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):
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:
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)
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).