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

Input Stage

Symmetry of the input stage is key to its operation. Each transistor pair, Q1-Q2 and Q3-Q4, is matched as closely as possible.

Q3 is diode connected. This forces the collector current in Q3 to equal IC1. The base-emitter junctions of Q3 and Q4 are in parallel so they both see the same VBE. Because Q4 is matched to Q3, its collector current is also equal to IC1. This circuit is called a current mirror.

Current source 2IE is divided between Q1 and Q2. This division depends on the input voltages, Vp and Vn.

When Vp is more positive than Vn, Q1 carries more current than Q2, and IC1 is larger than IC2. The current mirror action of Q3-Q4 causes IOUT1 to flow into the collector-collector junction of Q2-Q4.

When Vn is more positive than Vp, Q2 carries more current than Q1 and IC2 is larger than IC1. The current mirror action of Q3-Q4 causes IOUT1 to flow out of the collector-collector junction of Q2-Q4.

IOUT1 is the single-ended signal out of the first stage and is proportional to the differential input, Vp - Vn. IOUT1 = gm1(Vp - Vn). The term gm1 is called the transconductance of the input stage. The input stage is a transconductance amplifier.