technology and zen of life

“A heisenbug (named after the Heisenberg Uncertainty Principle) is a computer bug that disappears or alters its characteristics when an attempt is made to study it.”

Reverse-engineering the cascode

(from –

Cascode (shown in Fig. 1) is well known and widely used circuit for creating large-impedances in integrated circuits. Cascode circuits also provide another advantage i.e. isolation between input and output ports and thereby reducing the Miller-effect and as a result cascodes have a good frequency performance. And when impedances achieved from cascode circuits are not good enough, we go on to use active-cascodes or regulated-cascodes. Even though cascodes are affected by the limitations of headroom (voltage swing), there are work-arounds for this problem that have been figured out by circuit designers (the smart engineers again). The question that I want to discuss here is – how did someone (must be a genius) think of this circuit which provides an elegant area-efficient solution for achieving large impedances consequently large voltage gains?

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Why do we perform small signal analysis?

Why do we perform small signal analysis, when we are analyzing or designing analog circuits? This is a fundamental question that I think, is not discussed rigorously enough in the classes or text books on analog circuit design. I will attempt to qualitatively address this question here in this blogpost.

Fig. 1 a) Small-signal MOSFET model and b) small-signal bipolar model

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