In BiFET operational amplifiers, phase reversal may be prevented by adding an appropriate resistance in series with the amplifiers input to limit the current Bipolar input devices can be protected by using a Schottky diode to clamp the input to within a few hundred millivolts of the negative rail For a complete description of the output voltage phase reversal effect, please consult Reference 1 BEWARE OF AMPLIFIER OUTPUT PHASE REVERSAL a Sometimes Occurs in FET and Bipolar Input(Especially Single Supply )Op Amps when Input Exceeds Common Mode Range Does Not Harm Amplifier, but may be Disastrous in Servo Not Usually Specified on Data Sheet, so Amplifier Must be Checked Easily Prevented BiFETs: Add Appropriate Input Series Resistance Determined Empirically, Unless Provided in Data Sheet) Bipolars Use Schottky Diode Clamps to the Supply Rails Figure 7. 4 Rail-to-rail operational amplifiers present a special class of problems to the integrated circuit designer, because these types of devices should not exhibit any abnormal behavior throughout the entire input common-mode range. In fact, it is desirable that devices used in these applications also not exhibit any abnormal behavior if the applied input voltages exceed the power supply range. One of the more recent vintage rail-to-rail input/output operational amplifiers, the OPX91 family(the OP191, the OP291, and the OP491), includes additional components that prevent overvoltage and damage to the device. As shown in Figure 7.5, the input tage of the opX9l devices use six diodes and two resistors to clamp the input terminals to each other and to the supplies. Di and D2 are base-emitter nPn diodes which are used to protect the bases of Q1-Q2 and Q3-Q4 against avalanche breakdown when the applied differential input voltage to the device exceeds 0.7V Diodes D3-D6 are diodes formed from substrate PNp transistors that clamp the applied input voltages on the opX9 1 to the supply rails7 In BiFET operational amplifiers, phase reversal may be prevented by adding an appropriate resistance in series with the amplifier’s input to limit the current. Bipolar input devices can be protected by using a Schottky diode to clamp the input to within a few hundred millivolts of the negative rail. For a complete description of the output voltage phase reversal effect, please consult Reference 1. BEWARE OF AMPLIFIER OUTPUT PHASE REVERSAL Sometimes Occurs in FET and Bipolar Input (Especially Single￾Supply) Op Amps when Input Exceeds Common Mode Range Does Not Harm Amplifier, but may be Disastrous in Servo Systems! Not Usually Specified on Data Sheet, so Amplifier Must be Checked Easily Prevented: BiFETs: Add Appropriate Input Series Resistance (Determined Empirically, Unless Provided in Data Sheet) Bipolars: Use Schlottky Diode Clamps to the Supply Rails. Figure 7.4 Rail-to-rail operational amplifiers present a special class of problems to the integrated circuit designer, because these types of devices should not exhibit any abnormal behavior throughout the entire input common-mode range. In fact, it is desirable that devices used in these applications also not exhibit any abnormal behavior if the applied input voltages exceed the power supply range. One of the more recent vintage rail-to-rail input/output operational amplifiers, the OPX91 family (the OP191, the OP291, and the OP491), includes additional components that prevent overvoltage and damage to the device. As shown in Figure 7.5, the input stage of the OPX91 devices use six diodes and two resistors to clamp the input terminals to each other and to the supplies. D1 and D2 are base-emitter NPN diodes which are used to protect the bases of Q1-Q2 and Q3-Q4 against avalanche breakdown when the applied differential input voltage to the device exceeds 0.7V. Diodes D3-D6 are diodes formed from substrate PNP transistors that clamp the applied input voltages on the OPX91 to the supply rails