IC Biasing - Basic MOSFET current source
Using the transistors geometries (W/L)1 and (W/L)2as design parameters, we want to create a DC current Io, as long as transistor Q2 is in Saturation Mode The Drain of transistor Q2 is connected to a load circuit, not necessarily a resistor. The load circuit typically involves one or more additional MOSFET transistors. Depending on the load, transistor Q2 may be in any of three modes: Saturation, Triode or Cutoff. Of course, only when it is in Saturation it will work as originally planned (DC current source) The current Io always goes away from the load circuit and into Q2. Such a DC current source is said to be a sink.
· MOSFET current mirror is widely used for ICs with good device matching.
· Q1 and Q2 are identical and in saturation:
Current gain or current transfer ratio:
Effect of VO on IO
Current mismatch due to channel-length modulation