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PNP Current Mirrors

The Basic Current Mirror with PNP Transistors

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.

There 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.

The 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.

Combining NPN and PNP Current Mirrors

A dual NPN and PNP current mirror. A dual current mirror with multiple outputs.

When we use both NPN and PNP transistors, we can use the same reference circuit for both, as shown to the left. Also, as shown to the right, either or both mirrors can have multiple outputs. Generally, it's a good idea to include the extra transistor to provide base current compensation if multiple outputs are used.

One important factor when using multiple outputs from a current mirror: some of the complex configurations cannot be used. For example, any version of the Wilson current mirror is limited to a single output, because the diode-connected reference transistor in the mirror is in the ouput circuit. Putting multiple references in multiple output lines would cause some very strange behavior. However, we can put resistors in all of the emitter circuits, or we can use the cascode mirror configuration.

Another way to provide different current magnitudes at different mirror outputs is to change the areas of the transistor emitters. A larger emitter area will transmit more current for a given value of VBE.

Driving One Current Mirror From Another

Using one current mirror as reference for another.

Another way to link NPN and PNP current mirrors is shown to the right. Here, an output (in some cases the only output) of the NPN mirror is used as the reference for the PNP mirror. Although this would seem to be unnecessary, and no different from using a common reference for both polarities, there are several possible reasons for doing it this way instead.

In the first place, by doing it this way, we can have a circuit where +V1 is not the same as +V2. Either can be higher than the other; as long as the ground point is common to all circuits and the supply voltages are both high enough for the purpose, all mirrors will work.

In some cases, +V1 is heavily regulated and filtered. This ensures that the mirror currents are always the same, regardless of any changes in +V2. Another possibility is that the primary NPN reference may be part of a control circuit of some kind, while the output mirrors are involved with data.

Prev: The Modified Wilson Current Mirror Next: FET Current Mirrors

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