Calculations and Simulations with Pads and Feedback Amplifiers
This plot shows the impedances created at the circuit input with the output terminations described. There is no pad present for this simulation. Each calculation produces a small rectangle that is plotted within the Smith Chart circle. Regions near the left edge are precluded because the resistance does not drop below 5 Ohms. The blank region at the right extreme results because there are no resistances above 500 Ohms. “Dots” above the chart center line correspond to a positive, or inductive input impedance while the negative reactance capacitive impedances are below the center line. Several thousand calculations were used for this simulation.
A1 dB pad is now used in the circuit. The effect is that no impedances close to the edge of the chart are allowed. This simulation used 2000 calculations.
The third example, a simulation with only 1000 calculations, uses a 10 dB pad. Impedances are now confined to a small region around the 50 Ohm center of the chart.
A Feedback Amplifier
We now consider a feedback amplifier simulation using SW Cad, a free version of SPICE distributed by Linear Technology, a Bay Area Semiconductor house. We are grateful to LT for this wonderful tool that they maintain and provide, free of charge. The schematic for the amplifier considered here is from Fig 2.57 of EMRFD with the simulation circuit shown:
The amplifier is driven by a 50 Ohm generator with an open circuit voltage of 2. Note that if this 50 Ohm generator is loaded with 50 Ohms, the voltage that results at the load is 1. A floating (non grounded) 1 volt voltage source is attached to the generator output. If the generator was terminated in an exact 50 Ohms, the floating generator voltage would subtract from the signal at the output of the 50 Ohm source to produce zero volts at the point marked “S11.” The voltage at this point is the voltage reflection coefficient.
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