Class AB audio amplifier for portable equipment

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Dr. Gärtner Péter
Department of Electron Devices

The subject of this thesis is the design of a class-AB audio amplifier chip. During the semester, I learnt the basics of analog circuit design. I familiarised myself with the necessary basic analog elements, e.g. the common source amplifier circuit. I designed the chip in 180 nm technology. This has the advantage of less problems due to deviation of technology parameters. The load of the amplifier is a pair of Sennheiser headphones with an impedance of 70 ohm and a maximum power rating of 200 mW. In order to be able to provide this power on the output, the amplifier needs 5 V supply voltage. These amplifiers are needed because sound cards generally used in laptops or mobile phones are optimised for driving smaller impedance devices, so they could not provide enough power on a device with such an impedance. Since the operation of the device has to be ensured on a big input signal range, its input stage is a rail-to-rail stage consisting of pMOS and nMOS differential pairs. However, the behaviour of a circuit with such an input is greatly influenced by the value of the common mode signal. If the DC level of the input signal is around half of the supply voltage, the nMOSes and pMOSes can operate in saturation. As a result, the transconductance of the input stage changes depending on the common mode voltage. Therefore, a gm compensation circuit was necessary. Due to time constraints, the design of a class-AB output stage was chosen instead of a switched-mode output stage. Since a wide output voltage range is important, an inverting amplifier was chosen.

Three versions of the output stage were designed. The first one was ruled out because it could only operate appropriately in an asymmetrical structure. The second version was capable of symmetric operation, but the problem here was that the control signal is sent to the output by two opposite-switched non-ideal current generators, which resulted in inappropriate behaviour in case of extreme input signals. The third version is based on the structure of the second one. The difference is that the output is voltage-controlled. In order to be able to produce an output signal with an amplitude of 5V in the chosen technology, two amplifiers were bridged, so the signal value can be read between the outputs of the two amplifiers. To ensure stable operation, the system was expanded with two additional circuits: a common mode voltage buffer and a constant gm . During the simulations, the adequate operation of the circuit was checked in 64 corners.


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