Rev - Apr 25, 2016
Project: Pinguino Single Transistor Audio Amplifiers
Author: Jan Zumwalt
License: Not copyrighted
Source: Public domain content (more than three public sources) see: http://zoomaviation.com/pinguino/hardware-projects/1-transistor-audio-amp-5/
Connecting a very small speaker directly to the output pin of a Pinguino PIC is possible but the volume is feeble. In fact only the mid-range frequencies can be heard a few inches from the speaker. Adding a simple single transistor audio amplifier will increase the volume and frequency response to the level we are accustom to.
All of the circuits presented here will work with suitable performance with a Pinguino PIC chip. However, for optimum performance a circuit can be designed to maximize the PIC pin current capabilities. The first circuit we present will do just that. We demonstrate how to calculate the values for a simple 2n3904 transistor amplifier.
A PIC18F2455/2550/4455/4550 can source up to 300ma to an amplifier or speaker. For improved speaker output a single transistor audio amp will be a great improvement.
So what are our options?
The maximum output to a 8Ω speaker can be calculated using the Ohms Law formula P = V * I, or in this case P = 5v * 300ma = 1.5w. It is desirable for Vout to be 1/2 of V+ to drive the transistor in the middle of its operating range.
Vout = R2 * R1 + R2 * Vin
can be re-arranged to give:
R1 = R2(Vin * Vout − 1)
We know the speaker R2, that's 8Ω, Vin is 5V and Vout is 2.83V. So substitute the values and we have:
R1 = 8(52.83 − 1)
which gives us 6.134Ω. The closest standard resistor size would be 6.8Ω, which would be ideal. Of course, you need a nice chunky resistor, at least 1W, preferably a little more.
Your schematic could look like Fig-1.