Section2 High Speed Op Amps.pdf

SECTION 2
HIGH SPEED OP AMPS
Driving Capacitive Loads
Cable Driving
Single-Supply Considerations
Application Circuits
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SECTION 2
HIGH SPEED OP AMPS
Walt Jung and Walt Kester
Modern system design increasingly makes use of high speed ICs as circuit building
blocks. With bandwidths going up and up, demands are placed on the designer for
faster and more power efficient circuits. The default high speed amplifier has
changed over the years, with high plementary bipolar (CB) process ICs
such as the AD846 and AD847 in use just about ten years at this writing. During
this time, the general utility/availability of these and other ICs have raised the “high
speed” common performance denominator to 50MHz. The most recent extended
plementary bipolar (XFCB) process high speed devices such as the
AD8001/AD8002, the AD9631/9632 and the AD8036/AD8037 now extend the
operating range into the UHF region.
Of course, a traditional performance barrier has been speed, or perhaps more
accurately, painless speed. While fast IC amplifiers have been around for some time,
until more recently they simply haven’t been the easiest to use. As an example,
devices with substantial speed increases over 741/301A era types, namely the 318-
family, did so at the expense of relatively poor settling and capacitive loading
characteristics. Modern CB process parts like the AD84X series provide far greater
speed, faster settling, and do so at low user cost. Still, the application of high
performance fast amplifiers is never entirely a cookbook process, so designers still
need to be wary of many inter-related key issues. This includes not just the amplifier
selection, but also control of parasitics and other potentially performance-limiting
details in the surrounding circuit.
It is worth underscoring that reasons for the "speed revolution" lie not just in
affordability of the new high speed ICs, but is also rooted in their ease of use.
Compared to earlier high speed ICs, C