Is there good ways to switch on/off current source?

[wizardz]

I'm think about changing the bias current of an amplifier to achieve different output impedance.

The problems lies in the switching on/off the current source feeding the MOSFET of the amplifier. For example, I can cascode a CMOS switch on the current mirror source and then cut-off/open the current branch.

I find there involves some design considerations, it is hard to define the DC voltage at the node where switch and current source is conected during "on" state. Also, cascoding a MOSFET can consume voltage headroom.

Is there any better ways?

Thanks!

 

[dick_freebird]

Might think about steering rather than switching, if you
want fast settling or low glitching.

You could have a mirror rack where two weighted mirrors
are fed by two independent, matched source and you
shunt the intermediate gate node to ground or release,
sum the drains of the output devices for a composite
current. That is probably your minimum headroom option.

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[mdcui]

to correct a concept, cascode a switch to turn on/off a current mirror won't cost any voltage head room, if the device is big enough, the VDS drop across it shall be at a few mV, so totally not a problem, you can simulate it out and verify my words.
we use this kind of connection everywhere to enable/disable current mirrors, very common.

 

[wizardz]

Thanks dick_freebird,

your idea seems quite good