LTspice Sub_Circuits Illustrating Cascode-Circuit Effect – EKV Simulator

The supply and reference voltages for this project are as follows for all circuits evaluated here. This provides for a variety of circuit configurations.

The following sub-circuits are created for the supply and references.

The supply voltages are for this case internal to the sub-circuit.

The library files are located as shown.

A simulation of the above circuit produces the net file, as shown, from which the reference-circuit file can be copied.

Basic Amp – Fig. 1

1.) Symmetrical Diff amp input stage, 2.) single-ended diffamp gain and output-stage adapter, 3.) output stage. Diffamp output voltage should be about zero for minimizing common-mode input to the following single-ended output diffamps but slightly positive vp and vn improves the performance of m1 and m2.

With sub-circuits.

X2 calls power-supply voltages, X4 calls reference voltages, both in vref.lib.

Subckt example. Symmetrical Diffamp sub-circuit. Label input and output nodes, vg1, vg2, von, and vop, (F4). Simulate and obtain net file (below).

Create dansym subckt.

From LTspice, lib, sym, opamps, open opamp2, for example.

Edit for this application and name and save.

When creating the above circuit with subckts, use F2 for opamps, and place the subckt symbol.

Do a right mouse, and bring up this window and enter value (subckt name).

Perform DCsweep (Fig. 1). In this case, we read the LTspice file with LabVIEW and compute the DCtransfer.

MOSFET Parameters (EKV)  generated by LabVIEW for given NSUB, etc.

Symmetrical diffamp with common-mode feedback circuit (right).

This version includes CMFB circuit for holding  outputs (vp and vn) near zero. Note that ms2 and ms3 are not necessarily required and that m5 and m6 are diode-connected PMOS devices.

With Subckt CMFB and subckt reference voltages and power supply.

PMOS Current-Source Diffamp. We note here, that with vo = 0 V, Vds of m1 and m2 is small.

DCtransfer

Add common-gates, m5 and m6.

DCtransfer – Output resistsance dominated by rds of m1 and m2.

Folded cascode load. m9, m10, m7, and m8 are in the common-gate mode. In this circuit, the common-gate devices have large source-degeneration resistance.

Opamps with Common-Source Gain Stage

Basic

Add cascode NMOS

Add PMOS Cascode