IEEE Solid-States Circuits Magazine - Winter 2021 - 30

Brokaw is a consummate master of the use of
on-chip feedback loops that exploit the unique
benefits of monolithic realization.
high common-mode rejection, and a
well-defined gain to a single-ended
output with a user-defined common
voltage. This is the cornerstone of
most sensor interfaces. The first
instrumentation amplifier employed
a nested feedback loop that is hard
to compensate for stability (Figure 3).

Although monolithic, to a certain degree, its topology reflects
a discrete op amp legacy. In 1975,
Brokaw advanced it [3] by breaking
the nested feedback into two independent feedback loops (Figure 4)
that are much easier to stabilize
individually. In the presence of large

V+
Out

-
A
+
-In

B
Q1

+In

Q3
RS

Sense
Ref.

l4
V-

FIGURE 3: A pre-Brokaw Analog Devices instrumentation amplifier, which requires two pairs
of well-matched, differentially controlled current sources. Image from [3].

V+

-In

+In

Sense
Ref.

RS
l1
l3

Out

-
A
+

l6
V-

FIGURE 4: Brokaw's instrumentation amplifier. This provides fully differential signal processing up to the output. The topology anticipates the folded cascode.

W I N T E R 2 0 2 1

IEEE SOLID-STATE CIRCUITS MAGAZINE

differential input voltages, the input
stage transistors carry a constant
current, transferring the input voltage accurately to a degeneration
resistor. This method would be reinvented in the 1980s for linear CMOS
transconductors used in continuoustime Gm-C analog filters. The input
stage is specially designed to reject
common-mode stimuli very effectively, reaching a common-mode
rejection ratio of 120 dB.
Perhaps Brokaw's least well-known
contribution to the community-
although it may well be his second
most important-is the development
in 1977 of a 10-b analog-to-digital converter (the AD571), followed shortly
by a 12-b converter (the AD574) that
uses a successive approximation
algorithm and in which the digital-toanalog converter, successive approximation and control logic, comparator,
clock, and, of course, voltage reference are, for the first time, all integrated on a single chip [4]. Previous
products were multichip modules.
The AD574 has been called the " 741 of
data converters, " referring to the part
number of the most famous IC of all
time, the 741 op amp. To this day, this
converter and its derivatives remain
listed in the Analog Devices, Texas
Instruments, and Maxim catalogs.
There are numerous uses for such a
converter, especially when it is priced
as a commodity item.
12-b linearity in a monolithic realization is hard to achieve at the best
of times: that Brokaw obtained it is
due largely to a brilliant innovation,
which enables the realization of a
binary-weighted array of bipolar current sources using a thin-film, lasertrimmed R-2R ladder. This requires
equal emitter voltages across 12 BJTs
biased at the same base voltage and
degenerated by the resistor ladder,
where the currents through adjacent
transistors change by a ratio of 2:1.
To maintain equal current density,
the emitter area would have to be
scaled at 2:1 as well, which means
that the largest emitter area would
be 4,000 times the smallest. This
was completely impractical at the

IEEE Solid-States Circuits Magazine - Winter 2021

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