SimElectronics 1.3

JFET Amplifier and Frequency Response Analysis

This audio amplifier is 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 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 Tools->Control Design-> Linear Analysis to open the Control and Estimation Tools Manager. Change the plot type to Bode response plot, and then click on Linearize Model. The linearization points are defined by right-clicking on a Simulink® line, and selecting Linearization Points. If you look at the measurement block labelled 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:

bode(a,b,c,d)

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