vivkr
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Hi allanxu,
I agree fully with your suggestion. I too use PMOS input transistors for the gain boosting amps in such a situation. The biasing becomes much easier in that case.
Also, Vref can be easily generated using a replica structure if you want to track the process. I do the following:
1. First forget the gain boosting amp altogether, and design a bias structure to set the cascode in the correct operating range. You can use low-voltage cascode biasing schemes. This includes 2 branches (see Johns & Martin or Razavi for examples). One of the branches is a diode-connected cascode pair and the other is a diode-connected transistor sized to keep the first cascode stack in saturation.
2. Now, tap Vref from the source of the cascode transistor in the above stack.
Regards Vivek
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