SLOA277B january   2019  – july 2023 LM124 , LM124-N , LM124A , LM158 , LM158-N , LM158A , LM224 , LM224-N , LM224A , LM258 , LM258-N , LM258A , LM2902 , LM2902-N , LM2902-Q1 , LM2902K , LM2902KAV , LM2904 , LM2904-N , LM2904-Q1 , LM2904B , LM2904B-Q1 , LM2904BA , LM321 , LM324 , LM324-N , LM324A , LM358 , LM358-N , LM358A , LM358B , LM358BA , TS321 , TS321-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Devices Covered in Application Note
    1. 1.1 Common Schematic
    2. 1.2 Base Part Numbers
    3. 1.3 Input Voltage Offset Grades
    4. 1.4 Maximum Supply Voltage
    5. 1.5 High Reliability Options
    6. 1.6 HBM ESD Grade
    7. 1.7 LM358B, LM358BA, LM2904B, LM2904BA, LM324B, LM2902B
  5. 2Input Stage Considerations
    1. 2.1 Input Stage Schematic
    2. 2.2 Input Common Mode Range
    3. 2.3 Input Impedance
    4. 2.4 Phase Reversal
  6. 3Output Stage Considerations
    1. 3.1 Output Stage Schematic, VOL and IOL
    2. 3.2 IOL and Common Mode Voltage
    3. 3.3 Output Stage Schematic, VOH and IOH
    4. 3.4 Short Circuit Sourcing Current
    5. 3.5 Output Voltage Limitations
  7. 4AC Performance
    1. 4.1 Slew Rate and Bandwidth
    2. 4.2 Slew Rate Variability
    3. 4.3 Output Crossover Time Delay
    4. 4.4 First Crossover Example
    5. 4.5 Second Crossover Example
  8. 5Low VCC Guidance
    1. 5.1 Low VCC Input Range Supporting –40°C
    2. 5.2 Low VCC Output Range Supporting –40°C
    3. 5.3 Low VCC Audio Amplifier Example
  9. 6Comparator Usage
    1. 6.1 Op Amp Limitations
    2. 6.2 Input and Output Voltage Ranges
    3. 6.3 Overload Recovery
    4. 6.4 Slew Rate
  10. 7Unused Amp Connections and Inputs Connected Directly to Ground
    1. 7.1 Do Not Connect Inputs Directly to Ground
    2. 7.2 Unused Amplifier Connections
  11. 8Conclusion
  12. 9Revision History

2.3 Input Impedance

The input stage uses a Darlington PNP configuration with dedicated internal current sources on all of the transistor emitters. Therefore the input appears as a current source to the application. This current is the input PNP transistor's base current whose value is specified in the data sheet as IIB. This effective input current source has a high impedance of approximately 1 GΩ and a wide voltage range compliance from 0 V to one diode drop below VCC, including cases where a large differential input voltage is present (see Figure 2-3 region B). When the input voltage is greater than one diode voltage drop below VCC or is greater than VCC, the input becomes high impedance and no current flows other than a reverse diode leakage current (see Figure 2-3 region C). Matching the input resistance can reduce the input offset voltage from IIB. This can be done by placing matching resistors on the inputs. Note that the consequential increase in resistor thermal noise may be unacceptable.

The input current profile for a unity gain buffer powered by a 5-V supply, shown in Figure 2-2, can be seen in Figure 2-3. Applying a negative input voltage will forward bias a diode formed from the input PNP transistor's base to the die’s grounded substrate on the input pin. The result will be high current flow as indicated by the vertical slope near –0.5 V (region A). An input between 0 V and 3.5 V (region B) has a fixed current (IIB) with an impedance of approximately 1 GΩ. Region C spans from 4.5 V to 7 V and beyond, continuing up to a maximum of VCC where there is no input current flow.

GUID-2B90F179-CE18-47CF-916D-894A9B0D87D5-low.gif Figure 2-2 Unity Gain Buffer Test Configuration
GUID-CB33954E-C1D6-4201-AECA-D1EFE14E8D8A-low.gifFigure 2-3 Input Current vs Input Voltage
GUID-689EE26A-74E2-4A92-8832-496C8B783A90-low.gifFigure 2-4 Output Voltage vs Input Voltage