SLOA011B January   2018  – July 2021 LF347 , LF353 , LM348 , MC1458 , TL022 , TL061 , TL062 , TL071 , TL072 , UA741

 

  1. 1Introduction
    1. 1.1 Amplifier Basics
    2. 1.2 Ideal Op Amp Model
  2. 2Non-Inverting Amplifier
    1. 2.1 Closed Loop Concepts and Simplifications
  3. 3Inverting Amplifier
    1. 3.1 Closed Loop Concepts and Simplifications
  4. 4Simplified Op Amp Circuit Diagram
    1. 4.1 Input Stage
    2. 4.2 Second Stage
    3. 4.3 Output Stage
  5. 5Op Amp Specifications
    1. 5.1  Absolute Maximum Ratings and Recommended Operating Condition
    2. 5.2  Input Offset Voltage
    3. 5.3  Input Current
    4. 5.4  Input Common Mode Voltage Range
    5. 5.5  Differential Input Voltage Range
    6. 5.6  Maximum Output Voltage Swing
    7. 5.7  Large Signal Differential Voltage Amplification
    8. 5.8  Input Parasitic Elements
      1. 5.8.1 Input Capacitance
      2. 5.8.2 Input Resistance
    9. 5.9  Output Impedance
    10. 5.10 Common-Mode Rejection Ratio
    11. 5.11 Supply Voltage Rejection Ratio
    12. 5.12 Supply Current
    13. 5.13 Slew Rate at Unity Gain
    14. 5.14 Equivalent Input Noise
    15. 5.15 Total Harmonic Distortion Plus Noise
    16. 5.16 Unity-Gain Bandwidth and Phase Margin
    17. 5.17 Settling Time
  6. 6References
  7. 7Glossary
  8. 8Revision History

3.1 Closed Loop Concepts and Simplifications

Substituting a = ∞ Equation 1 into Equation 24 results in

Equation 27. GUID-D62868A6-BBCB-4E58-AA37-DCB45A1ABC31-low.gif

Recall that in equation Equation 6 we stated that Vd, the voltage difference between Vn and Vp, was equal to zero so that Vn = Vp. Still they are not shorted together. Rather there is said to be a virtual short between Vn and Vp. The concept of the virtual short further simplifies analysis of the inverting op amp circuit in Figure 3-1.

Using the virtual short concept, we can say that

Equation 28. Vn = Vp = 0

In this configuration, the inverting input is a virtual ground.

We can write the node equation at the inverting input as

Equation 29. GUID-A68BA8D4-017E-43E1-863F-AAE7D68ACA53-low.gif

Since Vn = 0, rearranging, and solving for A we get

Equation 30. GUID-8FB54939-74DF-4CF3-A342-E414F01D83A9-low.gif

The same result is derived more easily than in (Equation 24. Using the virtual short (or virtual ground) concept reduced solving the inverting amplifier, shown in Figure 3-1, to solving a single node equation.