Lab Manual at www.docircuits.com - common source characteristics of JFET

1. Access this lab-manual at:
http://www.docircuits.com/lab-manual/16/common-source-jfet-characteristics
Common Source JFET Characteristics
Aim
Aim is to bias an FET in common source configuration
and plot its characteristics
Related Experiments
JFET
JFET BASICS
COMMON SOURCE CHARACTERISTICS
Description
Theory:
DC biasing of an FET is done to produce the required gate-to-source voltage (VGS) to get the desired value of drain
current (ID). Common-source configuration offers high input impedance, low output impedance, high voltage gain
and the output voltage is 180 ? out-of-phase with the input voltage. The commonly used common-source biasing
configurations include fixed-bias configuration, self-bias configuration and voltage-divider configuration. In this
experiment we’ll use the fixed-bias configuration. The fixed-bias configuration is not used much as the wide
differences in the minimum and maximum values of the JFET parameters make drain current levels unpredictable
with simple fixed-bias circuits.
Procedure:
1. Connect the circuit as shown.
2. Connect DCSource1 as a voltage source – it will supply a gate voltage – as the FET is a voltage-controlled
device. The DCSource0 supplies the VDS voltage. Note the polarity of the VGS voltage. It should be
negative. Choose R1 of value 952 Ω and R0 of value 2.05 kΩ.
3. Click on Run > DCAnalysis. Vary VDS from 0 to 10V. Vary the VGS from 0 to 2V.
4. Click on “Run” and observe the characteristics on the plotter
Conclusion
Note the variation of ID with increase in VDS. ID increases linearly with increase in VDS till the VDS reaches a value
where the saturation effect sets in. The value of VDS where the saturation effect sets in is referred to as the pinchoff voltage (at VGS=0). For VDS > VP, JFET has characteristics of a constant current source. It can be further seen
that the VGS is the control voltage for JFETs in the same way as the base current (IB) is for BJTs. As an exercise can
you draw the dc load line for this plot and find a suitable Q-point?