For example, I was thinking about building a small current sensing device that has an display to measure small current draws on some of my circuits. The design would be such that a very small resistance (~1m Ohm) is placed in-line and the voltage take across it amplified. However, I have always had trouble with amplifier circuits.
As you know, the gain constant of a transistor varies greatly even within the same batch of devices. I know that certain designs can be made such that their transfer functions depend only slightly on the gain constant, but are all gain applications implemented in this way?
Operation Amplifiers can be a bit simpler because their open-loop gain is so high – but similar to any amplifier circuit the gain is a direct function of the values of resistances (which vary largely!).
So how can amplifier circuits be used effectively in a particular application that requires the exact gain coefficient of the circuit (i.e. the current sensing design I mentioned earlier). Of course you can measure the resistance values before you implement them into your design – but what about commercial designs? The software edited based on the resistance values for every unit manufactured – at least it seems very impractical.
Best Answer
One option not mentioned is to use packaged difference amplifiers or instrumentation amplifiers, where all the internal resistors are very effectively trimmed to high accuracy. This effectively minimized the risk of mismatched resistors, which can really kill common mode rejection (which is often a bigger problem than non-exact gains).
If high precision gain is a real must, an instrumentation amplifier sets the gain using only one resistor, and that can be as high precision as you're willing to pay for. Alternatively, a lower resistor and a trimmer pot can be used for adjustment