TABLE OF CONTENTS Walt Jung, Walt Kester
This section of the chapter features miscellaneous op amp applications, within a format of amplifier ideas. They range broadly across the spectrum, illustrating many innovative op amp uses that don't otherwise fit categories. Some of the concepts have been inspired by publication elsewhere. In such cases, an appropriate original reference is given.
Conventional video line drivers use a series or back-termination resistor, selected to match the transmission line characteristic impedance. Although simple, this scheme is inherently inefficient, as both load and series termination resistors drop the same voltage. This isn't usually a problem with 1 V p-p video signals operating on high voltage supplies, such as 12 V or 15 V. However, with lower voltage supplies, particularly 5 V or less, driver headroom is definitely an issue. For such conditions, a conventional driver may simply not be able to accommodate a signal of twice V OUT without distortion.
Figure 6-152 illustrates a solution to this driver efficiency problem. In this line driver (adapted from a circuit by Victor Koren, see Reference [1]), a Howland type of feedback configuration is used. This allows the series termination resistor R5 to be appreciably smaller, thus dropping less voltage and improving stage efficiency. Both positive and negative loop feedback paths are used around the op amp, R3 and R4, plus R1 and R2. An AD817 is chosen for its video characteristics, and line driving capability. The circuit also...
