Page 151 - The Art and Science of Analog Circuit Design

Carl Battjes

T-Coils in Transistor Interstages
The 150MHz 454 evolved from the 50MHz 453 oscilloscope by adding distributed
deflection plates to the cathode ray tube and, among other things, using a new
output amplifier. This amplifier employed T-coil peaking in the interstages. The T-
coil design was based on a lossless virtual capacitance, a very big approximation.
This virtual capacitance at the base was dominated by the transformation of the
emitter feedback admittance into the base. The emitter feedback cascode connec-
tion made two transistors function more like a pentode. The initial use of transis-
tors in the early '60s showed us that, most of the time, vacuum tube techniques
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didn't work with "those blasted transistors.' After all, vacuum tubes had a physical
capacitance that was measurable on an "off"tube; transistors had this 'Virtual
capacitance thing/! The conventional thinking in the design groups atTek in the
early and mid '60s was that inductive peaking and transistor high-fidelity pulse
amplifiers were not compatible. Despite this, the "toils and transistors did work,
the 454 worked and the 454 was a "cash cow" for Tektronix for several years.
Since then, ICs have displaced discrete transistors and the 'scope bandwidths
translated upwards, with and without T-coils. The fastest amplifiers, however, are
always produced with the aid of some T-coil configuration. Tektronix 454
Vertical-Output
Amplifier and
Interstage T-coil,

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