FYI, my application was also an analog multiplier circuit (used in a wattmeter circuit). With the original separate transistors, I would adjust the DC zero pot when the unit was first switched on, and then you could see it immediately start to drift just from junction heating due to quiescent current. When I switched to the dual transistors, the drift disappeared.
Nice
Transistors fabricated on the same die should be inherently well matched. If they aren't, there has to be something seriously wrong with the process.
I am not sure about that, but then I am no expert in semiconductor fabrication. From a human perspective the transistors should be identical if, as you say, they are fabricated on the same die, but at a molecular level they are miles apart. Some of the variables are, penetration, local doping levels, local impurities and mask tollerances.
In view of the fantastic things that the semiconductor manufacturers do inside chips, you would think it would be simple for them to fabricate two transistors with very close matching of all parameters on the same substrate, which would give excellent thermal tracking too. But this seems to be a very expensive process. The cheap transistor pairs, although in the same tiny pak, are all individual (discrete).
If you take a typical small signal BJT, the BC546, there is a wide spread of parameters, especially hFE and VCEmax, between individual transistors even those from the same die (I think). Some transistors families are produced from the same die but are just selected by the manufacturer. This is possibly the case with the ubiquitous BC546 to BC550 family of NBJTs and the complementary BC556 to 560 PBJTs.
The beautiful TI (National) LM194/LM394 super match pair shows what can be done with transistors on the same substrate, but they were always very expensive and are obsolete now- sadly.
**broken link removed**
The Analog Devices MATxxx series of matched transistors are still available but, even though AD are the best analog chip makers, in my humble opinion, their stuff is pricey.
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