2
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 speed complementary 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
frequency complementary 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, CB process devices are generally more stable
with capacitive loads (with higher phase margins in general), have lower DC errors,
consume less power for a given speed, and are all around more "user friendly".
Taking this a step further, XFCB family devices, which extend the utility of the op
amp to literally hundreds of MHz, are understandably less straightforward in terms
of their application (as is any amplifier operating over such a range). Thus, getting
the most from these modern devices definitely stresses the “total environment”
aspects of design.
Another major ease of use feature found in today's linear ICs is a much wider range
of supply voltage characterization. While the older ±15V standard is still much in
use, there is a trend towards including more performance data at popular lower
voltages, such as ±5V, or +5V only, single supply operation. The most recent devices
using the lower voltage XFCB process use supply voltages of either ±5V, or simply
+5V only. The trend towards lower supply voltages is unmistakable, with a goal of
squeezing the highest performance from a given voltage/power circuit environment.
These "ease of use" design aspects with current ICs are illustrated in this chapter,
