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

2.1 Closed Loop Concepts and Simplifications

Substituting a = ∞ Equation 1 into Equation 16 results in,

Equation 17. GUID-D309C1E3-6066-46F5-8EEF-D2DC0361D22B-low.gif

Recall that in equation Equation 6 we state that Vd, the voltage difference between Vn and Vp, is equal to zero and therefore, 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 non-inverting op amp circuit in Figure 2-1.

Using the virtual short concept, we can say that,

Equation 18. Vn= Vp = Vi

Realizing that finding Vn is now the same resistor divider problem solved in Equation 12 and substituting Equation 18 into it, we get,

Equation 19. GUID-B67BD57A-FE45-49F5-9028-0F1FD517E2E2-low.gif

Rearranging and solving for A, we get,

Equation 20. GUID-B553C145-7199-4795-BA24-6E8FF6123E6A-low.gif

The same result is derived in Equation 17. Using the virtual short concept reduced solving the non-inverting amplifier, shown in Figure 2-1, to solving a resistor divider network.