Understanding Operational Amplifier Parameters

Aug 22, 2024

Review flashcards

Parameters of Operational Amplifier

1. Output Offset Voltage

  • Present at the output without any input applied (input should be zero).
  • Ideally, should be zero when both inputs are grounded.
  • Due to imperfections in components, there will be some output, referred to as output offset voltage.

2. Input Offset Voltage

  • Voltage applied at input to make output offset voltage zero.
  • Necessary to nullify output offset voltage when both inputs are grounded.
  • Can be applied to either non-inverting or inverting input.

3. Input Offset Current

  • Algebraic difference between the currents flowing into inverting (I-) and non-inverting (I+) terminals.
  • Ideally, the difference should be zero due to infinite input resistance, but practically it is not infinite, leading to some current.

4. Input Bias Current

  • Average of I+ and I-:
    • Formula: ( I_B = \frac{I_+ + I_-}{2} )
  • Ideally, it should be zero, but due to non-infinite input impedance, there will be some current.

5. Common Mode Gain

  • Defined as the ratio of common mode output (VM) to common mode input (VCM).
  • Formula: ( A_{CM} = \frac{V_M}{V_{CM}} )
  • Ideally, should be zero because the input difference should be zero when a common signal is applied.

6. Common Mode Rejection Ratio (CMRR)

  • Ratio of differential gain to common mode gain.
  • Formula: ( CMRR = \frac{A_{D}}{A_{CM}} )
  • Ideally, differential gain should be infinite, making CMRR ideally very high.
  • Can also be calculated based on input offset voltage and common mode voltage.

7. Supply Voltage Rejection Ratio (SVRR)

  • Change in input offset voltage caused by variations in supply voltages.
  • Often referred to as Power Supply Rejection Ratio (PSRR) by manufacturers.

8. Thermal Drift

  • Changes in operational amplifier parameters (Vio, Iio, IB) with respect to temperature, supply, and time.
  • Parameters should ideally remain constant but can vary due to temperature changes, aging, etc.
  • For calculations:
    • Thermal voltage drift: Variation in input offset voltage with respect to temperature.
    • Thermal drift current: Variation in input offset current with respect to temperature.
    • Thermal drift in input bias current: Variation in bias current with respect to temperature.

9. Slow Rate

  • Maximum rate of change of output voltage per unit time, expressed in volts per microsecond.
  • Indicates how quickly the output can respond to input changes.
  • Ideally, should be as high as possible to ensure fast response from the operational amplifier.

Conclusion

The parameters discussed are crucial for understanding the behavior and performance of operational amplifiers in circuits. For any questions or further clarifications, please feel free to comment.