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APPLICATION NOTE 2 Theory The circuit can be analyzed by injecting a current to measure the current gain, and a voltage to measure the voltage gain of the circuit. Previously, the circuit to be analyzed was treated as a subcircuit, with the signal path, broken to make measurements, pulled out. The subcircuit was then used twice: once so that a voltage could be injected, and again so that a current could be injected. Note: This approach of measuring loop gain relies on the voltage and current feedback theory as described in detail in A Guide to Circuit Simulation and Analysis Using PSpice, references [1] and [2]. Another approach of measuring the loop gain is to create a subcircuit for the device that is used to make the measurements, then reinsert this subcircuit into the circuit to be analyzed. The loop gain subcircuit, consisting of two voltage sources and one current source, is as follows: .SUBCKT LGT A B PARAMS: VOLTS=0 V1 1 A DC 0 AC {VOLTS} V2 1 B DC 0 AC 0 I1 1 0 DC 0 AC {1-VOLTS} .ENDS Calculating the Current Gain When the value of the VOLTS parameter is equal to 0, the I1 current source will have a value of 1, and voltage sources V1 and V2 will have a value of 0. This condition allows measurement of the current gain of the circuit. Sources V1 and V2 will be used to sense the input current and the output current of the circuit. The current gain of the circuit is determined with the following equation: Ti = I(V1)/I(V2) Calculating the Voltage Gain When the value of the VOLTS parameter is equal to 1, the V1 voltage source will have a value of 1, and the I1 current source will have a value of 0. This condition allows measurement of the voltage gain of the circuit. The voltage gain is the ratio of the voltages measured at the nodes of the subcircuit. Tv = V(A)/V(B) Note: To perform the current gain and the voltage gain portions of the analysis, it is necessary to perform a parametric analysis in addition to the AC analysis. This can be accomplished by placing a PARAM symbol on the schematic. In the example circuit shown in Figure 2, the parameter is named ACVAL, and the default value is set to 0. The parametric sweep is set to run the AC analysis with the global parameter ACVAL set to each of the values defined in the value list. The value list for this analysis contains two values: 0 and 1.