In a previous post, I mentioned using a MOSFET to short out a resistor. So how does that work?

Well, a MOSFET is like a voltage-controlled switch. To be more precise, an N-channel enhancement type MOSFET is like an infinite resistance when the gate-to-source voltage is zero, and turns into a very low resistance when the gate-to-source voltage is a few volts positive.

To examine this in more detail, I created a test setup like this:

By applying a linear ramp voltage on the gate, we can see what it does with varying voltages. When open, the output should be 5V, and when conducting, it should drop to almost 0V. Let’s examine this in real life:

The blue line is the input voltage on the gate (by definition a sloped straight line), and the yellow line is the voltage on the output (i.e. between drain and resistor). Let’s try and read this:

• the gate voltage takes 10 divisions to reach 3V, so that’s 0.3 V/div
• the MOSFET starts conducting at around 1.8V and is fully on at ≈ 2.4V
• at slightly over 2.1V, the drain-to-source resistance is about 1 kΩ

The red trace is the derivative of the output, so the output change is maximal at just over 2.2V.

There’s no linear behavior, in terms of gate-to-source voltage (the derivative is never constant, except in the fully-open and fully-closed regions), but what you can see is that the MOSFET will switch just fine with a logic signal (anything switching between under 1.8V and over 2.4V will work perfectly).

There are more ways to look at this. Here’s an X-Y plot, with the linear ramp on the horizontal axis:

Note that – if you think about it – in X-Y mode, it doesn’t really matter what sort of signal is placed on the gate as long as it has the same voltage range. Here’s a sine wave to illustrate this perhaps somewhat surprising property:

It’s a good exercise to try and understand exactly why the two above screenshots are the same.

Lastly, here is a zoomed-in measurement, to get more precise data using the scope’s cursor features:

As you can see, a 0.33V change on the gate is all it takes between the “almost-OFF” and “almost-ON” states.

I’ll leave it as exercise for the reader to plot the resistance of this particular MOSFET at different gate voltages. With a bit more setup, the scope’s math functions should in fact be able to display that plot on-screen.

So there you have it: a MOSFET switches on voltage, and a scope + function generator makes it easy to see that behavior. Note that even without these instruments, with nothing more than a potentiometer and a multimeter, you could in fact derive exactly the same information. It would merely be a bit more work.