any input appreciated.


RonL


If you are making current mirrors, Vbe is the only important parameter.
In fact, one can mismatch Vbe by a specific value to give a 2:1 or
3:2 or some other ratio.
In this manner, emitter "balancing" resistors are not needed, and the
current ratio is kept over a decently wide temperature range.

Amazing! What is this "decently wide temperature range" you speak of
?:-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: "skypeanalog" | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

America: Land of the Free, Because of the Brave
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Robert Baer
Posted: Thu May 01, 2008 10:39 pm
Guest
Jim Thompson wrote:
Quote:
On Wed, 30 Apr 2008 23:38:25 -0700, Robert Baer
robertbaer@localnet.com> wrote:


r.laury@comcast.net wrote:


Hi Group:
I normally match pairs using the Hfe method. But this would be a repair
situation only.
But soon I'll need to do matching of some TO126's on a small production
scale.
I also will be matching some TO92 current mirrors.It seems to me that Vbe
would
be the more important requirement. I can't see matching both Vbe and Hfe.
I don't think you'll find many pairs that match both parameters. It would
also be
very time consuming. What do you guys recommend. I'll be building a test
fixture for the test
and can build what I need for the project.

Any input appreciated.


RonL



If you are making current mirrors, Vbe is the only important parameter.
In fact, one can mismatch Vbe by a specific value to give a 2:1 or
3:2 or some other ratio.
In this manner, emitter "balancing" resistors are not needed, and the
current ratio is kept over a decently wide temperature range.


Amazing! What is this "decently wide temperature range" you speak of
?:-)

...Jim Thompson
Will MIL spec do?
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