mikeyB

Transistor matching
« on: October 28, 2018, 08:29:38 PM »
I'm planning on building a transistor matching test jig based around the GM328 tester, many of them on ebay.
Quick question is: is there an equation or correction factor regarding transistor hfe at different temperatures?
The temperature is the only variable as everything else is set by the tester. Obviously, I need a thermometer and need to note this along with the test measurements.

Thanks in advance
making better music!


squarewave

Re: Transistor matching
« Reply #1 on: October 29, 2018, 03:11:58 AM »
Obviously, I need a thermometer and need to note this along with the test measurements.
Or use a little Peltier plate sandwiched between a heat sink and a block of aluminum with holes for various transistor packages. Then you just rest that on top of the transistor to set it's temperature. Use a thermistor mounted on the aluminum block to servo the DC converter powering the plate.

abbey road d enfer

Re: Transistor matching
« Reply #2 on: October 29, 2018, 05:21:47 AM »
I'm planning on building a transistor matching test jig based around the GM328 tester, many of them on ebay.
Quick question is: is there an equation or correction factor regarding transistor hfe at different temperatures?
The temperature is the only variable as everything else is set by the tester. Obviously, I need a thermometer and need to note this along with the test measurements.

Thanks in advance
What is the application that needs transistor's hfe to be matched so closely? Assuming you do the measurement at ambient, that may vary over about 20°C (35°F), the hfe variation would be about +/-5%. I understand that controlling Vbe with accuracy is important in many applications, but hfe?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

mikeyB

Re: Transistor matching
« Reply #3 on: October 29, 2018, 11:41:10 AM »
What is the application that needs transistor's hfe to be matched so closely? Assuming you do the measurement at ambient, that may vary over about 20°C (35°F), the hfe variation would be about +/-5%. I understand that controlling Vbe with accuracy is important in many applications, but hfe?
It's for matching for discrete opamps, in particular the differential input pairs
making better music!

JohnRoberts

Re: Transistor matching
« Reply #4 on: October 29, 2018, 11:59:33 AM »
It's for matching for discrete opamps, in particular the differential input pairs
For LTP (long tail pair) use Vbe matching is far more important than beta matching. Devices from the same production batch should be similar enough beta... note: Vbe changes with temperature so ideally Vbe is measured at similar temperature.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

mikeyB

Re: Transistor matching
« Reply #5 on: October 29, 2018, 07:41:44 PM »
Thanks for the info guys :)
making better music!

boji

Re: Transistor matching
« Reply #6 on: October 31, 2018, 08:58:32 PM »
I may have read / misread  PRR saying something to the effect that once the thing is fired up and pushing audio in its native environment temperature is going to vary, so don't lose your mind seeking a perfect match.  Can't hurt to get them close?

abbey road d enfer

Re: Transistor matching
« Reply #7 on: November 01, 2018, 05:55:18 AM »
I may have read / misread  PRR saying something to the effect that once the thing is fired up and pushing audio in its native environment temperature is going to vary, so don't lose your mind seeking a perfect match.
Indeed, transistors operating inside an enclosed piece of gear will drift significantly from the measurements done at ambient, but they will drift identically, as long as they are close enough to be at the same temperature.

Quote
Can't hurt to get them close?
Indeed. I used to superglue transistors in the log amp in my 1979  31-band RTA.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

JohnRoberts

Re: Transistor matching
« Reply #8 on: November 03, 2018, 10:38:24 AM »
You can buy transistor arrays where the transistors are on a single substrate (not unlike ICs) for superior thermal tracking.  Over the decades I used lots of inexpensive transistor arrays (lm3086) with 5 small NPNs.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

mikeyB

Re: Transistor matching
« Reply #9 on: November 07, 2018, 07:39:25 PM »
Thanks John,
But they are for use in various types of DOA  with the 2520 footprint
making better music!


rackmonkey

Re: Transistor matching
« Reply #10 on: November 11, 2018, 12:10:57 PM »
Alpha RPAR is manufacturing LM394/LM194 copies now (AH194/AH394).  Pretty reasonable prices.  TO-5 packages.

https://www.cabintechglobal.com

BT
Whether you think you can or you think you can't, you're probably right.

JohnRoberts

Re: Transistor matching
« Reply #11 on: November 11, 2018, 12:38:54 PM »
Thanks John,
But they are for use in various types of DOA  with the 2520 footprint
That corp sells some decent arrays http://www.thatcorp.com/300-series_Matched_Transistor_Array_ICs.shtml, but it should be easy to source modern off the shelf op amps with superior performance to DOAs.

The last time I designed a DOA was in the 70's (80's?) and I used a LM394 in that one (for a bus summing amp). 

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Matador

Re: Transistor matching
« Reply #12 on: November 11, 2018, 03:23:19 PM »
The best way I've found to do this is to build the front end of the DOA on a bread board, and then you can swap individual BJT's in to the circuit. 

For example, you can build up this circuit:



Set up the current source to whatever you will be running in your actual implementation, then ground the two bases of T1 and T2.  You can then make a measurement of the DC output between T2 and T4, and swap in different T2 transistors and find a pair that provide the same DC output, then you can place both in the T1 and T2 positions.

The other thing you can do is drive T2 with a precision op-amp, with the non-inverting input grounded and the inverting input tied to the output of the circuit.  You then measure the output of the op-amp (T2's base), and find two transistors that have the same input voltage applied that makes the circuit output ground.  These two transistors should have the same Vbe.

JohnRoberts

Re: Transistor matching
« Reply #13 on: November 11, 2018, 04:52:47 PM »
I hate to sound like I am supporting this (DOA) endeavor but you can pretty easily make a more precise current mirror using 3 active devices and degeneration resistors to force better current matching.

That crude current mirror will distort your matching attempt.

JR 
John Roberts
http://circularscience.com
Tune it, or don't play it...

Matador

Re: Transistor matching
« Reply #14 on: November 11, 2018, 08:55:03 PM »
That crude current mirror will distort your matching attempt.
Why would it distort it?  Shouldn't two matched devices result in the same collector voltage when tested in the T2 position?


 

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