CURRENT SOURCES, CURRENT SINKS

1
CURRENT SOURCES, CURRENT SINKS AND CURRENT
MIRRORS
2
(No Transcript)
3
(No Transcript)
4

• CURRENT SINKS AND SOURCES
• Characterization of Current Sinks and Sources
• Minimum voltage (VMIN) across sink or source for
  which the current is no longer constant.
• Output resistance which is a measure of the
  flatness of the current sink or source.

5
(No Transcript)
6
(No Transcript)
7
Increasing the output impedance ROUT
of a Current Source
8
Normally gm ? 10gmbs ? 100gds giving us
9
CURRENT MIRRORS AND CURRENT AMPLIFIERS
10
Sources of Errors
11
If the devices are matched
If VDS1 VDS2 then
12
Matching Accuracy of Current Mirrors
13
(No Transcript)
14
(No Transcript)
15
Layout Technique to Remove Layout Error
16
(No Transcript)
17
(No Transcript)
18
WILSON CURRENT MIRROR
19
This modification to the Simple Current mirror
does ensure an increased output impedance, but
does not ensure VDS1 VDS2. Vmin obtained in
this configuration is the sum of the voltage,
VDS1(sat) and VDS3(sat), the minimum drain to
source voltage required to keep M3 and M1 in
saturation. Defining VON VGS VTH, and
assuming that all transistors have same W/L
ratio, we have VDS1 VGS1 VON VTH and
VDS3(min) VON and Vmin 2VON VTH.
20
MODIFIED WILSON CURRENT MIRROR
21
CASCODE CURRENT MIRROR
22
REGULATED CASCODE CURRENT MIRROR
If all transistors have the same size, VGS2
VDS1 VGS VDS4, giving us a very good match in
mirroring.
23
(No Transcript)
24
CURRENT MIRROR WITH LOW Vmin and GOOD MIRRORING
PROPERTY
We see that in the Cascode Current Mirror, the
voltage VGG2 was obtained as 2VGS and hence we
had a Vmin of 2VGS VTH 2VON VTH. However if
we can obtain VGG2 2VON VTH, Vmin can be
reduced to 2VON.
25
(No Transcript)
26
WIDE RANGE CASCODE CURRENT MIRROR
27
COMPARISON OF VARIOUS CURRENT MIRROR ARCHITECTURE
Current Mirror Accuracy Output impedance Vmin
Simple Poor rds VON
Wilson Poor 2VON VTH
Improved Excellent 2VON VTH
Cascode Excellent 2VON VTH
Regulated Cascode Good 2VON VTH
Cascode Excellent 2VON