Biasing questions - what is the bias point?

[kidmanbasha]

Biasing question

Do designers size up the transistors according to the specs then simply hook them up in current mirrors to bias these transistors?

How do you guarantee the current mirror is giving the right bias point?

How do you know what the right bias point even is? (concerning a load transistor or tail transistor)

 

[kishore2k4]

Re: Biasing question

Do designers size up the transistors according to the specs then simply hook them up in current mirrors to bias these transistors?

How do you guarantee the current mirror is giving the right bias point?

How do you know what the right bias point even is? (concerning a load transistor or tail transistor)

1) When you size a transistor in your design you are indirectly picking a DC bias-point for it. Yes, generating the bias voltages can be done by hooking them with current mirrors.

2) You don't, especially in short-channel processes. Neglecting second-order effects like channel-length modulation, Vth variations etc it will, in theory, generate the right bias voltage as it is a simple ratio of W/L 's. But it is good enough(with design margin).

3) If you are biasing your MOSFET to be in saturation then the right bias point would be in the middle of strong saturation region but this may not always be possible depending on the required voltage-swing, depends on VDsat, etc.