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156 2. Tutorial
Replacing the load capacitor is done by editing our via ReadNetlist imported netlist. Therefore,
we first remove the netlist entry for CL using the Analog Insydes command DeleteElements
(Section 3.6.2) and in a second step we add an independent current source IZ using the command
AddElements (Section 3.6.1).
In[20]:= cmosdiffamp2 = DeleteElements[cmosdiffamp, CL];
cmosdiffamp2 = AddElements[cmosdiffamp2,
{IZ, {0, 5}, IZout}];
After the modification of the netlist we set up the modified nodal equations and solve for the voltage
at the output port, i.e. V$5.
In[22]:= mnaZout = CircuitEquations[cmosdiffamp2,
ElementValues −> Symbolic,
DefaultSelector −> LowFrequency]
Out[22]= DAEAC, 9 9
In[23]:= zout = Together[V$5 /. First[Solve[mnaZout, V$5]]
/. {IBIAS −> 0, VDD −> 0, V1 −> 0, V2 −> 0, IZout −> 1}]
Out[23]=
Gds$M1 Gds$M2 Gds$M1 Gds$M3
Gds$M2 Gds$M3 Gds$M3 gm$M1 Gds$M1 gm$M2 Gds$M3 gm$M2
Gds$M1 gm$M3 Gds$M2 gm$M3 gm$M1 gm$M3 gm$M2 gm$M3
Gds$M1 Gds$M2 Gds$M3 Gds$M1 Gds$M2 Gds$M4
Gds$M1 Gds$M3 Gds$M4 Gds$M2 Gds$M3 Gds$M4
Gds$M2 Gds$M3 gm$M1 Gds$M3 Gds$M4 gm$M1 Gds$M1 Gds$M4 gm$M2
Gds$M3 Gds$M4 gm$M2 Gds$M1 Gds$M2 gm$M3 Gds$M1 Gds$M4 gm$M3
Gds$M2 Gds$M4 gm$M3 Gds$M2 gm$M1 gm$M3 Gds$M4 gm$M1 gm$M3
Gds$M4 gm$M2 gm$M3 Gds$M1 Gds$M2 gm$M4 Gds$M2 gm$M1 gm$M4
For matching the MOS transistors forming the differential pair and the current-mirror load we again
apply the command MatchSymbols:
In[24]:= zoutmatch = Together @ MatchSymbols[zout,
{{"$M1", "$M2", "12"}, {"$M3", "$M4", "34"}}]
Out[24]=
Gds12 2 Gds34 2 gm34
2 Gds12 Gds34 Gds34 gm34
As usual, this expression contains many numerically small terms which are identified and discarded
by ApproximateTransferFunction. The output impedance is then given by the following simple
formula in terms of the drain-source conductances Gds12 and Gds34.
In[25]:= zoutSAG = ApproximateTransferFunction[
zoutmatch, s, dpcmos, 0.05] // Simplify
Out[25]=
1
Gds12 Gds34