Operational Amplifier Applications - Amplifiers - Non-inverting Amplifier

Non-inverting Amplifier

Amplifies a voltage (multiplies by a constant greater than 1)

• Input impedance
  • The input impedance is at least the impedance between non-inverting and inverting inputs, which is typically 1 MΩ to 10 TΩ, plus the impedance of the path from the inverting input to ground (i.e., in parallel with ).
  • Because negative feedback ensures that the non-inverting and inverting inputs match, the input impedance is actually much higher.
• Although this circuit has a large input impedance, it suffers from error of input bias current.
  • The non-inverting and inverting inputs draw small leakage currents into the operational amplifier.
  • These input currents generate voltages that act like unmodeled input offsets. These unmodeled effects can lead to noise on the output (e.g., offsets or drift).
  • Assuming that the two leaking currents are matched, their effect can be mitigated by ensuring the DC impedance looking out of each input is the same.
    • The voltage produced by each bias current is equal to the product of the bias current with the equivalent DC impedance looking out of each input. Making those impedances equal makes the offset voltage at each input equal, and so the non-zero bias currents will have no impact on the difference between the two inputs.
    • A resistor of value

    which is the equivalent resistance of in parallel with, between the source and the non-inverting input will ensure the impedances looking out of each input will be matched.

    • The matched bias currents will then generate matched offset voltages, and their effect will be hidden to the operational amplifier (which acts on the difference between its inputs) so long as the CMRR is good.
  • Very often, the input currents are not matched.
    • Most operational amplifiers provide some method of balancing the two input currents (e.g., by way of an external potentiometer).
    • Alternatively, an external offset can be added to the operational amplifier input to nullify the effect.
    • Another solution is to insert a variable resistor between the source and the non-inverting input. The resistance can be tuned until the offset voltages at each input are matched.
    • Operational amplifiers with MOSFET-based input stages have input currents that are so small that they often can be neglected.