Page 150 - The Art and Science of Analog Circuit Design

Who Wakes the Bugler?

In contrast with the artificial delay lines and the distributed amplifiers,
the individual peaking applications usually needed a coil with more cou-
pling (k = 0.4 to 0.5), which was realized by a coil shorter than its diame-
ter. When the coil value is near or below 100 nanohenries, the goal is
then to get as much coupling as possible so that the lead inductance of
the center tap connection can be overcome. Flat pancake or sandwich
coils of thin PC board material, thin films, or thick films are used to
achieve high coupling.

The Importance of Stray Capacitance in T-Coils

The stray interwinding capacitance of a T-coil can be crudely modeled by
one bridging capacitance C bs across the whole coil. It is defined by the
coil self-resonance frequency "f res."

where Lj is the coil total inductance. If C B is the required bridging capaci-
tance for constant-resistance proportions, then C x=C b-C bs needs to be
added. This is an effective working approximation. The recent coils built
for high-frequency 50 Ohm circuits usually need additional bridging ca-
pacitance. On the other hand, the old nominally m-derived circuits never
needed any added bridging capacitance. They were high-impedance cir-
cuits with very large coils and probably had enough effective bridging
from the stray interwinding capacitance. They were probably constant-
resistance coils in disguise. Capacitance to ground of the coil body is al-
ways a significant factor also.

Interstage Peaking

The Tektronix L and K units of the '50s were good examples of inter-
stage T-coil peaking. The T-coils were used to peak, not the preamp input
or the output, but in the middle of the amplifier. The interstage bandwidth
was boosted well above the

f, _ 1 _ 8* ^ g« f'
linterstage = ~
2nRiCtotai gain 2nCt«tai gain 2nCsubtotai gain
The individual pre-amp bandwidths are 60MHz. This is amazing be-
cause the effective f t of the tubes was only 200MHz or so. Both inductive
peaking and f t doubling techniques were needed to "hot rod" these plug-
ins to this bandwidth.

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