The UNETTO is a "minimalist" Class-AB audio power amplifier that uses only three amplifier stages and a couple of IGBT output devices to deliver up to 200W RMS (400W musical) on 4 ohms or 100W RMS (200W musical) on 8 ohms with a very low THD distortion. The amplifier is based on a differential input stage, a class-A driver and a class-AB output stage. The amplifier power capacity can be reduced by simply lower the power supply voltage.
The aim of the UNETTO power amplifier was to exploit today's semiconductor technologies to design an efficient, high power audio amplifier with interesting features such as a very limited number of components, low distortion figures, and high reliability. A class-AB output configuration was selected due to the following main reasons: high efficiency and power handling capacity, limited dissipation of power in form of heat, and low distortion.
The amplifier is based on a differential input stage, a class-A driver and a class-AB push-pull output stage. The next figure represents a general schematics of UNETTO.
Simplified diagram of the UNETTO power amplifier
IGBT type transistors
The IGBT or Insulated Gate Bipolar Transistor mix the best of the mosfet and bipolar transistor technologies. The next figure represents a general equivalent circuit of a IGBT, where the input can be considered a mosfet and the output a bipolar transistor.
The main advantage of such devices are high current handling up to 400 A, high switching speed (12A in less than 150 ns.), high input impedance, and less power dissipated as heat. IGBT are voltage sensitive devices, and a small variation in the input voltage generates a large variation in the output current. Due to its high input impedance, the device is sensible to static currents in the Gate.