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Design >> Analog Design >> combined Bipolar-MOSFET current mirror
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Message started by bki on Aug 14th, 2018, 7:42am

Title: combined Bipolar-MOSFET current mirror
Post by bki on Aug 14th, 2018, 7:42am

Hi everybody,

i was wondering what happens if i Combine a PMOS current mirror  (source connected to VDD) with a PNP current mirror (Collectors connected to the PMOS Drains) instead of a full PMOS cascode current mirror?

Does (PMOS current mirror with PNP current mirror connected as Cascode configuration) make sense to do it and does it bring any benefits compared to a full PMOS Cascode Current Mirror?

Thank you!


Title: Re: combined Bipolar-MOSFET current mirror
Post by Horror Vacui on Aug 20th, 2018, 8:42am

What is the idea behind the BJT cascode? Why would you do that?

I find BJTs inferior in current mirrors because you have base currents and the headroom is lower as well (Collector should be at a lower voltage than the base to keep it in forward active region). The only possible case I would think about it is if I would need a high voltage output and I have no high voltage MOS device, but bipolar.

Actually I would consider the opposite: using the BJT at the supply: I expect lower 1/f noise and the base terminal has lower impedance for the same current.

Title: Re: combined Bipolar-MOSFET current mirror
Post by RobG on Oct 21st, 2018, 11:14am

The base current of a bipolar device depends on the Vce voltage. For a bipolar cascode this voltage-dependent base current limits the amount of output resistance possible. Bipolars also don't recover smoothly when their base-collector junction is forward biased.

Bipolar mirrors are lousy because of their high gm (small mismatches result in large current changes). What you wind up doing is adding degeneration resistors, which are resistors between the emitters and the power supply. If you do that and have a good biCMOS process bipolar mirrors work well.

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