How to size transistors for cascoded current mirror?

• Mar 20, 2006

• #1

S

surreyian

Member level 3

Joined

Feb 10, 2006

Messages

63

Helped

0

Reputation

0

Reaction score

0

Trophy points

1,286

Activity points

1,796

cascoded current mirror

hi,

designing cascoded currrent mirror, how do we size the transistors?
i usually see (for NMOS current mirror) they usually have a large L at the bottom, why is it so?
having a large L will cause the vgs to increase, arent we trying to get a smaller vgs?

and, in cascoded mirror, the top transistor is the one affecting the mirroring effect?

 

• Mar 20, 2006

• #2

safwatonline

Advanced Member level 4

Joined

Nov 19, 2005

Messages

1,341

Helped

219

Reputation

438

Reaction score

48

Trophy points

1,328

Location

EGYPT

Activity points

9,038

Re: cascoded current mirror

hello,
being a newbie in analog design dont take my words as FACTS, anyway i think that large L is to improve mirroring (matching) as the current is directly prop. to (1+λVds) and u cannot make sure that Vds is the same for the two MOS , so we tend to decrease the effect of this term by decreasing λ (i.e. increasing L), also as L increase the o/p resistance increase which is also wanted " ideal current sources infinte R".
bye.
a.safwat

 

• Mar 21, 2006

• #3

H

Humungus

Full Member level 6

Joined

Jul 10, 2001

Messages

383

Helped

41

Reputation

82

Reaction score

15

Trophy points

1,298

Activity points

3,985

Re: cascoded current mirror

When designing cascoded current mirrors, the important thing is the output swing. Once you have determined the minimum allowable output voltage, size your transistors as to have an equivalent saturation voltage that satisfies your needs.

Actually, you can bias the mirror transistors to be in weak inversion as the W/L ratio increases faster than gm/Id. Also, use non-minimum L (~3-4 times minimum L will be OK) to reduce the posible mismatch of the Vds of the mirror transistors due to the Vgs mismatch of the cascode ones.

 

• Mar 21, 2006

• #4

obi1

Member level 2

Joined

Mar 2, 2006

Messages

43

Helped

1

Reputation

2

Reaction score

1

Trophy points

1,288

Location

Romania

Activity points

1,590

cascoded current mirror

if L rise , Ro rise and matching is better, but you consume more silicon beacause you must grow W.

 

• Mar 22, 2006

• #5

H

holddreams

Full Member level 6

Joined

Aug 2, 2005

Messages

351

Helped

15

Reputation

30

Reaction score

7

Trophy points

1,298

Location

Shanghai

Activity points

4,237

Re: cascoded current mirror

Page 257 to page 259,Ken Martin's book:

for the wide-swing cascode current mirror,the L of the bottom transistors are less than the top.

I have seen some design circuits, the L of the bottom transistors are larger than that of the top transistors.
I get confused.

 

• Mar 23, 2006

• #6

L

laglead

Full Member level 5

Joined

Feb 21, 2006

Messages

262

Helped

22

Reputation

44

Reaction score

8

Trophy points

1,298

Activity points

2,693

Re: cascoded current mirror

surreyian said:

hi,

designing cascoded currrent mirror, how do we size the transistors?
i usually see (for NMOS current mirror) they usually have a large L at the bottom, why is it so?
having a large L will cause the vgs to increase, arent we trying to get a smaller vgs?

and, in cascoded mirror, the top transistor is the one affecting the mirroring effect?

Click to expand...

Large L transistor has small gm which can improve matching and has low flick noise.
Small L has big gm which can boost the current mirror's output resistance.

 

• Mar 23, 2006

• #7

safwatonline

Advanced Member level 4

Joined

Nov 19, 2005

Messages

1,341

Helped

219

Reputation

438

Reaction score

48

Trophy points

1,328

Location

EGYPT

Activity points

9,038

cascoded current mirror

hello laglead,
i didnt get what u meant by decreasing L to get high gm
in order to get higher Rout , i think that increasing the L will increase the Rout and if u are talking about cascode then to increase the gm u may use large L along with large W.
regards,
a.safwat

 

• Mar 23, 2006

• #8

L

laglead

Full Member level 5

Joined

Feb 21, 2006

Messages

262

Helped

22

Reputation

44

Reaction score

8

Trophy points

1,298

Activity points

2,693

Re: cascoded current mirror

hello safwat,
Of course increasing L while keeping aspect ratio constant can get high Rout.
But it is area consuming in most cases. Generally the smallest length in a specific process such as 0.18 or 0.13um is used in upper N transitor in the cascode current mirror. Then large w of this transistor can increase gm. The upper transistor then can boost the lower long-length transistor Rout by gm*r0 times.
Choice of small L of the upper transistor is mainly due to the concern of area.

 

• Mar 26, 2006

• #9

M

montage3000

Member level 4

Joined

Mar 3, 2006

Messages

68

Helped

2

Reputation

4

Reaction score

1

Trophy points

1,288

Activity points

1,635

cascoded current mirror

at bottom and top the large size to reduce offset

 

• Oct 27, 2006

• #10

R

rajharvijay

Member level 1

Joined

Oct 27, 2006

Messages

36

Helped

3

Reputation

6

Reaction score

2

Trophy points

1,288

Activity points

1,503

Re: cascoded current mirror

hey guys this is the procedure of design of cascode current mirror...
i hope you would like it....
please email me back if you have any thing to say....


and ya thanks always welcommed...

bye

 

• Nov 7, 2010

• #11

T

turtlewang

Member level 2

Joined

Nov 7, 2010

Messages

46

Helped

1

Reputation

2

Reaction score

1

Trophy points

1,288

Activity points

1,605

Re: cascoded current mirror

rajharvijay said:

hey guys this is the procedure of design of cascode current mirror...
i hope you would like it....
please email me back if you have any thing to say....


and ya thanks always welcommed...

bye

Click to expand...

thanks very much!

 

• Feb 10, 2012

• #12

N

numberto

Newbie level 1

Joined

Aug 21, 2008

Messages

1

Helped

0

Reputation

0

Reaction score

0

Trophy points

1,281

Activity points

1,283

That's Great!
Thanks

 

• Mar 13, 2013

• #13

H

hujiaomianhao

Newbie level 4

Joined

Jun 28, 2008

Messages

5

Helped

1

Reputation

2

Reaction score

2

Trophy points

1,283

Activity points

1,299

hey guys this is the procedure of design of cascode current mirror...
i hope you would like it....
please email me back if you have any thing to say....


and ya thanks always welcommed...

bye
Attached Files
eee523proj1_1779.pdf (233.5 KB)



thanks for your sharing