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2.9 Frequency-Domain Analysis of Linear Circuits 155
Iin=1
V$in
circuit
Figure 9.6: Unit current source excitation for input impedance calculations
Likewise, an input admittance is the transfer function from a voltage across the port to the current
into the port. The same applies to output impedances and admittances which are, in fact, just the
input impedances and admittances at a port which is designated as an output. Consequently, to
calculate an output impedance we must connect a unit current source to the port, set up a system of
circuit equations, and solve for the port voltage.
Computing the Output Impedance of the CMOS Amplifier
To determine the output impedance of the differential amplifier (see Figure 9.4 in Section 2.9.2) we
replace the load capacitor by a stimulus current source as displayed in Figure 9.7.
6
VDD=0
M3
M4
5
1
M1
4
3
M2
2
IZOUT=1
V1=0 V2=0
IBIAS=0
Figure 9.7: Differential amplifier, small-signal configuration for computing the output impedance