IEEE Solid-States Circuits Magazine - Summer 2022 - 22

bias current, where the full scale is
jacked up to a whopping 12 V. This
is the maximum that the transistors
can withstand before the reversebiased
emitters go into Zener breakdown
. . . which is destructive.
The flatness of the parabola causes
-2
-1
Input Voltage (V)
FIGURE 9: The measured cross plot for the three outputs of a 4-b merged-function wiggler.
Maximum Output Amplitude
(Calculated Output at the Center of the Range)
-55 °C 25 °C 125 °C
1
-1
0.01
0.001
0.0001
0.00001
5 b6 b
1
2
3 b 4 b
another problem. At low full-scale
voltages, as base voltages get closer
and closer together, the idealized
notion that just one or two transistors
carry all of the tail current starts
to fail. In fact, the follower current
from one source can be spread over
three of four of the NPNs at the same
time. This explains the extra emitters
in Figure 1. A 4-b Gray code LSB has
eight transitions, so nine emitters
should do the trick. While this works
for large full-scale settings, the end
effects corrupt the output for the low
full scale-hence, the two extra emitters
on each end of Figure 1. (This
effect is not accounted for in Figure
10, so it is slightly optimistic.)
110
Number of Emitters
FIGURE 10: Here is what really kills this ADC. Each additional bit of resolution (vertical lines)
requires twice the number of " wiggles " in the LSB and hence twice the number of emitters on the
base resistor. This reduces the output amplitude dramatically, making it impractical beyond 4 bits.
TECHNICAL TAKEAWAYS
Well, the merged-function, superintegrated wiggler was ultimately denied the Nobel Prize. The
failure to win universal acclaim was a cruel injustice that would play out many times in the
career of our underappreciated grad student. However, there are still several good ideas in the
wiggler design that you can make use of:
■ Bit-slice architecture: Among the highest-speed options for ADCs, this one is arguably the
smallest, but it does depend on how the wiggles are made. It has been implemented in
all kinds of creative ways and lends itself to unusual technologies, like electro-optics [7].
■ The moving parabola: The ability to move a voltage profile around in space is a neat trick
to have up your sleeve. My favorite application: the alphanumeric character readout in
the old Tektronix oscilloscopes [2].
■ Wired-or connection: This is a useful hack for all sorts of analog functions. Use it to make an absolute
value circuit or find the maximum/minimum within a group of signals. It can be blazingly fast.
■ Custom transistor design: The extreme lithography constraints and the strain interactions
among transistor pieces in modern ICs certainly discourage improvisation. Merged-function,
as it was practiced in 1981, may not be desirable or even possible anymore. However, the
basic theme, which is that layout is itself a design variable, should not be forgotten. It is still
alive at the very highest frequencies where engineers craft exotic structures for microwaves
and beyond. Who knows? Future technologies may one day open that door again for all of us.
22 SUMMER 2022
100
Thesis Defense
Stop booing for a minute and remember:
this is all happening back in 1981,
when the world is shrouded in darkness.
Ignorance walks the Earth, and
there is much wickedness and bipolar
technology among the people.
Just two years earlier, Advanced
Micro Devices launched its landmark
AM6688, also a 4-b ADC, which
boasted about its large input range
of 6 V! This beast drew up to 100 mA
from 13-V supplies. By comparison,
our nascent wiggler would require
only a quarter of the comparators,
plus another 10 mA for biasing, in
just a fraction of the die area. (See
" How Good Is It? " ) That's why our
young hero is not being laughed out
of graduate school.
Also, the superintegrated prototype-without
being explicitly optimized
for speed-can settle after
large transients in 10 ns. So, it is at
least in the ballpark to compete with
the breathtaking 100 MS/s that the
Am6688 can do. Not bad.
Epilogue
Time moves on. Four bits soon
became inadequate. Bipolar fell out
IEEE SOLID-STATE CIRCUITS MAGAZINE
Output (V)
Output Voltage (Mixed Scales)

IEEE Solid-States Circuits Magazine - Summer 2022

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