Rail-to-rail output swing is a hot topic with single-supply amplifiers. Most single-supply amplifier manufacturers claim that they have devices that can perform this extraordinary output characteristic. But what does"rail-to-rail" output swing really mean? First and foremost, it is important to realize that there isn’t an amplifier on the market today that can actually achieve rail-to-rail output operation. In reality, the manufacturers of operational amplifiers make the "rail-to-rail output" claim in an effort to aspire to obtain that lofty goal. So, a system designer should be well informed when it comes to this specification. Not only is the terminology somewhat misleading, but different manufacturers specify output swing in different ways. If it appears that the output swing capability of the amplifier could be a showstopper in the application, a second look at this specification could be productive. Better yet, a third look may be warranted because this specification may or may not provide the information the designer needs.

Specifically, What Does the Specification Mean? -- In its simplest form, the output swing specification of a single-supply operational amplifier defines how close the output terminal of the amplifier can be driven towards the negative or positive supply rail. Furthermore, the output swing performance of an amplifier is dependent on the amount of loading at the output. Given these conditions, the rail-to-rail output swing operation is primarily dependent on the transistors in the amplifier’s output stage. This test scenario only states that the amplifier will slightly surpass the specified output voltages. It does not imply that the operational amplifier is operating in its linear region as the output approaches the extremes.

The output swing test condition can be stated in a variety of ways:

The conditions shown in Table 1 seem to have enough diversity to make it difficult to compare one condition to another. But in reality, these conditions have one thing in common: the amplifier output current. Although the output current is specifically stated in case 1d and 2d it can easily be calculated in the other cases by subtracting the output voltage from the load reference voltage. For instance in case 1a, the amplifier is sourcing ((VDD - VOH ) - VDD/2)/10 kW . If you assume that the amplifier output (VDD - VOH) is driven fairly close to the rail, the current coming out of the amplifier into the 10 kW load would be ~250 m A. In the case of 1b, the amplifier is sourcing twice the current as compared to case 1a.

The effect of these conditions is shown in the last column of Table 1 where a single supply amplifier was tested (see Fig. 1) under the stated conditions using one sample. These data demonstrate the effects of the output conditions on the output swing performance of that amplifier. The conditions outlined in 1c and 2b make the amplifier look pretty good. If you calculated the sink or source current, you will quickly discover that the amplifier is driving near zero microamperes out of its output terminal.

Is the Amplifier Still an Amplifier?-- Taking this specification alone could give the user a false sense of security that the amplifier will operate as an amplifier, i.e. in a linear fashion over the full output range. In fact, some manufacturers don’t mean to imply that at all. Some only intend to guarantee that the amplifier can swing that far. What isn’t stated in this specification is what happens to the rest of the amplifier. In reality, if an amplifier is pushed to the rails, the offset voltage of the amplifier will dramatically change and eventually the circuit will stop being an amplifier.

With this phenomenon (see Fig. 2) it is noticeable that the linearity of the amplifier starts to degrade long before the output swing maximums are reached. If the output of an amplifier is operated beyond the linear region of this curve, the input-to-output relationship of the signal will, of course, be non-linear.

The Open-Loop Gain (AOL) specification can be used to determine the linear operating output range of the amplifier. The definition of the open-loop gain is:
AOL (dB) = 20 log (d VOUT / d VOS),
where,
VOUT is the output voltage,
VOS is the input offset voltage,
d VOUT is (VOH - VOL ), and
d VOS is the change in offset voltage given the change in output voltage.

The open-loop gain test is performed with two stated conditions: 1) the output swing range and 2) the output load.

The difference between the AOL conditions (use the test circuit in Fig. 1, again) and the Output Swing conditions can be profound. The AOL specification validates the voltage-output swing test by implying that the operational amplifier is operating within its linear region. Table 2 shows some examples of this specification as well as the output swing specification as stated by various manufacturers who claim rail-to-rail output operation.

From Table 2, manufacturer A states different conditions between the amplifier output swing and open loop gain specifications. In this example, the amplifier is required to drive 30 m A to achieve the stated open-loop gain specification but the output swing conditions call out 100 m A load currents. Clearly, the amplifier cannot be in its linear region while driving to the stated VOH and VOL limits. Although this amplifier has been advertised as having"rail-to-rail output" performance, these specifications would indicate that this particular amplifier could only be used to drive the inputs of a 12-bit ADC between the ranges of VO = 1 V to 4 V and still achieve linear performance.

The conditions and specifications given by manufacturer B (Table 2) are more conservative than those stated by manufacturer A. With this device, the output swing conditions and open-loop gain conditions are the same. So, given the previous example, this amplifier could drive the inputs of a 12-bit ADC between the ranges of VO = 100 mV to 4.9 V while achieving good linear performance.

The third manufacturer (in Table 2) has stated its specifications in the best light (for the manufacturer) possible. The output swing test demonstrates good rail-to-rail operation with a load of approximately 250 mA. The 20 mV specification for VOH and VOL is the best of the three amplifiers in Table 2. The open-loop gain specification of this amplifier is also the best of the three; however, careful examination of the conditions for this test reveals that the amplifier has been specified across its most stable and linear region. This is done with extended power supply voltages and output voltage swings 2.5 V from the rail. The question one would ask," Would this amplifier be able to drive a 12-bit converter in zero to 5V single supply environment?" It’s hard to say.