Download Current sources and current mirrors

Current sources and
current mirrors
 Current sources
- the current is independent of the load resistance
- the current is not influenced by the supply voltage or
temperature variations or other operating conditions
 Current mirrors
- Generates dc currents in direct ratio with a reference current
- Used to bias integrated circuits
 Current sources and current mirrors with
- MOSFET
- BJT
MOSFET current sources
 pseudo-sources, consume power
=> voltage sources are necessary
Current
sink
I   n VGS  VThn 
2
VO  VPS  RL I
VO  VDSsat ;
VPS  RL I  VDSsat ;
VPS  RL I  VG  VThn
Current
source
I   p VGS  VThp 
2
VO  VPS  RL I
MOSFET current mirror (integrated transistors)
 if the gate-to-source voltages of two identical MOST are
equal, the currents through the transistors are equal
VGS1  VG  VGS 2
W  W 
   
 L 1  L  2
I O  I REF
How can IREF be obtained?
MOSFET current mirror
 if the gate-to-source voltages of two identical MOST are
equal, the currents through the transistors are equal
VGS
T1 – MOS diode
I REF
VPS  VGS

R
To set a certain IREF value:
V  VGS
R  PS
I REF
I REF
K1  W 
2



V

V
  GS
Th1
2  L 1
VPS  VTh1
1
R

I REF
I REF ( K1 2)(W L)1
 Current multiplier mirrors
(different W/L ratios of the transistors)
K1=K2 VTh1=VTh2
VGS
K1  W 
2


I REF 
V

V
  G Th1
2  L 1
K2  W 
2


IO 
V

V
  G Th 2
2  L 2
W L 2
IO 
I REF
W L 1
Numerical
example
What is the dc biasing
current of the T transistor
in the CS amplifier?
0  VGS 1   VPS  VPS  VGS 1


I1 
R
R

I1  K  W  VGS 1  VTh 2 ;

2  L 1
I1  0.2mA
W L 2
I  I 2  I1
W L 1
VPS=12V; K=0.04mA/V2,
VTh=1.2V, R=42.8KΩ; (W/L)1=2,
(W/L)2=10, (W/L) =4
10
I  0.2 
 1mA
2
Numerical
example – cont.
Are the transistors in the
current source in the active
region?
T1 being a MOS diode,
it is always in aF
K W
I 
2L
VGS  VTh 

2


V

V
 GS
Th

I
 1.2 
K W 
 
2L
1
 4.7V
0.04
4
2
VS  4.7V; VDS 2  4.7   12  7.3V
VPS=12V; K=0.04mA/V2,
VTh=1.2V, R=42.8KΩ; (W/L)1=2,
(W/L)2=10, (W/L) =4
VGS 2  3.4V
VDSsat 2  2.2V, T2  (a F )
Biasing a MOST integrated circuit
Optional
BJT current sources
Current
sink
T must stay in aF (VCE < VCEsat)
VPS  VCEsat
RL 
I
Current
source

VB  VBE
I
IE; IE 
 1
RE
 VB VBE VB VBE
I

  1 RE
RE
VB  VBE
I
RE
BJT current sources
the base potential can be set with diodes
2VD  VBE 0.7V
I

RE
RE
VZ  VBE
I
RE
Thermal
compensation
Adjustable
current
source?
BJT current mirror
I REF
IO 

IE
 2

IE  2

IE
 1
 1  1

 1
IE
1
IO 
I REF  I REF
1 2 
Biasing a BJT integrated circuit
I REF 
2VPS  VBE1  VBE2
R
Optional