The need for higher voltage swings in applications such as test-and-measurement instruments is constantly increasing, but the power supplies impose limitations on the operational amplifier rails make the high-voltage need a challenge for designers. How do you deliver high-voltage swings to a load without increasing the voltage levels of the power supplies of the operational amplifier? In other words, for example, how can you produce a ±16V or greater signal swing across the load using only ±15V power supplies? The circuit in Figure 1 uses a fully differential amplifier to offer an answer to this problem. Fully differential amplifiers enable you to deliver an output-voltage swing beyond the rails into the load. One of the common problems in working with operational amplifiers is the limit that the power-supply rails impose. The standard since the days of analog computers has been ±15V. Analog computers are gone, but their legacy remains in the power-supply voltages. These voltages find widespread use in transducer interfaces and applications in which voltage swing and dynamic range are of primary importance.

This ±15V supply voltage notwithstanding, applications exist that require a higher swing range beyond the power-supply limits. Typical ±15V operational amplifiers are seldom optimized for rail-to-rail operation, and their useful output-voltage range may be only 24 to 26V. Many audio consoles use older technology 741 op amps with ±18, ±20, and even ±22V power supplies to obtain more voltage swing and, therefore, more dynamic range from their systems in light of this limitation. The op amps in these systems often run hot and have heat sinks. The advent of fully differential op amps has given designers a better way to extend the output-swing range. Fully differential operational amplifiers 40 to 50 years ago were tube or discrete-transistor units. They have recently re-emerged as a way to interface to fully differential A/D converters and applications in which the load needs differential drive for better swing range or to reduce the noise effects in the systems.

The outputs of fully differential op amps have a characteristic that makes them useful for doubling the swing. The two outputs are 180° out of phase: When one output swings positive, the other swings negative and vice versa. The net effect is similar to what happens in a bridged amplifier: The effective output-voltage range doubles. The price of doubling this output-voltage swing is that you can no longer connect the output load to ground. Designers of automotive audio amplifiers are familiar with this concept: Audio-power bridged amplifiers have a fully differential output. Many installers have learned the hard way that the minus speaker connection cannot connect to ground. When the output voltage doubles, power quadruples. This feature is useful for audio power amplifiers in a limited-power-supply-voltage application.

To illustrate the advantage of fully differential outputs, assume that a fully differential op amp has a voltage-rail limitation of ±13V when operated from ±15V supplies. The absolute- maximum output range of each output is ±13V. But when the top output is at 13V, the bottom output is –13V: (13V)–(–13V=26V. When the top output is –13V, the bottom output is 13V. As a result, the output voltage is (–13V)–(13V)=–26V. Therefore, the output-voltage range is ±26V, which enables the output to swing from –26V to +26V, resulting in a doubled voltage-swing range. Fully differential op-amp designs require that the two feedback loops be symmetrical; the components in the top and the bottom sections must be the same. In Figure 1's schematic, the components in the top feedback path are labeled "A," and those in the bottom are labeled "B." When this design references a designator, the comment applies to both paths A and B.

The gain of the overall circuit is V_OUT/V_IN=R₃/R₁. The location of the load is often at the end of a balanced line. You cannot discount the effect of the wire; it affects the amplitude across the load, reducing the expected gain of the system. The sense lines help compensate for the voltage drop across the lines, resulting in the delivery of the targeted voltage to the load. The two sense amplifiers in the schematic are each composed of an op amp and resistors R₄ through R₇. R₂ acts as a summing component, adding small-signal amplitude equal to the voltage drop of the wire back into the input, boosting the op amp's output such that the expected gain appears at the load. If you make R₄ though R₇ equal in value to R₁, then it is easy to calculate R₂: It takes the same value as R₃. If R₄ through R₇ cannot be equal to R₁, then R₃ should be proportional to R₂. You should be aware that the resistors in the sense amplifiers change the load characteristics. For 600Ω audio-distribution systems, this fact is less of a problem than it is in 50Ω systems. You should be aware, however, of the output-drive characteristics of your operational amplifiers, if you wish to deliver the exact desired voltage swing to the load.

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