The .tf line defines the small-signal output and input for the dc or ac small-signal analysis. The first parameter is the small-signal output variable (node voltage or name of inductor or voltage source for branch current) and insrc is the small signal input source. If this line is included, WRspice computes the dc or ac small-signal value of the transfer function (output/input), input resistance or impedance, and output resistance or impedance. For the first example, WRspice would compute the ratio of v(5,3) to vin, the small signal input resistance at vin, and the small-signal output resistance measured across nodes 5 and 3. If the ac parameters are given, the .tf line produces output vectors representing the impedance and other parameters at each frequency point.
The optional dc sweep is a dc analysis specification which will cause the transfer function analysis to be performed at each point of the dc sweep. The small-signal parameters are reevaluated at every sweep point, and the output vectors will be multidimensional.
In interactive mode, the tf command, which takes the same arguments as the .tf line, can be used to initiate transfer function analysis.