Page 153 - The Art and Science of Analog Circuit Design

Carl Battjes

1 stopped my slide into obsolescence in 1971 by doing a little downward mobil-
ity. I left the small portable oscilloscope group I headed, and joined George Wilson
in the 1C group as a designer. This foresight on my part was most uncharacteristic.

rCoiis with Integrated Circuit Vertical Amplifiers

The initial use of integrated circuits in the vertical amplifiers of Tektronix
'scopes supplied a huge bandwidth boost, but not just because of the high
f t. New processes included thin film resistors that allowed designers to
put the small value emitter feedback resistors on the chip, thus eliminat-
ing the connection inductance in the emitters of transistors. That emitter
inductance had made a brick wall limit in bandwidth for discrete transis-
tor amplifiers. That wall was pretty steep, starting in the 150-200MHz
area. In order to have flat, non ripple, frequency response at VHP and
UHF, the separately packaged vertical amplifier stages needed to operate
in a terminated transmission line environment. T-coils were vital to
achieve this environment. Thor Hallen derived formulas for a minimum
VSWR T-coil. Packaging and bond wire layout made constant-resistance
T-coil design impossible. Hallen's T-coil incorporated and enhanced the
base connection inductance. The Tektronix 7904 achieved 500MHz
bandwidth by using all of the above, along with 3GHz transistors and
an ft-doubler amplifier circuit configuration.
In 1979, the IGHz 7104 employed many of the 7904 techniques but,
in addition, had 8GHz f, transistors, thin film conductors on substrates,
and a package design having transmission line interconnects. It also had
a much more sensitive cathode ray tube. Robert Ross had earlier devel-
oped formulas for a constant-resistance T-coil to drive a non-pure capaci-
tor (a series capacitor-resistance combination). John Addis and Winthrop
Gross made use of the Ross type T-coils (patterned with the thin film
conductor) to successfully peak the stages and terminate the inter-chip
transmission lines.
I have lumped Thor Hallen's and Bob Ross's T-coils together in a class
I call "lossy capacitor T-coils."

Dual Channel Hybrid withT-Coils

In 1988, the digitizing IGHz Tektronix 11402 was introduced. A fast
real-time cathode ray tube deflection amplifier was no longer needed.
T-coils were employed, however, in the 11A72 dual-channel plug-in pre-
amp hybrid (Figure 10-12), where all of the two-channel analog signal
processing took place. The T-coils peaked frequency response and mini-
mized input reflections in the 50 Ohm input system. As in the 7904
'scope, Hallen used a design technique for the T-coils that minimized
VSWR. To realize this schematic, a T-coil was needed which had

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