I'm new to electrical engineering, and I'm a little stuck. I've got this circuit that I want to make, but I don't have any bjt transistors, and I can't buy any right now. I was wondering how base, collector, and emitter would relate to gate, drain, and source.

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    \$\begingroup\$ This is too broad. You need to understand how both types of transistors work, then you can tell what the relation is, and it is a big topic. \$\endgroup\$
    – Eugene Sh.
    Feb 4, 2021 at 15:23
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    \$\begingroup\$ What little circuit? You haven't shown it. \$\endgroup\$
    – Andy aka
    Feb 4, 2021 at 15:25
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    \$\begingroup\$ They are quite different. Just for starters, the MOSFET is activated by voltage between the gate and source, while a BJT is activated by current through the base. This is just the tip of the iceberg, however, and to make any meaningful replacement you need to be pretty familiar with the differences and what you are trying to accomplish. \$\endgroup\$ Feb 4, 2021 at 15:26
  • \$\begingroup\$ First decide ,how to linearize or how to switch function , then how to bias base current vs Gate Voltage above threshold \$\endgroup\$ Feb 4, 2021 at 16:42

1 Answer 1


Well, they relate quite straightforwardly, in theory: Base becomes Gate, Collector become Drain, Emitter become Source.

However they work in a substantially different way: a BJT controls the current between collector and emitter more-or-less proportionally to the current from base and emitter. A MOSFET instead controls the current between drain and source (similar working here) with the voltage between gate and source (totally different thing). A BJT takes a current and gives a current, a MOSFET takes a voltage and gives a current.

They are not really easily interchangable and the polarization areas have different effects (also the parasitics are wildly different).

That said, most (if not all) the circuit topologies have their equivalent: the linear amplifier topologies and the simple switch are the same and 'only' need a recomputed bias. In fact the simple low side load switch usually only need the base/gate resistors to be changed.

Post the circuit and we could elaborate on that.


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