domingo, 23 de mayo de 2010

A Differential Op-Amp Circuit Collection - Parte IV

5 Audio Applications

5.1 Bridged Output Stages

The presence of simultaneous output polarities from a fully-differential amplifier solves a problem inherent in bridged audio circuits – the time delay caused by taking a single-ended output and running it through a second inverting stage.

The time delay is nonzero, and a degree of cancellation as one peak occurs slightly before the other when the two outputs are combined at the speaker. Worse yet, one output will contain one amplifier's worth of distortion, while the other has two amplifier's worth of distortion. Assuming traditional methods of adding random noise, that is a 41.4% noise increase in one output with respect to the other, power output stages are usually somewhat noisy, so this noise increase will probably be audible.

A fully-differential op-amp will not have completely symmetrical outputs. There will still be a finite delay, but the delay is orders of magnitude less than that of the traditional circuit.

This technique increases component count and expense. Therefore, it will probably be more appropriate in high end products. Most fully-differential op-amps are high-speed devices, and have excellent noise response when used in the audio range.

5.2 Stereo Width Control

Fully-differential amplifiers can be used to create an amplitude cancellation circuit that will remove audio content that is present in both channels.

The output mixers (U2 and U4) are presented with an inverted version of the input signal on one input (through R6 and R14), and a variable amount of out-of-phase signal from the other channel.

When the ganged pot (R5) is at the center position, equal amounts of inverted and noninverted signal cancel each other, for a net output of zero on the other input of the output mixers (through R7 and R13).

At one extreme of the pot (top in this schematic), the output of each channel is the sum of the left and right channel input audio, or monaural. At the other extreme, the output of each mixer is devoid of any content from the other channel – canceling anything common between them.

This application differs from previous implementations by utilizing fully-differential op-amps to simultaneously generate inverted and noninverted versions of the input signal. The usual method of doing this is to generate an inverted version of the input signal from the output of a buffer amp. The inverted waveform, therefore, is subject to two op-amp delays as opposed to one delay for the non-inverted waveform. The inverted waveform, therefore, has some phase delay which limits the ultimate width possible from the circuit. By utilizing a fully-differential opamp, a near perfect inverted waveform is available for cancellation with the other channel.

6 Summary

Fully-differential amplifiers are based on the technology of the original tube-based op-amps of more than 50 years ago. As such, they require design techniques that are new to most designers. The performance increase afforded by fully differential op-amps more than outweigh the slight additional expense of more passive components. Driving of fully differential A/D converters, data filtering for DSL and other digital communication systems, and audio applications are just a few ways that these devices can be used in a system to deliver performance that is superior to single-ended design techniques.

Hernández Caballero Indiana
Asignatura: CAF

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