In yesterday's post, we discussed the differences in transistor types: BJTs and FETs. Today I thought we'd dig a little deeper.
On paper, a JFET is just another transistor, but it didn't behave like the BJTs I was used to implementing in our designs. It reminded me more of a vacuum tube than a solid-state device—and once I listened to it, I understood why.
FETs—field-effect transistors—operate differently from bipolar junction transistors. Where a BJT is controlled by current, a FET is controlled by voltage. That one shift changes everything. The result is a device that behaves more like a tube: graceful overload characteristics, sweet and warm, and a kind of openness in the midrange that’s hard to get with anything else.
There are different kinds of FETs, and knowing the differences is key if you care about how a circuit sounds. I’ve always loved JFETs—junction field-effect transistors—for input stages. They’re quiet, linear, and natural-sounding. When you drop a JFET at the front end of a phono stage or DAC, the music gains this dimensionality. You hear body. You hear space. The sound breathes.
MOSFETs—metal-oxide FETs—are a different animal. They’re stronger, able to pass a lot more current, which makes them better suited for output stages or drivers in amplifiers. But they come with a tradeoff: higher input capacitance and trickier bias settings (how you turn them on).
There’s another layer too: depletion-mode vs. enhancement-mode. Depletion-mode FETs conduct by default—you have to bias them to shut off. Enhancement-mode FETs are the opposite; they’re off until you bias them on. That changes how you design the circuit, how it starts up, how it recovers from overload.
For me, designing audio gear isn’t just about choosing the right devices for the job. It’s about understanding how those devices behave, how they interact, and how they sound when they’re working together. That’s the art of it. That’s what turns a schematic into something you can feel.
Each type of FET brings something unique to the table. The trick is knowing where to use which—and how to listen when it’s right.
Want to dig deeper? I still have a limited number of complete Audiophile's Guide sets avaialble. I think I am down to the last 20 or so. Once they are gone, that's it.
