|The UNETTO circuit|
The UNETTO input stage is composed by a differential pair with an active load and a current source to fix bias current. This configuration guarantees a high level of common noise rejection, low distortion, and high linearity. The bias current of the input is fixed to approximately 1 mA, and can be easily changed modifying the values of R3 according to the following formula:
[ R3 (Kohm) = 0,7 / new bias current (mA) ]
The driver stage is composed by a medium power bipolar transistor operating in class-A and loaded with a current source that fix the collector current around 8 -10mA.
The next figure shows the complete diagram of UNETTO.
The output stage is based on a push-pull configuration, operating in class-AB and using IGBT type out put devices. The Q8 monitor the temperature and control the static bias current of the IGBT output devices. It must be physically located close to the output devices and on the same heat sinks.
The overall gain of the amplifier is controlled by the values of R15 and R18 and it is fixed around 18 to provide a full 100W output power in an 8 ohms load with an input signal of 1.6 Volt. The overall gain can be easily changed using the following formula:
[ Av = R15/R18 ]
The UNETTO is very simple to build because its "minimalist" design and uses a limited number of electronic components. The only critical point is that the IGBT devices are very sensible to variations of the bias current, so the R8 variable resistor must be carefully adjusted for a bias current of 90-120 mA. R8 must be a high quality multi-turn potentiometer.
For the setup phase, use a current meter connected in series with the positive voltage source to monitor the value of the absorbed (bias) current.
Connect the + input pin to ground. Fix R8 at the middle of its course.
Turn-on the power supplies.
The bias current must be lower than 100mA, if not turn off the power supply and check the circuit. If all is OK, slowly route R8 until the bias current reach 90 mA. Wait some minutes and check and adjust again. Control the temperature of the heat sinks, they must be below 70-80 °C.
The UNETTO schematics