High-Speed Circuit Design Principles:Large-Signal Transconductance

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Large-Signal Transconductance

Device transconductance, the change in collector (or drain current) with change in the input voltage, must be determined by the circuit context for the particular device. That is, if the input voltage varies little, as in the case of an emitter follower, then a small signal gm is most relevant. For a bipolar transistor

High-Speed Circuit Design Principles-0017

and the current in each device, DIC, varies from zero to ICS since the device switches between cutoff and ICS. Because GM is governed by the internal device VBE and there is always some parasitic emitter resistance, REX, then large signal GM is given by

High-Speed Circuit Design Principles-0018

Note that GM will be considerably smaller than the small signal gm because the voltage swing DVLogic is much greater than kT/q. This is beneficial for delay as shown in the next section.

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