Selecting Resistors for Op-Amp Gain

[johnerrington]

To save time drawing circuits I'm referring to this document

Using Operational Amplifiers in your Arduino project

3: BASIC CIRCUITS USING OP AMPS: Op amps can be used in many different configurations, but the simple circuits described here cover most applications. Unity gain buffer, inverting and non-inverting amplifier, difference amplifier If you feel some of this is getting too technical you may wish to skip any bits in italics. Unity gain buffer. A "unity gain buffer" gives an output voltage that is the same as the input voltage See also here The important feature of this circuit is that it takes…

Non-inverting amplifier

Like the unity gain buffer, this circuit does not load the input - but it does provide a precise amount of voltage gain, set by resistors R1 & R2.

Vout = Vin (R2 + R1 / R1) or Vout / Vin = 1 + (R2 / R1)

The input resistance is VERY HIGH (ideally "infinite")

With no other option I1 = I2 and I1 = Va /R1
If you choose very LOW values for R1, R2 the op amp may not be able to provide enough current to maintain gain linearity as Va increases. So the total feedback resistannce needs to exceed say 500 ohms (5V 10mA)

If you choose very HIGH values the input bias current becomes significant.
For a "741" (747 in the table is a dual '741) Ibias = 80nA
so a resistor of 1 Mohm will cause an offset of 80mV