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Current mirror with channel length modulation

  In Section 4.1, we showed that the current mirror realises a constant, process independent current amplification, when channel length modulation is disregarded. In this Section, the consequences of channel length modulation are considered.

Assuming that a common gate-source voltage, VIN, is applied to both transistors, the current amplification can be calculated using Equation (3.1):
 eqnarray983

  figure1002
Figure 4.2: Equivalent (large signal) diagram for the output of the MOS current mirror.

For a specific input current, IIN, the output current, IOUT, depends on the voltage VOUT over the output of the current mirror. Equation (4.7) may be rewritten as follows:
 equation1011
Figure 4.2 shows an equivalent diagram, from which the dependence of IOUT on VOUT may be found. The output conductance 1/ROUT is calculated using Equation (4.8)
equation1024
Not very surprisingly, the output conductance corresponds exactly to the channel conductance, gds, of M2. The channel conductance can be calculated from Equation (3.7).



Flemming Stassen (Lektor)
Wed Jan 21 13:35:14 MET 1998