The Single NPN Transistor Audio Preamp

Here’s a great little project that goes well with the LM386 audio amp. It’s a good first time transistor project because it’s simple and demonstrates the common emitter class A amplifier circuit with only six components in the signal path.

Here’s an excerpt from a great tutorial I found on NPN transistors:
A “Class A Amplifier” operation is one where the transistors Base terminal is biased in such a way as to forward bias the Base-emitter junction. The result is that the transistor is always operating halfway between its cut-off and saturation regions, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of any AC input signal superimposed upon this DC biasing voltage. Without this “Bias Voltage” only one half of the input waveform would be amplified. This common emitter amplifier configuration using an NPN transistor has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages.

I powered this circuit with a single 3V coin battery I salvaged from an old computer motherboard. It works just fine at this low voltage because it’s just a preamp. Go build one and keep on hackin!

Parts List:
1 – 2N3904 NPN transistor
2 – 10K resistors
1 – 100 K resistor
1 – 0.1 uF ceramic capacitor
1 – 1 uF ceramic capacitor
1 – 3V – 9V battery and holder