Page 142 - The Art and Science of Analog Circuit Design

Carl Battles

to feed the resistor, also producing a voltage equal to IR. No current is
wasted in the resistor while the capacitor is charging.
A current step applied to the constant-resistance bridged T-coil yields
the same capacitor voltage risetime, 0.8 RC, as the elf circuit. In both
cases, during the rise of voltage on the capacitor, the voltage waveform
on the termination resistor is negative, zero, or at least low. Without the
helpful elf, or without the T-coil, the risetime would have been 2.2 RC,
With these risetime enhancers, the risetime is lowered to 0.8 RC. This is
a risetime improvement factor of 2.75. If there are two or more capacitor
lumps, Principle Number One can combine with Principle Number Two
to. obtain even higher risetime improvement factors.
When both principles are working optimally, reflections, overshoot,
and ringing are avoided or controlled. This is a matter of control of en-
ergy flow in and out of the T-coil section reactances. A T-coil needs to be
tuned or tolerated. In the constant-resistance T-coil section, given a load
capacitance, there is only one set of values for the inductance, mutual
inductance, and bridging capacitance which will satisfy one set of speci-
fications of the driving point resistance (may imply reflection coefficient)
and desired damping factor (relates to step response overshoot).

T-Coils Peaking Capacitance Loads
A cathode ray tube (CRT) electrostatic deflection plate pair is considered
a pure capacitance load. In the '50s and '60s, T-coils were often used in
deflection plate drive circuits. Usually a pentode-type tube was used as
the driver, rather than a transistor, because of the large voltage swing
required. The pentode output looked like a eapacitive high-impedance
source. A common technique was to employ series peaking of the driver
capacitance, cascaded with T-coiled CRT deflection plate capacitance.

The 10-MHz mtrnnix 3A6
The 3A6 vertical deflection amplifier works really hard. The 3A6 plug-in was de-
signed to operate in the 560 series mainframes, where the plyg«s drove the
CRT deflection plates directly. The deflection sensitivity was poor {20 volts per
division) and the capacitance was high. To cover the display serein linearly and
allow sufficient overscan, the output beam power tube on each side had to tra-
verse at least 80 volts. The T-coils on the 3A6 made the bandwidth and dynamic
range possible without burning up the large output vacuum tubes.

A RealT-Coil Response

A vertical-output deflection-amplifier designer has a unique situation—
the amplifier output is on the screen—no other monitor is needed. This
is the case with the 3A6 circuit shown here. The input test signal is clean
and fast. The frequency and step response of the entire vertical system
is dominated by the "tuning" of the T-coil L384 and its opposite-side

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