When we apply an alternating signal to the gate terminal of a JFET, that would be amplified in the drain circuit. The proper amplification of a gate signal depends on proper biasing of both gate and drain section of the JFET. Proper biasing ensures that the JFET is being operated in the active or saturation zone during its amplification action. If during amplification either swinging peak (negative or positive peak) of the output signal comes under ohmic or cut off region the signal gets clipped and distorted and this is not desirable. To identify proper biasing voltages in input as well as in output circuit we need load line analysis and a properly located Q point on the characteristic curve of the device.
Q point is the intersection between DC load line and the characteristic curve of the JFET.
For better understanding, let us consider an n channel JFET with applied input biasing voltage VGG and output biasing voltage VDD. VDS and ID are the drain to source voltage across the JFET element and drain current through the JFET respectively. Applying Kirchhoff Voltage Law we get, From here, we can say that VDS gets its maximum value when current ID is zero and the maximum VDS is,The maximum drain current occurs when VDS becomes zero and then the maximum value of drain current is,Now we will connect the coordinate of maximum VDS and maximum ID in JFET characteristic by a straight line. This line is called dc load line. This line is so called because during determining the line no ac signal present in the circuit only dc components are there for biasing purpose.
During operation of a JFET as an amplifier operating point can be chosen somewhere well within the active zone of the characteristic. But when it is determined by dc load line analysis it would be the most optimized position of Q point.
The characteristic curve of JFET for a dc gate bias voltage VG (VG = – 2 V as shown in the given characteristic above) cuts the load line at point Q. At that Q the corresponding drain current world be ID and drain to source voltage VDS as indicated by dotted lines in the characteristic graph above.