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Input Overvoltage Considerations

previous Rail-Rail Input Stages
In order to achieve the performance levels required, it is sometimes not possible to provide complete overdrive protection within IC op amps. Although most op amps have some type of input protection, care must still be taken to prevent possible damage against both CM and differential voltage stress.
This is most likely to occur when the input signal comes from an external sensor, for example. Rather than present a cursory discussion of this topic here, the reader is instead referred to Chapter 7, Section 7-4 for a detailed examination of this important issue.
Output Stages
The earliest IC op amp output stages were NPN emitter followers with NPN current sources or resistive pull-downs, as shown in Figure 1-31A below. Naturally, the slew rates were greater for positive-going than they were for negative-going signals.
While all modern op amps have push-pull output stages of some sort, many are still asymmetrical, and have a greater slew rate in one direction than the other. Asymmetry tends to introduce distortion on AC signals and generally results from the use of IC processes with faster NPN than PNP transistors. It may also result in an ability of the output to approach one supply more closely than the other in terms of saturation voltage.
Some traditional op amp output stages
Figure 1-31: Some traditional op amp output stages
In many applications, the output is required to swing only to one rail, usually the negative rail (i.e., ground in single-supply systems). A pulldown resistor to the negative rail will allow the output to approach that rail (provided the load impedance is high enough, or is also grounded to that rail), but only slowly. Using an FET current source instead of a resistor can speed things up, but this adds complexity, as shown in Fig. 1-31B.
With modern complementary bipolar (CB) processes, well matched high speed PNP and NPN transistors are readily available. The complementary emitter follower output stage shown in Fig. 1-31C has many advantages, but the most outstanding one is the low output impedance. However, the output voltage of this stage can only swing within about one VBE drop of either rail. Therefore an output swing of +1V to +4V is typical of such a stage, when operated on a single +5V supply.
The complementary common-emitter/common-source output stages shown in Figure 1-32A and B (opposite) allow the op amp output voltage to swing much closer to the rails, but these stages have much higher open-loop output impedance than do the emitter follower-based stages of Fig. 1-31C.
In practice, however, the amplifier's high open-loop gain and the applied feedback can still produce an application with low output impedance (particularly at frequencies below 10Hz). What should be carefully evaluated with this type of output stage is the loop gain within the application, with the load in place. Typically, the op amp will be specified for a minimum gain with a load resistance of 10kΩ (or more). Care should be taken that the application loading doesn't drop lower than the rated load, or gain accuracy may be lost.
It should also be noted these output stages can cause the op amp to be more sensitive to capacitive loading than the emitter-follower type. Again, this will be noted on the device data sheet, which will indicate a maximum of capacitive loading before overshoot or instability will be noted.
The complementary common emitter output stage using BJTs (Fig. 1-32A) cannot swing completely to the rails, but only to within the transistor saturation voltage (VCESAT) of the rails. For small amounts of load current (less than 100μA), the saturation voltage may be as low as 5 to 10mV, but for higher load currents, the saturation voltage can increase to several hundred mV (for example, 500mV at 50mA).
"Almost" rail-to-rail output structures
Figure 1-32: "Almost" rail-to-rail output structures
On the other hand, an output stage constructed of CMOS FETs (Fig. 1-32B) can provide nearly true rail-to-rail performance, but only under no-load conditions. If the op amp output must source or sink substantial current, the output voltage swing will be reduced by the I×R drop across the FETs internal "on" resistance. Typically this resistance will be on the order of 100Ω for precision amplifiers, but it can be less than 10Ω for high current drive CMOS amplifiers.
For the above basic reasons, it should be apparent that there is no such thing as a true rail-to-rail output stage, hence the caption of Fig. 1-32 ("Almost" Rail-to-Rail Output Structures). The best any op amp output stage can do is an almost rail-to-rail swing, when it is lightly loaded.
Op amps built on foundry CMOS processes have a primary advantage of low cost. Also, it is relatively straightforward to design rail-to-rail input and output stages with these CMOS devices, which will operate on low supply voltages.
Figure 1-33 below shows a simplified schematic of the AD8531/8532/8534 (single/dual/quad) op amp, which is typical of these design types. The AD8531/8532/8534 operates on a single 2.7V to 6.0V supply and can drive 250mA. Input offset voltage is 25mV maximum at +25°C, and voltage noise is 45nV/√Hz.
AD8531/8532/8534 CMOS rail-to-rail op amp simplified schematic
Figure 1-33: AD8531/8532/8534 CMOS rail-to-rail op amp simplified schematic
This type of op amp is simple and cost effective, and the lack of high DC precision is often no disadvantage. To the contrary, the high output drive available can be an overriding plus, particularly in AC-coupled applications.
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