differential and multistage amplifiers elz 303
TRANSCRIPT
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Differential and MultistageDifferential and Multistage
AmplifiersAmplifiers
ELZ 303 ELZ 303 -- ElektronikElektronik IIII
Dr. Mehmet SiraDr. Mehmet Siraçç ÖÖzerdemzerdem
ElektrikElektrik ElektronikElektronik MMüüh. Bh. Bööllüümmüü
Dicle Dicle ÜÜniversitesiniversitesi
Microelectronic Circuits – Fourth Edition
Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press
Copyright 2004 by Oxford University Press, Inc.
The basic BJT differential-pair configuration.
Microelectronic Circuits - Fifth Edition Sedra/Smith
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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Different modes of operation of the BJT differential pair
The differential pair with a common-mode input signal vCM
Mode 1
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/SmithThe differential pair with a “large” differential input signal.
Mode 2
Different modes of operation of the BJT differential pair
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The differential pair
with a large differential
input signal of polarity
opposite to that in
mode2.
Microelectronic Circuits - Fifth Edition Sedra/Smith
Different modes of operation of the BJT differential pair
Mode 3
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
The differential pair
with a small differential
input signal vi. Note that
we have assumed the
bias current source I to
be ideal (i.e., it has an
infinite output
resistance) and thus I
remains constant with
the change in vCM.
Different modes of operation of the BJT differential pair
Mode 4
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Large-Signal Operation of the BJT Differential Pair
Microelectronic Circuits - Fifth Edition Sedra/Smith
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Large-Signal Operation of the BJT Differential Pair
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Large-Signal Operation of the BJT Differential Pair
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Small-Signal Operation of the BJT Differential Amplifier
Microelectronic Circuits - Fifth Edition Sedra/Smith
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Small-Signal Operation of the BJT Differential Amplifier
Microelectronic Circuits - Fifth Edition Sedra/Smith
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Small-Signal Operation of the BJT Differential Amplifier
Microelectronic Circuits - Fifth Edition Sedra/Smith
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An alternative viewpoint
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An alternative viewpoint with emitter resistances
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Input Differential Resistance
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Differential Voltage Gain
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Differential Voltage Gain
Microelectronic Circuits - Fifth Edition Sedra/Smith
If the output is taken differentially,
If the output is taken single-endedly,
For the differential amplifier with emitter resistances,
if the output is taken differentially,
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Equivalence of the Differential Amp. to a Common Emitter Amp.
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Microelectronic Circuits - Fifth Edition Sedra/Smith
The differential half-circuit
Equivalence of the Differential Amp. to a Common Emitter Amp.
Input resistance =
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Equivalence of the Differential Amp. to a Common Emitter Amp.
The differential amplifier fed in a single-ended fashion.
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(a) The differential amplifier fed by a common-mode voltage signal vicm.
(b) Equivalent “half-circuits” for common-mode calculations.
Common- Mode Gain (Assume that the circuit is perfectly symmetric)
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Common- Mode Gain (Assume that the circuit is perfectly symmetric)
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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
If the output is taken single endedly, the common mode gain
Since in this case the differential gain is
The common-mode rejection ratio (CMRR)
The common-mode rejection ratio (CMRR) in dB
Common- Mode Gain (Assume that the circuit is perfectly symmetric)
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Common- Mode Gain (Assume that the circuit is NOT symmetric)
For a mismatch Rc in the collector resistances
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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Common- Mode Gain
The circuit is symmetric The circuit is NOT symmetric
>
CMRR CMRR<
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
(a) Definition of the input common-mode resistance Ricm.
(b) The equivalent common-mode half-circuit.
Input Common-Mode Resistance (Ricm)
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Input Common-Mode Resistance (Ricm)
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Example
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Other nonlinear characteristics of the differential amplifier
The BJT differential pair with both
inputs grounded.
If the two sides of the differential
pair were perfectly matched, then
the current I would split equally
between Q1 and Q2, and Vo would
be zero.
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Practical circuits exhibit mismatches
that result in a dc output voltage Vo
even with both inputs grounded.
Vo : The output dc offset voltage
VOS = VO / Ad
Vos : The input offset voltage
Ad : The differential gain
Other nonlinear characteristics of the differential amplifier
If we apply a voltage –Vos between the input terminals, then the
output voltage will be reduced to zero.
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Other nonlinear characteristics of the differential amplifier
Let the transistors have a mismatch in the load resistance Rc1 and Rc2.
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Other nonlinear characteristics of the differential amplifier
Let the transistors have a mismatch in the load resistance Rc1 and Rc2.
Example
Consider the situation where the collector resistors are
accurate to within ±1%. Then the worst case mismatch will be
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Other nonlinear characteristics of the differential amplifier
Let the transistors have a mismatch in their emitter-base junction areas.
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Other nonlinear characteristics of the differential amplifier
Let the transistors have a mismatch in their emitter-base junction areas.
Example
An area mismatch of 4% gives rise to ( Is/Is)=0.04 and input
offset voltage of 1mV.
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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Other nonlinear characteristics of the differential amplifier
Let the transistors have a mismatch in
In a perfectly symmetric differential pair
A mismatch in make the two input dc current unequal. The
resulting difference is the input offset current, IOS .
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Other nonlinear characteristics of the differential amplifier
Let the transistors have a mismatch in
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Biasing in BJT integrated Circuits
Microelectronic Circuits - Fifth Edition Sedra/Smith
The diode-connected transistors
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Biasing in BJT integrated Circuits
Microelectronic Circuits - Fifth Edition Sedra/Smith
The Current Mirror
IREF=IO
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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/SmithAnalysis of the current mirror taking into account the finite of the BJTs.
Biasing in BJT integrated Circuits
The Current Mirror
IREF=IO
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Biasing in BJT integrated Circuits
A simple current source
IREF=IO
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Current
steering
circuits
Biasing in BJT integrated Circuits
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Biasing in BJT integrated Circuits
Improved current source circuits
1A current mirror
with base-current
compensation.
IREF=IO
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Biasing in BJT integrated Circuits
Improved current source circuits
2
The Wilson bipolar
current mirror
IREF=IO
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Biasing in BJT integrated Circuits
Improved current source circuits
3
The Widlar
current source
IREF IO
23
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Biasing in BJT integrated Circuits
Example
Microelectronic Circuits - Fifth Edition Sedra/Smith
Io=10μA (Const.)
R1=?
Assume that
VBE=0.7V at a
current of 1mA.
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Biasing in BJT integrated Circuits
Example
Microelectronic Circuits - Fifth Edition Sedra/Smith
Io=10μA (Const.)
R1=? R3=?
Assume that
VBE=0.7V at a
current of 1mA.
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Example
A four-stage
bipolar op amp
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Example
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MOS Differential Amplifiers
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MOS Differential Amplifiers
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MOS Differential Amplifiers
At the bias point, vid=0
Eqn. (1)
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MOS Differential Amplifiers
vGS1 - vGS2 = vid = 0 → vGS1 = vGS2 = VGS
Eqn. (2)
Reform of Eqn. (1)
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MOS Differential Amplifiers
For small-signal approximation
Reform of
Eqn. (2)
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MOS Differential Amplifiers
28
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(a) The MOS differential pair with both inputs grounded. Owing to
device and resistor mismatches, a finite dc output voltage VO results.
(b) Application of a voltage equal to the input offset voltage VOS to
the terminals with opposite polarity reduces VO to zero.
MOS Differential Amplifiers
Offset Voltage
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MOS Differential Amplifiers
Microelectronic Circuits - Fifth Edition Sedra/Smith
Let the transistors have a mismatch in load resistance
29
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MOS Differential Amplifiers
Microelectronic Circuits - Fifth Edition Sedra/Smith
Let the transistors have a mismatch in the W/L ratios
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MOS Differential Amplifiers
Microelectronic Circuits - Fifth Edition Sedra/Smith
Let the transistors have a mismatch in the W/L ratios
30
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MOS Differential Amplifiers
Microelectronic Circuits - Fifth Edition Sedra/Smith
Let the transistors have a mismatch in the Vt
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MOS Differential Amplifiers
Microelectronic Circuits - Fifth Edition Sedra/Smith
Let the transistors have a mismatch in the Vt
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MOS Differential Amplifiers
Current Mirrors
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MOS Differential Amplifiers
Current Mirrors
ID1
ID2
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MOS Differential Amplifiers
Current Mirrors
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MOS Differential Amplifiers
Current Mirrors
The Wilson MOS mirror
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MOS Differential Amplifiers
Current Mirrors
Modified circuit.
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Multistage Amplifiers
Example
a) input resistance ?
b) voltage gain
c) output resistance ?
= 100
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Multistage Amplifiers
Equivalent circuit for calculating the gain of the input stage
of the amplifier
Example (cont.)
a)
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Multistage Amplifiers
Equivalent circuit for calculating the gain of
the second stage of the amplifier
Equivalent circuit for
calculating the gain of the
input stage of the amplifier
Example (cont.)
b)
35
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Multistage Amplifiers
Equivalent circuit for evaluating
the gain of the third stage in the
amplifier
Example (cont.)
b)
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Multistage Amplifiers
Equivalent circuit of the
output stage of the amplifier
Example (cont.)
b)
36
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Multistage Amplifiers
Example (cont.)
c)
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Analysis Using Current Gains
The circuit of the multistage amplifier prepared for small-
signal analysis. Indicated are the signal currents throughout
the amplifier and the input resistances of the four stages.