OVERVOLTAGE EFFECTS Junctions may be Forward Biased if the current is Limited In general a safe Current limit is 5mA Reverse Bias Junction Breakdown is Damaging Regardless of the current level When in Doubt, Protect with External Diodes and series Resistances Curve Tracers Can be Used to Check the Overvoltage Characteristics of a device Simplified Equivalent Circuits in Data Sheets do not tell the Entire Story!!! Figure 7.3 AMPLIFIER OUTPUT VOLTAGE PHASE REVERSAL Some operational amplifiers exhibit output voltage phase reversal when one or both of their inputs exceeds their input common-mode voltage range. Phase reversal is usually associated with JFET (n-or p-channel)input amplifiers, but some bipolar devices(especially single-supply amplifiers operating as unity-gain followers)may also be susceptible In the vast majority of applications, output voltage phase reversal does not harm the amplifier nor the circuit in which the amplifier is used. Although a number of operational amplifiers suffer from phase reversal, it is rarely a problem in system design. However, in servo loop applications, this effect can be quite hazardous. Fortunately, this is only a temporary condition. Once the amplifiers inputs return to within its normal operating common-mode range, output voltage phase reversal ceases. It may still be necessary to consult the amplifier manufacturer, since phase reversal information rarely appears on device data sheets. Summarized as follows is a list of recent vintage Analog devices amplifier products that are now including output voltage phase reve characterization/commentary: Single-Supply/ Dual Supply Rail-to-Rail OP295/OP495 OP282OP482 OP113/OP213/OP413 OP285 OP183/OP283 OP467 OP292OP492 OP176 OP191/OP291OP491 BUF04 AD820/AD822/AD824 OP193/OP293/OP493
6 OVERVOLTAGE EFFECTS Junctions may be Forward Biased if the Current is Limited In General a Safe Current Limit is 5mA Reverse Bias Junction Breakdown is Damaging Regardless of the Current Level When in Doubt, Protect with External Diodes and Series Resistances Curve Tracers Can be Used to Check the Overvoltage Characteristics of a Device Simplified Equivalent Circuits in Data Sheets do not tell the Entire Story!!! Figure 7.3 AMPLIFIER OUTPUT VOLTAGE PHASE REVERSAL Some operational amplifiers exhibit output voltage phase reversal when one or both of their inputs exceeds their input common-mode voltage range. Phase reversal is usually associated with JFET (n- or p-channel) input amplifiers, but some bipolar devices (especially single-supply amplifiers operating as unity-gain followers) may also be susceptible. In the vast majority of applications, output voltage phase reversal does not harm the amplifier nor the circuit in which the amplifier is used. Although a number of operational amplifiers suffer from phase reversal, it is rarely a problem in system design. However, in servo loop applications, this effect can be quite hazardous. Fortunately, this is only a temporary condition. Once the amplifier’s inputs return to within its normal operating common-mode range, output voltage phase reversal ceases. It may still be necessary to consult the amplifier manufacturer, since phase reversal information rarely appears on device data sheets. Summarized as follows is a list of recent vintage Analog Devices amplifier products that are now including output voltage phase reversal characterization/commentary: Single-Supply/ Rail-to-Rail Dual Supply OP295/OP495 OP282/OP482 OP113/OP213/OP413 OP285 OP183/OP283 OP467 OP292/OP492 OP176 OP191/OP291/OP491 BUF04 OP279 AD820/AD822/AD824 OP193/OP293/OP493
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 rails
7 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 SingleSupply) 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
A CLOSER LOOK AT THE OP-X91 INPUT STAGE REVEALS ADDITIONAL DEVICES AAAI D3-D6: SUBSTRATE PNPs(COLLECTORS TO VNEG) Figure 7.5 An interesting benefit from using substrate PNPs as clamp diodes is that their collectors are connected to the negative supply; thus, when the applied input voltage exceeds either supply rail, the diodes energize, and the fault currents are diverted directly to the supply and not through or into the device's input stage There are al 5kohm resistors in series with each of the inputs to the opX9 1 to limit the fault that these kohm resistors are p-type diffusions placed inside an n-well, which lo current through D1 and D2 when the differential input voltage exceeds 0. 7V. Note then connected to the positive supply. when the applied input voltage exceeds the positive supply, some of the fault current generated is also diverted to Vpos and away from the input stage. As a result of these measures, the input overvoltage characteristic of the oPX91 is well behaved as shown in Figure 7.6. Note that the combination of the kohm resistors and clamp diodes safely limits the input current to less than 2mA, even when the inputs of the device exceed the supply rails by 10v
8 A CLOSER LOOK AT THE OP-X91 INPUT STAGE REVEALS ADDITIONAL DEVICES Figure 7.5 An interesting benefit from using substrate PNPs as clamp diodes is that their collectors are connected to the negative supply; thus, when the applied input voltage exceeds either supply rail, the diodes energize, and the fault currents are diverted directly to the supply and not through or into the device’s input stage. There are also 5kohm resistors in series with each of the inputs to the OPX91 to limit the fault current through D1 and D2 when the differential input voltage exceeds 0.7V. Note that these 5kohm resistors are p-type diffusions placed inside an n-well, which is then connected to the positive supply. When the applied input voltage exceeds the positive supply, some of the fault current generated is also diverted to VPOS and away from the input stage. As a result of these measures, the input overvoltage characteristic of the OPX91 is well behaved as shown in Figure 7.6. Note that the combination of the 5kohm resistors and clamp diodes safely limits the input current to less than 2mA, even when the inputs of the device exceed the supply rails by 10V
INTERNAL 5kQ RESISTORS PLUS IINPUT CLAMP DIODES COMBINE TO PROTECT OP-X91 DEVICES AGAINST OVERVOLTAGE 1mA 10V 10v 1mA Figure 7.6 As an added safety feature, an additional pair of diodes is used in the input stage across Q3 and Q4 to prevent subsequent stages internal to the oPX91 from collapsing(that is, forced into cutoff). If these stages were forced into cutoff, then th amplifier would undergo output voltage phase reversal when the inputs exceeded the positive input common mode voltage. An illustration of the diodes' effectiveness is shown in Figure 7.7. Here, the oPX91 family can safely handle a 20Vp-p input signal on 5V supplies without exhibiting any sign of output voltage phase reversal or other anomalous behavior. With these amplifiers, no external clamping diodes are
9 INTERNAL 5k RESISTORS PLUS IINPUT CLAMP DIODES COMBINE TO PROTECT OP-X91 DEVICES AGAINST OVERVOLTAGE Figure 7.6 As an added safety feature, an additional pair of diodes is used in the input stage across Q3 and Q4 to prevent subsequent stages internal to the OPX91 from collapsing (that is, forced into cutoff). If these stages were forced into cutoff, then the amplifier would undergo output voltage phase reversal when the inputs exceeded the positive input common mode voltage. An illustration of the diodes’ effectiveness is shown in Figure 7.7. Here, the OPX91 family can safely handle a 20Vp-p input signal on ±5V supplies without exhibiting any sign of output voltage phase reversal or other anomalous behavior. With these amplifiers, no external clamping diodes are required