BJTii

BJTii is a hypothetical, educational class of bipolar junction transistor variants intended to illustrate the behavior of composite transistor structures in analog circuit design. It does not correspond to a defined device in standard catalogs or industry practice, but appears in textbooks and circuit simulators as a teaching abstraction for studying current gain, input-output coupling, and thermal effects in multi-transistor configurations.

Conceptually, a BJTii consists of two bipolar junction transistors integrated in a single package with a common node (for example a shared collector) and separate bases (or emitters) that allow the two devices to be connected to realize a range of configurations such as cascaded gain stages or cascode-like arrangements. The overall transconductance and current gain depend on the gains (beta) of the constituent transistors and the chosen interconnection.

In practice, BJTii is used to illustrate how current gain can be amplified by combining devices while preserving or distributing base drive and how thermal coupling affects operating points. It is also used in simulations to compare with real devices such as the Darlington pair, Sziklai pair, or cascoded BJTs, highlighting tradeoffs in noise, bandwidth, and stability.

Because BJTii is a pedagogical construct rather than a standard component, actual physical implementations must rely on real devices. When designing circuits, engineers typically choose conventional configurations such as a Darlington pair, a Sziklai pair, or a true cascode depending on required gain, bandwidth, and drive capability.