This example shows a typical low-noise audio amplifier circuit. Resistor R2 provides negative feedback to stabilize the overall amplifier gain, making it independent of transistor open-loop forward transfer gain. Circuit gain is approximately defined by (R1+R2)/R1 = 101. The simulation shows that it takes the circuit a few seconds to settle to its steady operating point, and that the output is initially clipped. Input u and output y are included to support linearization.
This model can be used to obtain the frequency response of the system. The Solver Configuration block option "Start simulation from steady state" should be set to ensure that the model is linearized about its nominal operating point. MATLAB command linmod can be used to linearize the model.
If you have Simulink® Control Design™ installed, then to plot the frequency response, select Analysis->Control Design-> Linear Analysis to open the Linear Analysis Tool. Change the Plot Result to New Bode, and then click on Linearize. The linearization points are defined by right-clicking on a Simulink® line, and selecting Linearization Points.
The plot below shows the input and output voltages for the amplifier circuit. The gain is about 100. It takes a few seconds for the circuit to settle to its steady operating point due to the charging of the capacitors, shown in the lower plot.