Getting Hands-On with a MOSFET Common Source Amp
Alright, so the other day I got the itch to build something simple, get back to basics. Decided to mess around with a MOSFET common source amplifier. I always find it neat to see these little transistors work their magic, turning a tiny signal into a bigger one.
First things first, had to dig through my parts bins. Found a trusty N-channel MOSFET, I think it was a 2N7000, nothing fancy. Grabbed a handful of resistors – you always need more resistors than you think – and a couple of capacitors for coupling the signal in and out. Oh, and my trusty breadboard, power supply, signal generator, and scope. Can’t do much without those!
Putting it Together on the Breadboard
I didn’t overthink the circuit too much. The standard common source setup is pretty straightforward. Needed a couple of resistors to set the DC voltage on the gate – just a simple voltage divider connected to my power supply rail (I used 9V for this). Then a resistor on the drain, connected between the drain pin and the positive supply. This one’s important because the output signal develops across it. Added capacitors on the input and output to block any DC voltage from getting where it shouldn’t.
Actually putting it on the breadboard took a few minutes.
- Popped the MOSFET into the board. Had to double-check the pinout – Gate, Drain, Source – easy to mix up.
- Wired up the gate biasing resistors first. Connected one from 9V to the gate, the other from the gate to ground.
- Connected the drain resistor from the drain pin up to the 9V rail.
- Connected the source pin directly to ground for this simple version.
- Added the input capacitor, connecting my signal input point to the gate.
- Added the output capacitor, connecting the drain pin to my output point.
- Finally, connected the power supply +9V and ground rails to the breadboard.
Powering Up and Seeing What Happens
Okay, time for the smoke test… just kidding (mostly!). Turned on the power supply, set it to 9V. First thing, grab the multimeter. Measured the DC voltage at the gate – looked about right, matched what my resistor divider should have been giving. Then checked the drain voltage. Was aiming for something around half the supply voltage, maybe 4.5V, to give the signal room to swing up and down. It was a bit off initially, so I swapped out the drain resistor for a slightly different value until the DC voltage at the drain looked better. You often have to tweak things a bit in practice.
Now for the signal test. Hooked up my function generator to the input capacitor. Set it to a sine wave, about 1 kHz, and kept the amplitude pretty small to start. Connected one probe of my oscilloscope to the input point (after the capacitor) and the other probe to the output point (after the output capacitor).
And there it was! On the scope, I could see the small input sine wave, and right below it, a much larger sine wave at the output. Crucially, the output wave was upside down compared to the input – inverted, exactly what you expect from a common source amplifier. That inversion still trips me up sometimes conceptually, but seeing it on the scope makes it clear.
I played around a bit, increased the input signal amplitude. Eventually, I saw the output signal start to flatten out at the top and bottom – clipping. This means the amplifier couldn’t swing the voltage any further. Usually happens if the input signal is too large or the bias isn’t quite optimal. I reduced the input signal again to get a clean, amplified wave.
So yeah, it worked! A simple little amplifier built on a breadboard. Didn’t measure the exact gain, but it was definitely amplifying. It’s always satisfying building these fundamental circuits and seeing the theory actually play out with real components. Simple stuff, but a good way to spend an hour or two.