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Also an audio amplifier had to be made by our group with a gain of 30dB. This is a differential audio amplifier, so the design was chosen to contain a differential pair. Because the gain of this stage was only 19.5dB, an extra stage had to be added. It was chosen to connect the output of the differential pair to two common source amplifiers. With this second stage the gain was increased to 31.4dB with 1MΩ output impedance(for an active speaker). The input impedance of the amplifier should be 50Ω, so a resistor is added between the input and ground of 50Ω, because the input impedance of the differential pair is extremely high (transistor gate). The differential audio amplifier design is shown below:


 

Revision:


After discussion, it was found that the gain of 30dB was needed as a power gain, which relaxes the constraints a lot. The 19.5dB voltage gain is now enough for the total amplifier, however, the10kΩ load reduced this below 15dB thus something needed to improve. The second stage which was not needed as a gain stage could now be used as an output buffer. This was successfully changed and a voltage gain of 15dB+ was achieved. The added stage also allowed for common-mode feedback as mentioned in the lecture, by using the common-mode output as a bias in the first stage. Furthermore, it was noticed that the gain was higher than needed, this is useful to compensate for lack of gain in previous stages but it is unclear how much this should be. Therefore the current source of the second stage is made variable from an eternal voltage source to tune the gain. Also, margins are built in to have a working circuit when the threshold voltages are slightly different from the model. The final design is shown below:


  

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