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In this chapter, we will:
Microelectronics
Analyze and understand the characteristics of
various bipolar circuits used to provide a
constant output current.
Analyze and understand the characteristics of
various MOSFET circuits used to provide a
constant output current.
Analyze the dc characteristics of amplifier circuits
using transistors as load devices (active loads).
Analyze the small-signal characteristics of
amplifier circuits with active loads.
Design an NMOS current source circuit to provide
a specified bias current and output resistance.
Circuit Analysis and Design
Donald A. Neamen
Chapter 10
Integrated Circuit Biasing
and Active Loads
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-1
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-3
Chapter 10-2
DC Equivalent Circuit:
2-Transistor Current Source
2-Transistor Current Source
Neamen
Microelectronics, 4e
McGraw-Hill
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-4
1
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2-Transistor Current Source
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-5
2-Transistor Current Source
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-6
3-Transistor Current Source
3-Transistor
Current Sources
produce stable
Io as β changes.
=
=
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-7
Neamen
Microelectronics, 4e
McGraw-Hill
2
1+
(1 + )
− − − _
Chapter 10-8
2
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Problem-Solving Technique:
BJT Current Source Circuits
Other 3 T Current Sources
Widlar
Wilson
(Io not same as Iref)
(higher output resistance)
1. Sum currents at various nodes to find
relation between reference and bias currents
2. To find output resistance place test voltage at
output node and analyze small-signal
equivalent circuit. Since the reference current
is a constant some base voltages may be
constant or at ac ground.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-9
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-10
Widlar Current Source Analysis
$1%
2 = 23 Widlar Current Source
= 10 / = 5
= =3 = 6 %
Which is too large for most IC
designs
Microelectronics, 4e
McGraw-Hill
Chapter 10-11
<
sothat = E ln
6
3 = E ln
()%
6
− 3 = E ln
H%%I'I#'( − 3 = 3 ≅ Finally
)
= E ln(
"#$%&#'%()(
5 − 0.7 − (−5)
=
= 0.9/Ω
10,10 -
Neamen
789:
7;
>89?
<>
;
()% ≅ 5 = 6 %
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-12
3
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Multi-transistor current Mirror
Multi-output Current Mirror
1. 2 'HI%I(%H'%.
Q = + 3 + 2. %H(J$'H)%H 3. 'HLL$'%(M#(L#(
2N HH
Multitransistor
= 3 = ⋯ = P =
Neamen
(1 + M)
1+
Microelectronics, 4e
McGraw-Hill
Chapter 10-13
Neamen
Chapter 10-14
MOSFET Current Source
Generalized
Current Mirror
=
Microelectronics, 4e
McGraw-Hill
− − − Q = = Q
Q3 = 2
QR = 3
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-15
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-16
4
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MOSFET Current Source
MOSFET Current Source
Alternatively
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-17
MOSFET Current Source
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-19
Chapter 10-18
MOSFET Wilson Current Source
These provide higher output impedance.
Uses M3 instead of a resistor
Neamen
Microelectronics, 4e
McGraw-Hill
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-20
5
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BJT Amplifier with Active Load
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-21
BJT Amplifier with Active Load
BJT Amplifier with Active Load
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-22
BJT Amplifier with Active Load
In the forward active region the
Output voltage is in the range
=U(6VW) < Q < ( − =3
6VW
)
The output voltage from Eqn. 10.82 is
7:
<
SP ST
6Q % 7;
SP
=
1−
+
( − 3 )
SP + ST
SP + ST
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-23
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-24
6
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Voltage Gain of BJT Amplifier with
Active Load
Characteristics of Bipolar Circuit with
Active Load
The derivative of Eqn. 10.82 is
7[
<
SP ST
6Q % 7; 1
Y =
= −(
)
( )
Z
SP + ST
E
≅ 6Q %
7[
<7
;
SP ST
Y =
=−
Z
SP + ST
Neamen
1
=
E
1
− E
1
1
SP + ST
Microelectronics, 4e
McGraw-Hill
Chapter 10-25
MOSFET Amplifier with Active Load
=
Neamen
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-26
MOSFET Amplifier with Active Load
3
[1 + λ^ − _` ] cb (Z − EP )
−
λ^ + λb
(λ^ + λb )
Microelectronics, 4e
McGraw-Hill
Chapter 10-27
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-28
7
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Small-Signal Equivalent Circuit:
BJT Active Load
MOSFET Amplifier with Active Load
Y =
−2cb (Z − EP )
=
Z
(λb + λ^ )
H'I%1d = 2cb (Z − EP ) and = Z
e9f g
Y =
Neamen
−1d
= −1d (b ||^ )
1
1
+
b ^
Microelectronics, 4e
McGraw-Hill
The resistance looking into the
collector of Q2 is just = 3
Chapter 10-29
Fig. 10.37
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-30
Fig. 10.37
Y =
Q = −1d i ( 3 j )
Y =
= −1d ( 3 j )
k
= −1d ( 3 j )
k
Note: 1d = Zl<7; & = 7no<Zl &3 = 7np<Zl
Hqj = ∞;
1
−( )
E
Y =
1
1
+
SP
ST
As before!!
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-31
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-32
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Fig. 10.40
Small-Signal Equivalent Circuit:
MOSFET Active Load
Y =
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 10-33
Neamen
= −1d ( 3 j )
k
Microelectronics, 4e
McGraw-Hill
Chapter 10-34
9