FET Fundamentals Unit 4 – JFET Current Source
Exercise 2 – JFET Voltage and Power Distribution
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine voltage and power
distribution of a JFET current source by using a JFET test circuit. You will verify your results
with a multimeter.
DISCUSSION
- A JFET constant current source, when used in a circuit, is connected in series.
- Kirchhoff’s voltage law states that the sum of the voltage drops must equal zero.
- Current flowing through the load produces a voltage drop across the load (VRL).
- When the load voltage (VRL) is subtracted from the source voltage (VDD) the resulting
voltage is the drain to source voltage (VDS). - Load voltage will vary as load resistance varies, effecting the value of VDS.
- The value of VDS is critical. In order to maintain a constant current, VDS must exceed the
pinch-off voltage (VP). - Power dissipation of the JFET increases as VDS increases. A constant current source has 100
mW dissipated across Q1 with a 100Ω resistive load.
NOTES