Electronic – What’s the difference between field-effect transistors (FETs) marketed as switches vs. amplifiers


For instance, the J108 JFET is listed as "N-Channel Switch", and the datasheet mentions the RDS on resistance, while the J201 JFET is listed as "N-Channel General Purpose Amplifier" (and the on-resistance would have to be deduced from the IDS curves?)

Is there a difference in the way these are designed and manufactured? Can one type generally be used in the other application, but not vice versa?

Related, for BJTs: What's the difference between small signal bipolar junction transistors (BJTs) marketed as switches vs. amplifiers?

Best Answer

There are various choices that can be made in the design of transistors, with some tradeoffs being better for switching applications and others for "linear" applications.

Switches are intended to spend most of their time fully on or fully off. The on and off states are therefore important with the response curve of the in-between states being not too relevant.

For most applications, the off state leakage current of most transistors is low enough to not matter. For switching applications, one of the most important parameters is how "on" on is, as quantified by Rdson in FETs and the saturation voltage and current in bipolars. This is why switching FETs will have Rdson specs, not only to show how good they are at being fully on, but because this is also important for designers of the circuit to know how much voltage they will drop and heat they will dissipate.

Transistors used as general purpose amplifiers operate in the "linear" region. They may not be all that much linear in their characteristics, but this is the name used in the industry to denote the in-between range where the transistor is neither fully on nor fully off. In fact, for amplifier use you want to never quite hit either of the limit states. The Rdson is therefore not that relevant since you plan to never be in that state. You do however want to know how the device reacts to various combinations of gate voltage and and drain voltage because you plan to use it accross a wide continuum of those.

There are tradeoffs the transistor designer can make that favor a more proportional response to gate voltage versus the best fully on effective resistance. This is why some transistors are promoted as switches versus for linear operations. The datasheets then also focus on the specs most relevant to the circuit designer for the intended use.