PDF《运算放大器的单电源供电》

One of the most common applications questions on opera- tional amplifiers concerns operation from a single supply voltage.

Can the model OPAxyz be operated from a single supply? The answer is almost always yes. Operation of op amps from single supply voltages is useful when negative supply voltages are not available. Furthermore, certain ap- plications using high voltage and high current op amps can derive important benefits from single supply operation. Consider the basic op amp connection shown in Figure la. It is powered from a dual supply (also called a balanced or split supply). Note that there is no ground connection to the op amp. In fact, it could be said that the op amp doesn'

t know where ground potential is. Ground potential is some- where between the positive and negative power supply voltages, but the op amp has no electrical connection to tell it exactly where. VIN VOUT = VIN G = +1 +VS = 15V CVS = 15V VIN VOUT = VIN G =

1 +VS = 30V (a) (b) FIGURE 1. A simple unity-gain buffer connection of an op amp illustrates the similarity of split-supply op- eration (a) to single-supply operation in (b). The circuit shown is connected as a voltage follower, so the output voltage is equal to the input voltage. Of course, there are limits to the ability of the output to follow the input. As the input voltage swings positively, the output at some point near the positive power supply will be unable to follow the input. Similarly the negative output swing will be limited to somewhere close to CVS. A typical op amp might allow output to swing within 2V of the power supply, making it possible to output C13V to +13V with ±15V supplies. Figure 1b shows the same unity-gain follower operated from a single 30V power supply. The op amp still has a total of 30V across the power supply terminals, but in this case it comes from a single positive supply. Operation is otherwise unchanged. The output is capable of following the input as long as the input comes no closer than 2V from either supply terminal of the op amp. The usable range of the circuit shown would be from +2V to +28V. Any op amp would be capable of this type of single-supply operation (with somewhat different swing limits). Why then are some op amps specifically touted for single supply applications? Sometimes, the limit on output swing near ground (the negative power supply to the op amp) poses a significant limitation. Figure 1b shows an application where the input signal is referenced to ground. In this case, input signals of less than 2V will not be accurately handled by the op amp. A single-supply op amp would handle this particular application more successfully. There are, however, many ways to use a standard op amp in single-supply applications which may lead to better overall performance. The key to these applications is in understanding the limitations of op amps when handling voltages near their power supplies. There are two possible causes for the inability of a standard op amp to function near ground in Figure 1b. They are (1) limited common-mode range and (2) output voltage swing capability. These performance characteristics are easily visualized with the graphical representation shown in Figure 2. The range over which a given op amp properly functions is shown in relationship to the power supply voltage. The common- mode range, for instance, is sometimes shown plotted with respect to another parameter such as temperature. A ±15V supply is assumed in the preparation of this plot, but it is easy to imagine the negative supply as being ground. In Figure 2a, notice that the op amp has a common-mode range of C13V to +13.5V. For voltages on the input termi- nals of the op amp of more negative than C13V or more positive than +13.5V, the differential input stage ceases to properly function. Similarly, the output stages of the op amp will have limits on output swing close to the supply voltage. This will be load- dependent and perhaps temperature-dependent also. Figure 2b shows output swing ability of an op amp plotted with respect to load current. It shows an output swing capability of C13.8V to +12.8V for a l0k? load (approximately ±1mA) at 25°C. ? ?