I have seen in several schematics where a precision resistor, or matched resistors are called for, but every one of them talk about you going to purchase special resistors for this purpose.
I am building the Go, No-go transistor matching circuit shown here. It stated the just about all the resistors have to be matched to 1% at the most for it to work. I have cheapo "1% resistors", but don't trust them to be in spec. For this particular circuit it would seem that the closer I can get to precise resistances, the more accurate the vbe measurements will be.
Question: do I really need to find resistor "pairs" for this or can I simply put resistors in series until the total resistance is exactly what I want (my 20000 count multimeter would let me get to 0.1% or even 0.01% precision this way)? So for example, to get a perfect 20k resistor, I might use a 15k, 4.7k, 270 and maybe something small between 1-100 ohms all in series so that the total real resistance measured is 19.999-20,001 ohms. It seems like a simple no brainer, but every time I see schematics talking about precision resistors, none of them offer this approach, which got me to wonder if there was a reason.
Best Answer
This approach works well for 'one-off' instruments designed to work at room temperature. I use it all the time. It has two drawbacks.
1) It's labour intensive, which is why it's not used for mass produced equipment, where it's cheaper to buy 0.1% resistors and just fit them, rather than muck about measuring and matching.
2) 1% resistors won't necessarily have as good a tempco of resistance as 'precision' 0.1% or 0.01% resistors. This will be mitigated to some extent if you are only after tracking between multiple resistors, but even then there is no guarrantee cheaper resistors will track well. A set of precision resistors in a pack will be specified to track to a certain extent.