318 Chapter 10
The other kind could be called a controlled-current source type, which is, in essence, a
single emitter–follower with an active emitter load for adequate current sinking. If this
latter element runs out of current capability, it makes the output stage clip much as if it
had run out of output voltage. This kind of output stage demands a very clear idea of how
low an impedance it will be asked to drive before design begins.
Valve textbooks contain enigmatic references to classes of operation called AB1 and
AB2; in the former, grid current did not fl ow for any part of the cycle, but in the latter, it
did. This distinction was important because the fl ow of output-valve grid current in
AB2 made the design of the previous stage much more diffi cult.
AB1 or AB2 has no relevance to semiconductors, for base current in BJT always fl ows
when a device is conducting, whereas gate current in power FET never does, apart from
charging and discharging internal capacitances.
10.4.2 Class-AB
This is not really a separate class of its own, but a combination of A and B. If an amplifi er is
biased into Class-B and then the bias increased further, it will enter AB. For outputs below
a certain level, both output devices conduct and operation is Class-A. At higher levels, one
device will be turned completely off as the other provides more current, and the distortion
jumps upward at this point as AB action begins. Each device will conduct between 50 and
100% of the time, depending on the degree of excess bias and the output level.
Class-AB is less linear than either A or B, and in my view its only legitimate use is as a
fallback mode to allow Class-A amplifi ers to continue working reasonably when faced
with low-load impedance.
10.4.3 Class-B
Class-B is by far the most popular mode of operation, and probably more than 99% of the
amplifi ers currently made are of this type. Most of this book is devoted to it, so no more
is said here.
10.4.4 Class-C
Class-C implies device conduction for signifi cantly less than 50% of the time and is
normally only usable in radio work, where an LC circuit can smooth out the current