For those who want to design their own audio amplifier circuit, here’s a little guide. Be sure to have knowledge on basic transistor biasing. For all other concerns, we can ask help from our friend simetrix(it is actually a simulation software I usually use).
The Circuit
The circuit above is from sound.westhost.com that I used for simulating and analysis as a reference. The design is simple and easy to understand. Basically an audio amplifier is composed of 3 blocks, the input, driver and output stage. This circuit is easy to build and can deliver up to 60W, good enough for home audio system.
Input stage
Input stage is usually a differential amplifier in most cases. Some use a common emitter configuration for more simpler version of the circuit. But for this version I chose the most common differential circuit configuration. This is where the voltage is being amplified. R2 and D1 is a simple voltage regulator to provide 15V and R1 to provide an almost constant current supply to the differential amplifier Q1, and Q2. For better design, this is usually replaced with a NPN and PNP transistor circuit to provide better constant current.
Improved version using constant current circuit. D1 can be replaced with red LED.
There is a lot of calculations and may not be easy to explain, so I will just make the simulation file available for download in case you want to experiment. See the download link below this page. The simulation program I used is called simetrix. It is a free simulation software for individual use only, you will need a license if you want to use it for business. Learn more here: Simetrix website
Driver Stage
Driver stage is composed only of Q3, C2, D2, D3, R7, R6 and C3. This stage drives the complementary darlington pair Q4, Q5 and Q6, Q7. D2 and D3 is used to introduce voltage drop across high side and low side transistor to make sure they wont turn on at the same time. In some circuit it uses a transistor connected to a trimmer resistor to fine tune the bias voltage.
Output Stage
Unlike the usual darlington, complementary darlington or also known as “Sziklai pair” is combination of NPN and PNP transistor pair. Their total hfe is also equal to the product hfe of both transistor.
Final version
Simulation
Transient response
Frequency Response
more to come…
Downloads: