Guys, a question I've always had about operational amplifiers.
Let's say as an example you need a gain around 5. To obtain this gain using the standard formula and I select a 4K7 and a 1K resistor, this gives 4.7 plus 1 of the formula gives a gain of 5.7.
If I use a 47K resistor and another one of 10K, I will also end up with a gain of 5.7.
My question what if it is better with larger or smaller resistor values?
Virtually no difference.
Lower values will pull more current from the op-amp's output and higher values will tend to pick-up more noise. But both ranges of values are good mid-range values.
With an inverting op-amp the input resistor is the input impedance and that's often a consideration.
With a non-inverting design, the input impedance is very high (if there no added resistor to ground) and independent of the gain resistors.
To save time drawing circuits I'm referring to this document
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
There is a small current into (or out of) the op amp; this is the bias current, the difference between the current in/out of the inputs is the offset current. These currents add to the errors in the gain as well as affecting the offset voltage by adding to the opamps own offset voltage.
If you want to add capacitance to limit bandwidth (i.e. filter) then the resistor value may affect the capacitor choice.
Aslo, lower value resistors will dissipate more power than higher values.
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