Difference between simple cascode current source and modified Wilson current source

• Aug 24, 2004

• #1

R

rajath

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Hi all,
I've been going through some current source topologies for my work, and I acn't seem to find the difference between a simple cascode current source and a modified Wilson current source (see attached figure). Eventhough the textbooks claim that the negative feedback in Wilson increases the output resistance, the final output resistance for both circuits turn out to be the same after some approximations. Is there any advantage in using one over the other?

thanks.

 

• Aug 24, 2004

• #2

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guamak_menanak

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mos wilson current mirror

I had used 'cascode current source' for my DAC...
I think the it can mirror current very well!

I think there is not a very big issues on those current source...
'cascode" and 'wilson' is a good current source....

You can read form books for better explanation...
I just tell you from my previous experience!

 

• Aug 24, 2004

• #3

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sutapanaki

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wilson current mirror mos

Well, for DC and not very high frequencies they may be very comparable. I don't remember what the output resistance is for the Wilson one, anyway it can be easily derived. After all both of them provide large enough output resistance.
For high frequencies, though, I think that the cascode one is better. In Wilson mirror there is feedback which is always a potential problem for higher frequencies causing instability.

 

• Aug 24, 2004

• #4

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jordan76

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wilson mos mirror

If my memory is correct, it is said in some text that the output resistance for cascode current source is about twice the modified wilson current source and that is why cascode current source is preferred in practice.

regards,
jordan76

 

• Aug 24, 2004

• #5

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Question

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cascode current mirror bjt

This is one of my question when study current source.

 

• Aug 26, 2004

• #6

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gauchosuizo

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cmos wilson current mirror

hi people

I think the diference between the both bandgap circuits is the inside resistance(output resistance). By Wilson is greater than Cascode.

Aloha Pablo

 

• Aug 26, 2004

• #7

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sunking

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mos cascode current feedback lecture

Yes, the input and output Res. is different.
and the need voltage is different too

 

• Aug 26, 2004

• #8

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kyrandia

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bjt cascode current mirror

jordan76 said:

If my memory is correct, it is said in some text that the output resistance for cascode current source is about twice the modified wilson current source and that is why cascode current source is preferred in practice.

regards,
jordan76

Click to expand...

I calculate the output res. If take input current source's output res into accoutn, you are right. And the both minimal voltage of them are Vth + 2Vdsat. Then can you tell me what is the wilson's advantage?

thanks

 

• Aug 27, 2004

• #9

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jordan76

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wilson mirror output resistance

I once saw a similar structure like modified wilson current source to be used as current amplifier for its high speed.

Please correct me if I am wrong.

regards,
jordan76

 

• Aug 27, 2004

• #10

V

v_naren

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wilson current mirror mosfet

The wilson current source is particularly advantageous to use in the case of bipolar tech. There is no advantage for the case of MOSFETs. The explanation is given in more detail by Prof.Brodersen in his video lecture(EECS140 at https://webcast.berkeley.edu) when another person asked the same question as you did.

 

• Aug 31, 2004

• #11

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the wilson output resistance

hi v_naren,

Could you elaborate on your point since I cannot see the video with the link? Thanks.

regards,
jordan76

 

• Aug 31, 2004

• #12

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chinboon

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cascode current source using bjt

Well basically wilson current mirror has one significant advantage that i think most of you all missed it out. Remember the negative feedback in wilson configuration. It increase output resistance but in fact it also helps to reduce the distortion.
Imagine if at the output connecttion there is a distortion the ditortion would be feedback to form an inverted distortion which in returns cancel the original distortion.
One of the significant disadvantage about wilson is that the poor current mirroring as vds of both transistor would differ due to channel length modulation effect.
If you wan precise current match use improved wilson current mirror instead.

 

• Sep 1, 2004

• #13

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rajath

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modified wilson output resistance

Doesn't that mean that the output is low resistance? What you are saying is that any change in the output voltage
is countered by the negative feedback, and hence the voltage is being maintained a constant. This works against the
principle of a current source: it should supply contsant current and not bother about changing voltages.
Of course, my understanding of distortion is pretty weak, so I'm assuming that it is ome kind of voltage distortion.

 

• Sep 1, 2004

• #14

V

v_naren

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cascode bjt mirrors

hi chinboon
please kindly note that the o/p resistance is almost same for both wilson and cascode. then where is the advantage of wilson's negative feedback etc etc??

uiltimately the o/p resistance is the same.....then the negative feedback is not useful as u have said....I have mentioned before also Prof. Brodersen has discusssed that the advantage of wilson is only in case of BJT.

FOR CMOS IMPLEMENTATION WILSON HAS NO ADVANTAGE OVER CASCODE!!!

 

• Sep 1, 2004

• #15

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chinboon

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wilson mirror impedance

Thanks for the information provided. Basically what i mention here is what being taught by my lecturers. I am not sure of the thing you mention. To be practical i wil test it out my own. Sorry for the confusion caused.

 

• Sep 2, 2004

• #16

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jordan76

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bjt wilson current mirror

I checked the textbook about the cascode and wilson part and I reparaphrase it as below:
Modified Wilson current source is often preferred than cascode current source in BJT because BJT cascode current source suffers more due to finite base current.

For CMOS, cascode current source is more preferred than modified wilson current source for the reasons I mentioned before.

regards,
jordan76

 

• Sep 3, 2004

• #17

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li-jerry

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wilson current source cmos

Hi, the output swing is quite different, and i think this point is important because you only need 2Vds dropped on the two transitors in cascode but Vds+vt on Wilson current source.

 

• Sep 3, 2004

• #18

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improved wilson mirror

Hi li-jerry,

If you check any book dealing with cascode and wilson current source,
you will find that the minimum output voltage for both to stay in saturation region is 2*Vdsat+Vth. :wink: Of course, if adding some modifications,
you can make cascode current source minimum output voltage only 2*Vdsat.

regards,
jordan76

 

• Sep 30, 2005

• #19

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nuiscet

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modified wilson current mirror

I have the same puzzle before, by and large comprehend the difference beween cascode current source and modified Wilson current source. thanks.

 

• Sep 30, 2005

• #20

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wanily1983

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current source wilson output impedance

The main limitation of BJT cascode mirror is that the systematic gain error stemming from finite beta was large. To overcome this limitation, the wilson mirror is used. Maybe in MOS case, the performance is almost the same, since the mirror usually work at LF. The Vout(min) or Vin(min) of them in both BJT and MOS case are almost the same.