Question 10.3: Objective: Compare the variation in bias current between the...
Objective: Compare the variation in bias current between the two- and three-transistor current-source circuits as a result of variations in β. A PSpice analysis is used.
Figure 10.6(a) shows the two-transistor PSpice circuit schematic and Figure 10.6(b) shows the three-transistor PSpice circuit schematic used in this analysis.
In both circuits, the current gain β of all transistors was assumed to be equal, but the actual value was varied between 20 and 200. Since the change in β is very large, we cannot use derivatives to determine the changes in bias currents. Standard 2N3904 transistors were used, which means that the Early voltage is 74 V, and not infinite as in the ideal circuit. The Early voltage will influence the actual value of bias current, but has very little effect in terms of the change in bias current with a change in current gain.
Figure 10.6(c) shows the bias current versus current gain for both the twotransistor and three-transistor current-source circuits.
Comment: There is a significant decrease in the variation in bias current for the three-transistor circuit compared to that of the two-transistor circuit. For values of β greater than approximately 50, there is no perceptible change in bias current for the three-transistor current mirror.