JFET Amplifier and Frequency Response Analysis

This example shows an audio amplifier circuit based on an N-channel JFET. The desired operating point is taken to be Vds=5V, Id=2mA and Vgs=-2V. The manufacturer datasheet gives the JFET forward transfer conductance and output conductance values as 3mS and 50uS. These values are used to populate the mask of the N-Channel JFET block.

The bias resistor values are calculated as follows. Resistor R1 effectively ties the gate to ground. Hence the voltage across resistor R3 is -Vgs, and must be 2V. Hence R3=-Vgs/Id=2/2e-3 = 1K ohm. The total voltage across R3, the JFET drain-source connections and R2 must be 15V. Hence the voltage across R2 is 8V if Vds is 5V, and R2=8/2e-3 = 4K ohm. C3 has to be large enough such that at the lowest frequency of interest (20Hz), it is effectively a short circuit. C4 is chosen so that the loss in gain compared to the mid-band gain is about 6dB.

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. If you look at the measurement block labeled V4, 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_jfet_amplifier')

The frequency response can then be plotted using the Control System Toolbox™ by typing: