In case you miss it in the explanation below, you’ll want to remove R1 and C1 for most purposes as they will attenuate audio frequencies.
One thing I’ve noticed about electronics is the lack of simple, newbie-friendly information on the internet about specific topics. Then again, maybe I just wasn’t looking properly. To help fill this possibly existent information gap, I’m going to post a nice, simple schematic and an explanation on what it is and how it works.
This post goes along with a video that I posted. You don’t need to watch the video to understand the circuit, but the video demonstrates the circuit in action. Here it is:
So, let’s get down to business! So, here is the schematic image we’ll be working with:
The circuit you see here is a transistor amplifier circuit. It’s identical to the one seen in the video. This very simple circuit can be used to amplify signals of all kinds. Audio, radio, whatever. You may notice that it says “To antenna”. This is because in the video, it was used to amplify electrical noise.
The 1.5k resistor R1 and the 220nF capacitor C1 form a low-pass filter. A low pass filter, in brief, is a filter that only allows signals lower than the cut-off frequency to pass through. I used this because without the filter, it would pick up AM radio signals and distort the noise (hehe). If you remove that, then you’ll hear a bunch of jumbled AM radio signals. It’s pretty cool! The low-pass filter’s cutoff frequency is determined by a formula that involves the value of the resistor and capacitor used. It’s called an RC Filter (Because of the Resistor and the Capacitor). You can find many calculators online for the cutoff frequency of such a filter, just look up “RC filter calculator”. Unless you want to attenuate all frequencies above ~500Hz, you should remove or change these components for optimal performance.
The capacitor C2 is there to block DC from the circuit. The value of 220uF is good for passing audio frequencies with minimal impedance (Higher value = lower frequencies, take a look at capacitive reactance).
The two resistors R2 and R3 are a voltage divider, this is needed to bias the transistor. Biasing a transistor means to give the transistor enough voltage that it’s partly on, but not so much that it turns on fully. This is necessary for an amplifier.
Q1 is a 2N2222A NPN transistor. The collector resistor (The collector is the one without the arrow) and the emitter resistor set the gain of the transistor. The gain can be approximated with -Rc/Re (The minus is there because the signal is inverted with this amplifier). Rc is the collector resistor, and Re is the emitter resistor. In this case, the gain is ~100 (~ means approximately).
C3 blocks DC.
Q2 is the same as Q1, and the resistors around it have the same function. The voltage divider uses different value resistors, but that’s only because I didn’t have enough resistors of the same type as the voltage divider on Q1 (They were in use on some other projects). The Q2 voltage divider gives roughly the same voltage output as the Q1 divider.
There are two transistors because each one amplifies the signal more. You might think “Why can’t I just use one transistor with a really big collector resistor?”. That’s an excellent question. You can’t make the collector resistor (Rc) too big, because in a common-emitter amplifier (Which is what this circuit uses), Rc sets the output impedance. Basically, impedance is the concept of resistance applied to AC circuits. High output impedance will make the output signal’s current very low. So, you can’t make Rc too big or else you have too little current on the output.
There are two solutions to this (That I know of). One is to use a common-collector amplifier after the single transistor. A common-collector amplifier can be used to take a source with large output impedance and turn it in to a low output-impedance source. I didn’t do this because I couldn’t get it to work (But if I figure it out, I might make another updated article with that method!). The other method, which I used here, is to just use two common-emitter amplifiers which makes the output impedance less while still providing relatively large gain.
C4 blocks DC, and the resulting amplified signal is output!
I hope this post was informative, if you have any questions or comments or if I wasn’t clear on something, please comment in the form below.