For the BJT transistor, the output current IC and the input controlling current IB are related by beta, which was considered constant for the analysis to be performed. In equation form,
eq(2)
In Eq. (2) a linear relationship exists between IC and IB. Double the level of IB and IC will increase by a factor of two also. Unfortunately, this linear relationship does not exist between the output and input quantities of a JFET. The relationship between ID and VGS is defined by Shockley’s equation (see Fig. 16):
eq(3)
The squared term in the equation results in a nonlinear relationship between ID and VGS, producing a curve that grows exponentially with decreasing magnitude of VGS. For the dc analysis, a graphical rather than a mathematical approach will, in general, be more direct and easier to apply. The graphical approach, however, will require a plot of Eq. (3) to represent the device and a plot of the network equation relating the same variables. The solution is defined by the point of intersection of the two curves. It is important to keep in mind when applying the graphical approach that the device characteristics will be unaffected by the network in which the device is employed.
The network equation may change along with the intersection between the two curves, but the transfer curve defined by Eq. (3) is unaffected. In general, therefore:
The transfer characteristics defined by Shockley’s equation are unaffected by the network in which the device is employed.
The transfer curve can be obtained using Shockley’s equation or from the output characteristics of Fig. 11. In Fig. 17 two graphs are provided, with the vertical scaling in milliamperes for each graph. One is a plot of ID versus VDS, whereas the other is ID versus VGS. Using the drain characteristics on the right of the “y” axis, we can draw a horizontal line from the saturation region of the curve denoted VGS = 0 V to the ID axis. The resulting current level for both graphs is IDSS. The point of intersection on the ID versus VGS curve will be as shown since the vertical axis is defined as VGS = 0 V.
In review:
eq(4)
When VGS = VP = -4 V, the drain current is 0 mA, defining another point on the transfer curve. That is:
eq(5)
Before continuing, it is important to realize that the drain characteristics relate one output (or drain) quantity to another output (or drain) quantity—both axes are defined by variables in the same region of the device characteristics. The transfer characteristics are a plot of an output (or drain) current versus an input-controlling quantity. There is therefore a direct “transfer” from input to output variables when employing the curve to the left of Fig. 17. If the relationship were linear, the plot of ID versus VGS would result in a straight line between IDSS and VP. However, a parabolic curve will result because the vertical spacing between steps of VGS on the drain characteristics of Fig. 17 decreases noticeably as VGS becomes more and more negative. Compare the spacing between VGS = 0 V and VGS = -1 V to that between VGS = -3 V and pinch-off. The change in VGS is the same, but the resulting change in ID is quite different.
If a horizontal line is drawn from the VGS = -1 V curve to the ID axis and then extended to the other axis, another point on the transfer curve can be located. Note that VGS = -1 V on the bottom axis of the transfer curve with ID = 4.5 mA. Note in the definition of ID at VGS = 0 V and -1 V that the saturation levels of ID are employed and the ohmic region ignored. Continuing with VGS = -2 V and -3 V, we can complete the transfer curve.It is the transfer curve of ID versus VGS that will receive extended use in the analysis of the chapter “FET Biasing” and not the drain characteristics of Fig. 17. The next few paragraphs will introduce a quick, efficient method of plotting ID versus VGS given only the levels of IDSS and VP and Shockley’s equation.
eq(2)In Eq. (2) a linear relationship exists between IC and IB. Double the level of IB and IC will increase by a factor of two also. Unfortunately, this linear relationship does not exist between the output and input quantities of a JFET. The relationship between ID and VGS is defined by Shockley’s equation (see Fig. 16):
eq(3)The network equation may change along with the intersection between the two curves, but the transfer curve defined by Eq. (3) is unaffected. In general, therefore:
The transfer characteristics defined by Shockley’s equation are unaffected by the network in which the device is employed.
The transfer curve can be obtained using Shockley’s equation or from the output characteristics of Fig. 11. In Fig. 17 two graphs are provided, with the vertical scaling in milliamperes for each graph. One is a plot of ID versus VDS, whereas the other is ID versus VGS. Using the drain characteristics on the right of the “y” axis, we can draw a horizontal line from the saturation region of the curve denoted VGS = 0 V to the ID axis. The resulting current level for both graphs is IDSS. The point of intersection on the ID versus VGS curve will be as shown since the vertical axis is defined as VGS = 0 V.
In review:
eq(4)eq(5)If a horizontal line is drawn from the VGS = -1 V curve to the ID axis and then extended to the other axis, another point on the transfer curve can be located. Note that VGS = -1 V on the bottom axis of the transfer curve with ID = 4.5 mA. Note in the definition of ID at VGS = 0 V and -1 V that the saturation levels of ID are employed and the ohmic region ignored. Continuing with VGS = -2 V and -3 V, we can complete the transfer curve.It is the transfer curve of ID versus VGS that will receive extended use in the analysis of the chapter “FET Biasing” and not the drain characteristics of Fig. 17. The next few paragraphs will introduce a quick, efficient method of plotting ID versus VGS given only the levels of IDSS and VP and Shockley’s equation.
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