[ENGN2211 Home]

Switching and Load Line

Load the PSPICE file jfet-dc2.sch, Figure 42.

  

Figure 42: Circuit for JFET switching and load line.

Exercise:

1.

Try to calculate V_GS, V_G, I_D and V_D=V_DS using the DC model for the JFET. (assume I_G=0 A) for the circuit of Figure 42 (use V_DD=10V, R_G=50 k$\Omega$, R_D=3.3 k$\Omega$).

Simulate and compare with your calculations.

In your calculations, use the parameters I_DSS and V_p you measured above.

Are the assumptions for the use of the DC model valid? Explain.

2.

Enable DC sweep and simulate again, to obtain the graph of Figure 43 showing the switching characteristic (V_GG varies between 0 V and 5 V). At what value (approx.) of V_GG does switching occur? Discuss.

3.

Obtain the load line of Figure 44 from the same simulation.

In this case, the load line is given by

$$I_D = \frac{V_{DD} - V_{DS}}{R_D}$$

This is easy to sketch on your printout, by first determining the x and y intercepts.

  

Figure 43: JFET switching curve.

  

Figure 44: JFET load line for the circuit of Figure 42.

[ENGN2211 Home]