Building a Simple Transistor Amplifier

So… you’ve probably heard the word “transistor” before, right?. It’s one of those tiny things you find inside computers or other electronics. Great if you know what I’m talking about, but if not don’t worry! In this article we’re not going to crack open a CPU. This time, we’re going to play with one single transistor and see how it can take a small/weak signal and blow it up into something much stronger.

My brain had to struggle a bit to remember the workings of this circuit (cause it’s been four years since I touched any electronics) so please don’t feel intimidated at all. We all start somewhere and if today marks the beginning of your journey in electronic, it’s a privilege for me to be doing this exploration with you.

The Circuit we’re building

Here’s the diagram (don’t worry if it looks scary): Our main components are:

• 12-Volts power supply
• A simple NPN transistor BC107
• A few resistors and capacitors arranged in a particular way, which we’ll understand by the end of the post

On the input side

We start with a little AC signal, which is just a voltage that varies over time. But here’s the thing: transistors don’t like it if you just slam an AC signal right into them. They need to be “biased”, which means giving them a steady DC level to sit on. That’s why we use a coupling capacitor (C1 in the schematic). Without the capacitor, the input’s DC level would push the transistor out of its “sweet spot” called the bias point, and the whole amplifier would stop working properly. The capacitor solves this by blocking the DC while still letting the wiggly AC signal through.

Now, why is the C1 = 10µF there? Capacitors act like a kind of “frequency gate”: the smaller the capacitor, the more it resists low-frequency signals. The bigger it is, the more low frequencies it lets through. In designing a circuit, a good rule of thumb is to pick a value big enough so that even the lowest frequency of interest passes without much weakening.

But that’s not the only piece of the puzzle! Right after the capacitor you’ll see a 1 kΩ resistor (R6). This resistor sets the input impedance of the amplifier stage and works together with the capacitor to form what we call a high-pass filter (probably a topic for another time).

Biasing the Transistor

In order to get the transistor to work, we need to bias it; or in a more modern term, we need to “configure” it. The transistor has three legs:

• Base (B)
• Collector (C)
• Emitter (E)

For the transistor to amplify properly, we need to feed the base a steady little DC voltage. The exact value of the voltage we need in this case can be deducted from the datasheet of the BC107. I will dedicate an entire post on design calculations from datasheet otherwise this article might get too lengthy x)

Boosting the Signal

Now the magic happens. When a tiny current flows into the base, a much bigger current flows from the collector to the emitter. The ratio between these current values is called gain, and this is what we’re after.

On the output side

Finally, we connect a resistor (R5) and maybe an oscilloscope like in our case. On the scope you’ll see the input sine wave has grown way taller than our input, and that’s pretty much the transistor acting like a signal booster. If you bypass R3 with a capacitor (C3), you give the AC signal a shortcut around it, which increases gain even more.

Wrapping Up

Hopefully this post gave you an introduction to one of the most fundamental circuit blocks of electronics. If you got a simulation software (called SPICE software), you can literally build this circuit yourself and watch the waveform grow.

Back when I was in engineering school, my room used to be full of components and wires. But the cool thing about simulation software is that you can build and learn electronics at zero cost from the comfort of your bed. I use the Multisim software because it is the richest in terms of features in my opinion, compared to others like LTSPICE or QSPICE, but feel free to experiment and see what works for you!

Thanks for reading!