Op Amp Applications from Analog Devices - Get a Free Blog

Evolution of the Vacuum Tube Op Amp
OP AMP HISTORY
VACUUM TUBE OP AMPS
Nevertheless, Julie’s op amp design was notable in some regards. It had a better input
stage— due to the use of a long-tailed 6SL7 dual triode pair, with balanced loads. This
feature would inherently improve drift over previous single-ended triodes or pentodes.
As will be seen shortly, a truly key feature that Julie’s circuit held over previous singleended
input designs was the basic fact that it offered two signal inputs (inverting and noninverting)
as opposed to the single inverting input (Fig. H-3, above). The active use of
both op amp inputs allows much greater signal interface freedom. In fact, this feature is
today a hallmark of what can be called a functionally complete op amp— nearly 60 years
later! The differential input stage not only improved the drift performance, but it made the
op amp immeasurably easier to apply. Ironically however, there was still some time
before the application of op amps caught up with the availability of that second input.
Much other work was also done on the improvement of direct-coupled amplifiers during
the war years and shortly afterwards. Stewart Miller, Edward Ginzton, and Maurice Artzt
wrote papers on the improvement of direct-coupled amplifiers, addressing such concerns
as input stage drift stabilization against heater voltage variations, inter-stage coupling and
level shifting schemes, and control of supply impedance interactions (see References 43-
45). Some additional examples of improved dc amplifiers can be found in the Valley-
Wallman book (see Reference 46).
Before the 1940's expired, companies were already beginning to capitalize on op amp and
analog computing technology. Seymour Frost wrote about an analog computer developed
at Reeves Instrument Corporation, called REAC (see Reference 47). This computer used
as its nucleus an op amp circuit similar to the Swartzel M9 design. In the Reeves circuit
the first stage was changed to a 6SL7 dual triode, used in a Miller-compensated low drift
setup (Reference 43, again).
Chopper Stabilization of the Vacuum Tube Op Amp
But, even with the use of balanced dual triode input stages, drift was still a continuing
problem of early vacuum tube op amps. Many users sought means to hold the input
referred offset to a sub-mV level, as opposed to the tens to hundreds of mV typically
encountered. The drift had two components, warm-up related, and random or longer term,
both of which necessitated frequent re-zeroing of amplifiers. This problem was at least
partially solved in 1949, with Edwin A. Goldberg’s invention of the chopper-stabilized
op amp (see Reference 48).
The chopper-stabilized op amp employs a second, high gain, AC-coupled amplifier. It is
arranged as a side-path to the main amplifier. The chopper channel is arranged with the
input signal path AC-coupled to the inverting input of the main DC-coupled amplifier,
and a 60 or 400Hz switch periodically commutating to ground. The switching action
chops the small DC input signal to AC, which is amplified greatly (1000 or more). The
AC output of the chopper path is synchronously rectified, filtered, and applied to the main
H.17