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current mirrors are sth constant gm circuit which it means for those transistors
in saturation (which is quite a large voltage range) that the current is fixed. If you have V2I circuit, you can have constant voltage for other biasing.
If you use voltage bias directly, may i know where should the voltages coming from? Resistor divider from VDD? it seems not so easily to have a clean voltage biasing from this way, and in contrast, current mirror can give high PSRR
Think about it. first, the voltage is very hard to distribute (noise sensitive and poor matching). That's why bandgap voltage is not delivered for long distance, instead bandgap voltage is converted to current and delivered. On the other hand, the current can be matched very well by placing the mirrors next to each other.
roughly speaking, when we use mos transistor, we play around with I_DS. however, we do not have good control of I_DS since it varies with many factors, such as temperature, technology process, and so on so forth. I_DS cannot be precisely defined if you just set the voltage. that's why, you have to use a concept of "current copy" to obtain a "closer" value to what you want.
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