This example shows a differential pair amplifier circuit. The circuit can be used to explore the properties of a differential pair amplifier. Use the Manual Switch in the Source subsystem to switch between differential and common-mode inputs. The balanced output has zero gain in common-mode provided that the two transistors have identical properties.
The third transistor NPN 3 acts as a constant current source that helps stabilize differential-mode gain. The combination of R1, R3 and D1 sets the base voltage to about 1V above the negative power rail. The base-emitter saturation voltage is 0.4V, and hence the voltage across R2 is 0.6V giving an emitter current of 0.6/220=2.7mA. D1 provides temperature compensation, the forward voltage variation with temperature being similar to that of the base-emitter junction. Transistor component values are typical for a BC107, and the diode component values are typical for an IN4148.
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. Before linearizing, you need to select the Solver Configuration block option "Start simulation from steady state" to ensure that the model is linearized about its nominal operating point. 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. If you look at the Sensor block, you will see the linearization output symbol at the output of the PS-Simulink block. Similarly under the Source block you will see a linearization input point symbol on the output of the Sine Wave block.
If you do not have Simulink Control Design, then you can linearize the model between input port u and output port y by typing:
[a,b,c,d] = linmod('elec_bipolar_diff_pair')
The frequency response can then be plotted using the Control System Toolbox™ by typing: