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

5 Common-Mode Limitations Inside Bipolar Amplifiers

This section covers some of the details regarding the internal operation of the op amp. It is not necessary to have a deep understanding of the internal operation to design effective board-level amplifier circuits. However, developing a rudimentary understanding of the internal operation helps to provide a better insight into device selection, operation, and trade-offs for different technologies. This section provides some high-level details on internal operation, but for a complete coverage, consult integrated circuit design textbooks such as Analysis and Design of Integrated Circuits by Gray and Meyer(2).

From a common-mode perspective, CMOS and bipolar have different characteristics and need to be considered separately. A simplified version of a typical bipolar input stage is shown in Figure 5-1. The transistors Q1 and Q2 translate a differential input voltage to a single-ended output for the next stage. Transistor Q4 sets the bias current for the differential pair. The positive common-mode limitation of the input stage occurs when Q4 becomes saturated. For a bipolar transistor, saturation occurs at maximum collector current and minimum collector-to-emitter voltage (VCE(SAT) = 0.2 V to 0.3 V). The positive common-mode range can be calculated doing a Kirchhoff's walk from the input to ground. For this example, VIN_MAX = – VBE(Q1) –VCE(Q4-SAT) + VCC. For a 0.7-V VBE drop and a saturation voltage of 0.3 V the common-mode limit is approximately 1 V from VCC. If the common-mode input signal is equal to or greater than the common-mode limit, the transistor Q4 saturates and the amplifier becomes non-linear.

Figure 5-2 illustrates the negative common-mode limitation on a bipolar input stage. The negative supply becomes nonlinear when the input signal is driven near VEE so that Q1 becomes saturated. Doing a Kirchhoff's walk from VEE to the input shows that VIN_MIN = VEE + VD1 + VCE(SAT-Q1) – VBE(Q1), or VIN_MIN = VEE + 0.3 V.

It is evident from the bipolar common-mode examples that the input common-mode range is limited by 0.3 V from the negative rail and 1 V from the positive rail. Depending on the internal topology, the common-mode range can have even greater swing limitations. Some relatively unusual bipolar devices can have swing to VEE, but no bipolar has rail-to-rail input. If a wide common-mode range is required, CMOS rail-to-rail devices offer input swing from the negative to positive input supply (see Section 7).

GUID-20231017-SS0I-TNB8-CKTD-J8BF7FNTPQCD-low.svg Figure 5-1 Bipolar Input Stage Positive Common-Mode Limit
GUID-20231017-SS0I-LVDG-DRML-WKSK9XQBWR7M-low.svg Figure 5-2 Bipolar Input Stage Negative Common-Mode Limit