The book does not provide simulation results for these pages, thus this report is simply a brief lab note for future reference.

Bipolar Devices

given a bipolar current mirror, how does one scale output currents?

As noted in the book (refer to pages 3-5 below), the current ratio is determined by increasing or decreasing the geometry of one of the matching transistors (Q1 or Q2 for for the Widlar Current Mirror). More specifically for a bipolar transistor the ratio is determined by the size of the emitter – the active emitter length. This is accomplished by increasing the number (and size) of identical emitter diffusions as well as the number (and size) of the surrounding base diffusions, subsequently the single collector diffusion is increased proportionally in size (but not in number) to support larger currents. (refer to book pages 1-19 below)

For example:

  • Scale Up: By increasing the number of identical emitters to three for Q2, you multiply the output current by a factor of three relative to the current reference.

  • Scale Down: By increasing the number of identical emitters to three for Q1, you divide the output current by a factor of three.

Any ratio is possible by varying the emitter length of one of the current matching transistors. Keep in mind however that the base current of Q2 will change proportionally with the scaled collector current and this will have an effect in the base current error of the mirror.

MOS Devices

Given a MOS current mirror, how does one scale output currents?

As noted in the book (refer to pages 3-6 below), one can scale the output current by varying the channel widths for the current matching transistors (M1 or M2 for the Simple MOS Current Mirror). However a better alternative is to use multiple identical devices in parallel which provides better matching.

References



Leave your comments below (or comment directly here).

Thank you for your feedback.