I seem to remember Prof. Leach showed a simple circuit (or two) you can use to measure and match BJTs with. But after a quick look, I can't find it on his website.
http://users.ece.gatech.edu/~mleach/lowtim/
[EDIT] Found it! It's here...
http://users.ece.gatech.edu/~mleach/lowtim/part2.html
..Todd
http://users.ece.gatech.edu/~mleach/lowtim/
[EDIT] Found it! It's here...
http://users.ece.gatech.edu/~mleach/lowtim/part2.html
..Todd
Now I have a question about that Leach circuit. And I am a total newbie, so go easy on me.
I thought we were trying to measure/match current gain, but we're measuring the current through the base segment. Wouldn't it be better to provide a constant current to the base and measure the current gain on the collector/emitter segment? I'm sure there must be a simple reason for not doing that.
..Todd
I thought we were trying to measure/match current gain, but we're measuring the current through the base segment. Wouldn't it be better to provide a constant current to the base and measure the current gain on the collector/emitter segment? I'm sure there must be a simple reason for not doing that.
..Todd
Hi,
most amps using BJTs have a fixed collector current controlled by a fixed base emitter voltage (don't always believe those that say a BJT is a current to current amplifying device - but that's a different argument).
If you fix the collector current at your chosen value and test all your devices at this Ic, then you can measure the base current (indirectly by measuring the base resistor voltage).
Alternatively, fix the base current and measure the various collector currents. The disdavantage here is that different Ic result in different power dissipation and that in turn results in heating of the junction at different rates making it VERY difficult to repeat test at constant temperature. The gain will vary significantly as junction temperature changes.
You need a set up that let's you test quite quickly and at a fixed repeatable time interval, note your readings.
If you're a whiz kid with computers you could devise a computer controlled test procedure that takes your measurements at say 1 second intervals and watch what happens as the device heats up. Then select your matching parameters.
most amps using BJTs have a fixed collector current controlled by a fixed base emitter voltage (don't always believe those that say a BJT is a current to current amplifying device - but that's a different argument).
If you fix the collector current at your chosen value and test all your devices at this Ic, then you can measure the base current (indirectly by measuring the base resistor voltage).
Alternatively, fix the base current and measure the various collector currents. The disdavantage here is that different Ic result in different power dissipation and that in turn results in heating of the junction at different rates making it VERY difficult to repeat test at constant temperature. The gain will vary significantly as junction temperature changes.
You need a set up that let's you test quite quickly and at a fixed repeatable time interval, note your readings.
If you're a whiz kid with computers you could devise a computer controlled test procedure that takes your measurements at say 1 second intervals and watch what happens as the device heats up. Then select your matching parameters.
AndrewT said:
Andrew,
Does Vbe then affect current gain such that matching Vbe somewhat matches hFE as a result? Or by matching Vbe can the hFE still be wildly different between devices?
Sorry if that's pretty elementary, but I'm only in class 101.
Secondly, which of these parameters affect DC offset most?
..Todd
Hi Stefano
Jfets
I have a better cad drawn circuit of this but I can't find the file.
This is how I match Jfets and my circuits work. It is quite simple, measure gate voltage to GND with DC voltmeter and select R value for whatever Ids you are going to operate them.
Just change R to +14.6V and the drain pin to -14.6V for p-channels.
You will find a much greater degree of difference in the Jfet Vgs than variables in BJT's. This may have something to do as to why people pay extra for duel Jfets. I use SOT 23 Jfets, and stick them to the tabs to keep them sorted in more general Vgs groups. Then re-measure from each group to find more exact pairs. It is really not as bad as it sounds, but you must have a good supply of transistors. Some types are more various than others.
I know this has been discussed on this forum elsewere, but this is what I did and it seems to work. I could buy 10 or more of these Jfets for the cost of a duel.
Jfets
I have a better cad drawn circuit of this but I can't find the file.
This is how I match Jfets and my circuits work. It is quite simple, measure gate voltage to GND with DC voltmeter and select R value for whatever Ids you are going to operate them. Just change R to +14.6V and the drain pin to -14.6V for p-channels.
You will find a much greater degree of difference in the Jfet Vgs than variables in BJT's. This may have something to do as to why people pay extra for duel Jfets.
I use SOT 23 Jfets, and stick them to the tabs to keep them sorted in more general Vgs groups. Then re-measure from each group to find more exact pairs. It is really not as bad as it sounds, but you must have a good supply of transistors. Some types are more various than others.I know this has been discussed on this forum elsewere, but this is what I did and it seems to work. I could buy 10 or more of these Jfets for the cost of a duel.
Attachments
Vbe matching does not help hFE matching. It appears to be one or the other.taj said:
If you want to match both parameters you need a lot of devices to get just a few double matched pairs.
Match one and the range of variance in the other is almost a large as in the whole sample batch.
In an LTP, you can match the Vbe and the DC input offset will be in error. Similarly match the hFE and the offset still exists. I don't know which is worse. But I think gain matching is better for input offset.
For dual complementary LTP then gain matching definitely reduces input offset current, but this time the gain match is between respective PNP and NPN and this is often impossible to achieve due to the different manufacturing process for the complementary types. (Leach refers to first priority matching like this and using graded BC550/560 devices I could get nowhere close).
At the output end (or anywhere where devices are in parallel)where parallel devices are used Vbe is much more important.
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