My final project for my microelectronics class was to build a three-stage audio amplifier using only BJT transistors and basic components, i.e. resisters and capacitors. My design for this project was largely based off of a two-stage amplifier I had built for a previous lab exercise. I made initial calculations based on the design constraints provided, built a simulation of each stage as well as the final design using Multisim, revising resistor values as needed. Finally, I built the circuit and tested each stage and the final output using voltage signals, before finally hooking up an audio input signal and listening to the results.
The circuit operated as designed from the beginning, and not major adjustments had to be made. This was surprising, because in labs I usually had to adjust values in the lab even after doing calculations and simulations.
Here are the requirements I fulfilled for this project:
Amplifier’s gain must be 25V/V.
Since the load is a small impedance, you may need to combine two amplification stages with a common-collector stage.
Overall distortion must be less than -34dB; measure it in the lab!
Amplifier’s input impedance must be greater than 100 kΩ to avoid any connectivity issue.
For the last (power output stage) the transistor may need to manage over 1Watt. The 2N2222 should not manage more than 250mW (Vce-max*Ic-max) to be safe.
As a final test, connect the amplifier to a computer and the speaker for the final demonstration.