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The Basic Current Mirror with PNP Transistors
4 i9 p9 t5 v0 n$ C/ }A basic current mirror using PNP transistors. All of the current mirror circuits presented in previous pages in this group have used NPN transistors. This is by no means necessary; as shown to the left, we can easily use PNP transistors to build current mirrors. Every mirror configuration that can be built with NPN transistors can also be built with PNP transistors, and the general behavior, advantages, and disadvantages of each configuration will be the same.$ M5 E& Z+ V% K) M& l. p
5 k; m- N6 r9 G$ L" dThere are only two differences in using PNP rather than NPN transistors in current mirrors. The first is the obvious fact that the applied voltage polarity must be reversed. The emitters are connected to +V, while the collectors are referenced to a more negative voltage. This can be ground as shown here, or any voltage more negative than +V. In many practical circuits, both positive and negative power supplies are used to maintain balanced voltages, and in such circuits the PNP collector circuits are commonly referenced to -V rather than to ground.: m/ j: u" J7 E& ?" w
1 w* R- b- h vThe second factor to be considered is that in an IC, while it is always possible to manufacture both NPN and PNP transistors on the same die, it is not possible to make them the same way or to match their parameters. NPN transistors are made vertically, so the width of the base region can be closely controlled to enable high values of current gain. PNP transistors must be constructed horizontally (known as "lateral PNP transistors"), which means the base region must be wider, so the current gain is necessarily more limited. This does not preclude their use, but it does mean that their lower current gain must be accounted for, and in many cases compensated in some way.
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发表于 2018-4-23 18:04
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发表于 2018-4-24 08:50
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