DC Characteristics of JFET transistors

 

Introduction

This lab introduced a different device: The Junction Field Effect Transistor (JFET) also widely used in

electronics. We will examine the DC characteristics of JFETs and will generate the JFET’s curves.

Remember that your lab report will need to include your measurements, calculations, screenshots, etc.

as indicated at the end of this outline.

Procedure

1.- Measuring key parameters: Idss and Vp

1.1 Create the circuit shown in Figure 7.1. Remember to assign different keys to each potentiometer

and to change the increment to 1%

Figure 7. 1: Circuit to measure basic JFET parameters

ELEC 160 – Module 7 Laboratory – Page 2

1.2 Vary the 1 MΩ potentiometer until Vgs is as close to zero as possible.

1.3 Vary the 5 kΩ potentiometer until Vds is as close to 8V as possible.

1.4 Measure the current Id.

1.5 Because the current was measured with Vgs = 0, them Id = Idss.

1.6 Maintain Vds at 8 V. Change the value of the 1 MΩ potentiometer until Id = 10µA. Keep in mind

that every time you change the 1 MΩ potentiometer, you may have to readjust the 5 kΩ to keep Vds as

close to 8 V as possible. (We can consider 10 µA as a small enough current to be almost equal to zero.

The value of Vgs that resulted in Id = 10 µA is Vp).

1.7 Record the values of Idss and Vp for this JFET

1.8 Use Shockley’s equation to predict the value of Id for different values of Vgs from 0V to Vp at 0.5 V

intervals.

1.9 While keeping Vds as close to 8V as possible, measure the current Id for the same values of Vgs that

you used in Section 1.8. Remember to adjust both potentiometers: The 1 MΩ potentiometer is

adjusted to select the desired value of Vgs while the 5 kΩ potentiometer is adjusted to keep Vds = 8V (as

much as possible).

1.10 Plots Id vs Vgs for 1.8 and 1.9 on the same graph. Remember to use the Scatterplot option in Excel.

1.11 Are the graphs similar? What can you say about Shockley’s equation?

2.- Measuring JFET Transfer characteristics

2.1 On the same circuit, vary the potentiometers until Vgs = 0 V and Vds = 0 V (or as close to 0 V as

possible).

2.2 Maintain Vgs = 0 and increase Vds from 0V to 7 V at 0.5 V internals. Measure the current Id for each

step.

2.3 Vary the 1 MΩ potentiometer until Vgs = – 1V. Repeat 2.2

2.4 Repeat the procedure for Vgs = -2V and Vgs = -3V

ELEC 160 – Module 7 Laboratory – Page 3

2.4 For each value of Vgs, plot Id versus Vds. You should obtain a graph with a shape similar to the one

shown in Figure 7.2 that represents the DC characteristics of the JFET. Note that the values of current

and voltage will be different, but the shape should be very similar. If your graph appears to be very

different, check your work.

Figure 7. 2: Typical Id vs Vds for JFETs

Laboratory Report

Create a laboratory report using Word or another word processing software that contains at least these

elements:

– Introduction: what is the purpose of this laboratory experiment?

– Results for each section : Measured and calculated values, calculations, etc. following the outline.

Include screenshots for the circuits and waveforms as necessary — You can press Alt + Print_Screen

inside Multisim or if using Windows 7, you can use the “Snipping tool”. Either way, you can paste these

figures into your Word processor.

– Conclusion : What area(s) you had difficulties with in the lab; what did you lean in this experiment;

how it applies to your coursework and any other comments.