next up previous contents

[ENGN2211 Home]

JFET Characteristics

Load the PSPICE file jfet-dc1.sch, Figure 39.


  
Figure 39: Circuit for measuring JFET characteristics.
\begin{figure} \begin{center} \epsfig{file=images/clab7img1.eps}\end{center}\end{figure}

Exercise:

1.
Simulate the circuit for the given setup. Is the transistor J1 on or off? Why?

Lecture Notes : FET Transistor Circuits : Introductory FET Circuits : Switch

When on a substantial current flows (order mA), while when off only a small current flows (order few $\mu$A or less). So look at the value of ID.

Note also the values of VGS and $I_G \approx 0$.

2.
iD vs vGS curve. Enable DC sweep, and check that it is set so that VGS ranges from 0 V to -5 V in increments of 0.1 V. Simulate, and obtain the curve of Figure 40.

3.
Determine Vp, IDSS and calculate gm0.

Lecture Notes : FET Transistor Circuits : FET Models

4.
iD vs vDS curves. Now reset the DC sweep as follows.
VDS is set as the main sweep variable, ranging from 0 V to 10 V in increments of 0.1 V. VGS is the nested sweep variable, ranging from 0 V to -5 V in increments of 0.5 V.
Simulate, and obtain the curves of Figure 41. Determine the correct value of VGS for each of the curves.

Note the similarities with the BJT characteristic curves IC vs VCE.

In active mode, the BJT is like a current controlled current source: $I_C = \beta I_B$, a function of IB.

In active mode, the FET is like a voltage controlled current source: ID = IDSS(1-VGS/Vp)2, a function of VGS.

The parameter values Vp and IDSS you have found will be used below (same JFET).


  
Figure 40: JFET iD vs vGS curve.
\begin{figure} \begin{center} \epsfig{file=images/clab7img2.eps}\end{center}\end{figure}


  
Figure 41: JFET iD vs vDS curves.
\begin{figure} \begin{center} \epsfig{file=images/clab7img3.eps}\end{center}\end{figure}

next up previous contents

[ENGN2211 Home]

ANU Engineering - ENGN2211