I was just designing this Common emitter transistor amplifier this morning.
I have removed the input in the circuit intentionally. Here are a few words about the circuit:
I constructed initially with only one transistor (BC107) and tested by giving audio signal as input. The output from the 8 ohm speaker was pretty much distorted, which I think was due to impedance mismatch.
In order to overcome the distortion, I added a second stage using second transistor (2N3904). The second stage is a buffer circuit (Common collector mode) and it has been directly coupled to the first stage. I've tried the direct coupling because I studied it in my recent classes and want to test it practically.
Now, when I switched on the battery to give supply voltage, and connected the speaker at the output side, (without applying any input signal on the input side) I got a high frequency, audible static noise from the speaker like this:
When I changed the capacitor and tried out with values of 4.7uF, 10uF, 22uF and 100 uF, I found the frequency to be decreasing with increase in capacitance. I measured the sound frequency using an app in my cell phone (approximate frequency).
Now here are my questions: Why does this circuit produce such a noise? Can this circuit be further simplified in order to produce the same output. Also I've got the idea of developing this circuit as a musical instrument by adding several capacitor at the output so that, when I short the speaker to a capacitor manually it would produce a sound and then when I short it to another capacitor, it would produce a sound of another frequency. Given that I have the knowledge of the frequency of various musical notes, How do I calculate the capacitor values theoretically? Please provide a formula.