Operational amplifier applications

This article illustrates some typical applications of operational amplifier . A simplified schematic notation is used, and the reader is reminded that many details such as device selection and power supply connections are not shown.

Comparator

Compares two voltages and switches its output to indicate which voltage is larger.

• V_2 \\ V_{\text{S-}} & V_1 < src="http://upload.wikimedia.org/math/9/9/9/999b8b529bf4ba4625abaed3570f53b9.png">

(where V_s is the supply voltage and the opamp is powered by + V_s and − V_s.)

Inverting amplifier

An inverting amplifier uses negative feedback to invert and amplify a voltage. The R_f resistor allows some of the output signal to be returned to the input. Since t

he output is 180° out of phase, this amount is effectively subtracted from the input, thereby reducing the input into the operational amplifier. This reduces the

overall gain of the amplifier and is dubbed negative feedback.

• Z_in = R_in (because V ₋ is a virtual ground)
• A third resistor, of value , added between the non-inverting input and ground, while not necessary, minimizes errors due to input bias currents.

The gain of the amplifier is determined by the ratio of R_f to R_in. That is:

The presence of the negative sign is a convention indicating that the output is inverted. For example, if R_f is 10,000 Ω and R_in is 1,000 Ω, then the gain would be -10000Ω/1000Ω, which is -10.

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., R₁ in parallel with R₂).
  • 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 R₁ in parallel with R₂, between the V_in 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 V_in 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.