SBOA583 December   2023 OPA205 , OPA206 , OPA210 , OPA2206 , OPA2210 , OPA2392 , OPA2828 , OPA320 , OPA328 , OPA365 , OPA392 , OPA397 , OPA828

 

  1.   1
  2.   Abstract
  3. 1 Introduction
  4. 2 Circuit Configuration Impact on Common-Mode Range
  5. 3 Practical Input Limitations
  6. 4 Input Phase Reversal (Inversion)
  7. 5 Common-Mode Limitations Inside Bipolar Amplifiers
  8. 6 Common-Mode Limitations Inside CMOS Amplifiers
  9. 7 Rail-to-Rail CMOS Amplifiers
  10. 8 Output Swing Limitations Inside a Bipolar Op Amp
  11. 9 Linearity of Output Swing Specifications
  12. 10Output Voltage Swing vs Output Current
  13. 11Classic Bipolar vs Rail-to-Rail Output Stage for CMOS and Bipolar
  14. 12Rail-to-Rail Output and Open-Loop Gain Dependence
  15. 13Output Short-Circuit Protection
  16. 14Overload Recovery
  17. 15Supply Current During Input and Output Swing Limitations
  18. 16Summary
  19. 17References

15 Supply Current During Input and Output Swing Limitations

The majority of op-amp supply current flows through the amplifier output stage. For a bipolar amplifier driving the output stage beyond the output swing limitation causes one of the output transistors to saturate. Saturation of a bipolar transistor causes the current gain to dramatically decrease. The decrease in current gain in the output and preceding stages causes a significant increase in input base current of these transistors and consequently increases the overall op amp supply current. This increase in supply current is generally even higher with circuits that have a slew-boost circuit as the slew-boost is normally triggered when the output is driven to the supply rail. The OPA828 is an example of device that has a special feature that disables the slew-boost circuit when the output is driven into the supply rail. This feature significantly decreases the supply current compared to other similar devices that do not include this feature (see Figure 15-1).

GUID-20231017-SS0I-K0X4-TCF3-TCPKK181RVVH-low.svgFigure 15-1 Supply Current Change With Overloaded Output (OPA828 vs Competition)

When a CMOS output stage is driven into the supply rail, the transistors go into triode state (fully turned on). In this case, the biasing of the output stage is not disrupted in the same manor as the bipolar configuration, so output stage current does not significantly increase. The exception case is CMOS amplifiers with slew-boost that do not include the previously-mentioned disable feature.

From an input perspective, supply current is generally not impacted by common-mode range violations, but the input bias current (IB) of bipolar devices can be affected. Whenever a bipolar transistor is saturated, the current gain (beta) significantly decreases causing an increase in the base current. Since CMOS devices are voltage controlled, the non-linear operation of the input stage that occurs during common-mode violations does not impact the bias current. Figure 15-2 contrasts input bias current vs common-mode voltage for CMOS and bipolar amplifiers.

GUID-20231017-SS0I-KKXG-GV36-DWLHN1CXF2BL-low.svgFigure 15-2 Bias Current vs Common-Mode Voltage