question about current mirror?

[wanily1983]

source degeneration resistor current mirror

hi, guys
:?::?:the source degration Resistor can decrease the mismatch of the current mirror, one reason is that this method can decrease the difference of the Vds, any other reason? or some reason from the viewpoint of layout? hope for ur reply.
regards
wanily

 

[Syukri]

transistor current mirror

well in term of layout any addtional device is not that ok...it makes the circuit bigger, but in sertance cases it makes the environment similar...and thats good

 

[wanily1983]

source degeneration current

hi, Syukri
but it always uses dummy to make the cell matching.
anybody has else explain?

 

[dipswitch]

purpose of resistors in current mirror

The reason for a degeneration resistor in a current mirror is not for mismatch. The reason is only to increase output resistance.

It the texts, you'll see that the output resistance of a single transistor current mirror is Ro, and when you apply a source degeneration resistor Rs, the output resistance becomes gmRoRs.

Note that you may be confusing the issue of reducing mismatch with common source amplifiers. In a common source amplifier, you can reduce mismatch with source degeneration, since it reduces dependence on gm, however this is not the case in current mirrors.

Hope this helps

 

[avinash]

current mirror source degeneration

i do not agree with dipswitch.source degeneration in current mirror is used to reduce the effect of mismatch between transistor parameter.

 

[huojinsi]

source degeneration current matching

The purpose of source degeneration resistor in current mirror is twofold. First, it reduces the effect of mismatch. Second, it increases the output resistance of current source.

 

[dipswitch]

degeneration to reduce current mirror noise

Actually, huojinsi is correct as concerned with bipolar technology: by using low-valued resistors (and well matched resistors, of course) you reduce mismatch.

However with MOSFETs, there is no improvement in matching. And if one is not careful, the small mismatches in the resistors themselves will be effectively multiplied by gm, and can result in increased mismatch!

 

[wanily1983]

degeneration matching current source

i think dipswitch is right. in bjt case, the resistor connect the emitter is for reduce the mismatch of Is and in mos case, it has not this improvement. for both case, it can incraese the output resistance.

 

[safwatonline]

common source degeneration current source

i am not sure but i think it decrease the noise

Added after 2 minutes:

actually i have a question : does the the deg. resistance is one resistance common in the two branches cause if else i think there will be additional mismatch "in case two resistors used"

 

[flushrat]

degeneration emitter resistors in current mirror

Increases the output resistance will reduce the effect of mismatch between transistor parameter. They are same case.

 

[DenisMark]

mos mismatch source degeneration

This method is widely used bipolar technology for increase currents matching. But in mos technology using source degeneration resistor is not situable, because product Rs*gm*Ro is less than Ro for single transistor (if u don't use Rs in MegOhm range). For increase output resistence is better to use low W/L ratio or cascode connection.