Question A8.12: The following data were obtained while making measurements o...
The following data were obtained while making measurements on an N-channel field effect transistor:
| V_{GS} (V) | -1 | -2 | -3 | -4 | -5 | -6 |
| I_D (mA) | 8 | 6 | 4 | 2 | 0.4 | 0.1 |
Plot a graph showing how drain current ( I_D ) varies with gate-source voltage ( V_{GS} ) and use the graph to determine:
(a) the value of I_D when V_{GS} = 0 V, and
(b) the slope of the graph (expressed in mA / V) when V_{GS} = −2 V.
The data have been shown plotted in Fig. A8.10. Note that, since the values of V_{GS} that we have been given are negative, we have plotted these to the left of the vertical axis rather than to the right of it.
(a) We can use the graph to determine the crossing point (the intercept) on the drain current ( I_D ) axis. This occurs when I_D = 10 mA.
(b) The slope (or gradient) of the graph is found by taking a small change in drain current and dividing it by a corresponding small change in gate-source voltage. In order to do this we have drawn a triangle on the graph at the point where V_{GS} = −2 V. The slope of the graph is found by dividing the vertical height of the triangle (expressed in mA) by the horizontal length of the triangle (expressed in V). From Fig. A8.10 we can see that the vertical height of the triangle is 2 mA while its horizontal length is 1 V. The slope of the graph is thus given by 2 mA/1 V = 2 mA/V.
