Page 139 - The Art and Science of Analog Circuit Design

Who Wakes the Bugler?

of this basic circuit block are used with precision and finesse to do the
following:

Peak capacitive loads
Peak amplifier interstages
Form "loop-thru" circuits
Equalize nonlinear phase
Transform capacitive terminations to resistive terminations
Form distributed deflectors in cathode ray tubes
Form artificial delay line sections
Form distributed amplifier sections

I have successfully used T-coils in all of these applications except the
last two. Recently, however, some successful designers from the *40s and
'50s shared their experiences with those two applications.

Over My Head
While on a camping trip in Oregon in 1961,1 stopped at Tektronix and received an
interview and a job offer the same day. Tektronix wanted me. They were at a stage
where they needed to exploit transistors to build fast, high-performance 'scopes. I
had designed a 300MHz transistor amplifier while working at Sylvania. In 1961,
that type of experience was a rare commodity. Actually, I had designed a wide-
band 300MHz IF amplifier that only achieved 200MHz. What we (Sylvania) used
was a design that my technician came up with that made 300MHz. So I arrived at
this premier oscilloscope company feeling somewhat of a fraud. I was more than
just a bit intimidated by the Tektronix reputation and the distributed amplifiers and
artificial delay lines and all that "stuff" that really worked, The voltage dynamic
range, the transient response cleanliness, and DC response requirements for a
vertical output amplifier made my low-power, 50 Ohm, 300MHz IF amplifier seem
like child's play. Naturally, I was thrown immediately into the job of designing high-
bandwidth oscilloscope transistor vertical-output amplifiers. I felt tike a private,
fresh out of basic training, on the front lines in a war.

The Two Principles of Inductive Peaking
The primary and most obvious use of a T-coil section is to peak the fre-
quency response (improve the bandwidth, decrease the risetime) of a
capacitance load. Inductances, in general, accomplish this through the
action of two principles.

Principle Number One: Separate, in Time, the Charging of Capacitances
The coaxial cable depicts a limiting case of Principle Number One. A
coaxial cable driven from a matched-source impedance has a very fast
risetime. The source has finite resistance and the cable has some total
capacitance. If the cable capacitance and inductance are uniformly distrib-

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