13
e are injected to the base.
EB junction is forward-biased
Base is short
so before they recombine with holes in the base they will be swept by
electric field of reversed biased BC junction.
VBE = 0.7v
emitter
n+
E
B
collector
h
n+
n
e
base
sub-collector
Linear settling time
=
1
1
=
3dB ta
Folded cascade OPAMP ta =
g m1
Current Mirror OPAMP ta =
kg m1
CL
CL
Fully differential OPAMPs
differential-in differential-out
Use symmetry to reject substrate noise (unwanted coupling) because transistors are nonline
Modified version of wide-swing high-Z current mirror
Advantages compared to previous circuit:
Has smaller dissipated power
1 of the one +
2
dissipation Of
cascode stage.
And smaller area, and also more stable!
.
.
.
.
.
.
I in
.
.
Vcasc n
>
.
I bias
70
.
129
Advanced Current Mirrors and OPAMPs
Short channel length L
degraded output resistance
with high output resistance are needed
current mirrors such as cascode
signal swing is limited in cascode as 2
transistors have to stay in saturation.
Wide-swing Cas
113
A( j ) =
Av0
Av0 = 14000 in our case
j
1 +
p
Av0
A( j ) = = 1
1 + j
ta
= 1 ta = Av0 p
p
ta = 2 25MHz
Av0 = 14000
p = 2f p
f p 1.8 KHz ?
For p < < ta we can write:
A( j ) =
ta
j
When you put the opamp in a feedback loop:
<1
_
+
Vin
V x = V in
98
Operational Amplifier
Need for high gain stage:
.V
DD
Vin
A
Vout
Vout = AVin
problem is that with Vin having even small DC component
A
Vout will be saturated to either VDD or ground.
V DD = 3v
Assume A = 100,000
Vin = 1mvDC + 1v.Cos (2ft )
Vout = AVin
79
Noise in Resistors
Thermal noise (+ maybe flicker noise)
.
.
R
4kTRf
or
~
4kT
f
R
R
.
.
Noise in Diode
Shot noise (+ thermal noise from contact resistance + maybe flicker noise)
.
rd =
VT
ID
2
id = 2qI D f + k
.
RS
4kTRS f
~
.
Noise in MOS transistor
63
Noise in Integrated Circuits
Fundamental Noise Sources
Thermal Noise
(Johnson Noise, Nyquist Noise)
arises from thermally excited random motion of electrons
in a conductive medium
Noise Spectral
Density
Properties of thermal noise
1) noise is white (?)
57
Bipolar transistor Current Source
.V
.V .V
R
CC
I in
CC
CC
R
I out
I C1
I B1 + I B2
I C2
I B1
I B2
Current mirror with less current error
(not a function of )
Simple current mirror
In order to increase the output resistance:
Current mirrors with emitte
30
Output resistance of current mirror
V
.
DD
R
I D1
I D2
W
L 2
VDD VGS 2
R
I D2 =
W
L 1
= I D2
CoxW2
2 L2
Assuming both transistors is
in ideal MOS saturation
(VGS 2 Vt n ) 2
I D2 , VGS 2 are unknowns and you have two equations, therefore you can fin
22
Chapter 3
g m = n C ox
gm =
W
V
L eff
2 n C ox
W
ID
L
If source is at a different potential than body contact, you have body effect.
Vt n = Vt n 0 + ( V SB + 2 f 2 f )
You also introduce an additional trans-conductance known as body effect transconduct
8
As you know increase V DS
off.
your channel narrows down close to drain until it pinches
G
S
D
current saturates.
n+
n+
n
Depleted
I D = n C ox
Set V DS = VGS Vth in equation (I)
W
(VGS Vth ) 2 independent
2L
of V DS .
Condition for pinch off: VDS > VGS
1
Semiconductors
Intrinsic
e
EC
e
e
e
Si
EV
h
h
e
e
e
Si
Si
Si
Si
Si
hh
Si
Si
ni
Si
Si
Si
Si
N-material
EC
e
Si
Si
Si
Si
E donor
Si
P
Si
Si
EV
ND
Si
with minimum thermal energy this
becomes a free electron
density of Donor atoms like P.
Available free ele