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Introduction to Operational Amplifier Design vers 1.0 October 22, 2007 Copyright © 2007 Reprinted by permission
Introduction to Operational Amplifier Design
ii
Table of Contents 1 Preface ..................................................................................................... 1-1 2 Introduction............................................................................................... 2-1 3 The Ideal Voltage Feedback Op Amp.............................................................. 3-1
3.1 Ideal Characteristics .............................................................................. 3-2 3.2 Ideal Model with Feedback...................................................................... 3-3 3.3 Inverting or Non-inverting ...................................................................... 3-5 3.4 About the Transfer Function .................................................................... 3-6 3.5 About Input Impedances ........................................................................ 3-7
4 Linear Amplifiers and Attenuators.................................................................. 4-1 4.1 Voltage Follower.................................................................................... 4-1 4.2 Inverting Amplifier or Attenuator ............................................................. 4-2 4.3 Non-Inverting Amplifier .......................................................................... 4-3
5 Summation Amplifier................................................................................... 5-1 5.1 Inverting Summation Amplifier ................................................................ 5-1 5.2 Non-inverting Summation Amplifier.......................................................... 5-3
6 The Integrator............................................................................................ 6-1 6.1 Inverting Integrator............................................................................... 6-1 6.2 Non-inverting Low-pass Filter.................................................................. 6-3
7 The Differentiator ....................................................................................... 7-1 7.1 The Inverting Differentiator..................................................................... 7-1 7.2 Non-inverting High-pass Filter ................................................................. 7-2
8 Practical Considerations ............................................................................... 8-1 8.1 Device Parameters................................................................................. 8-1 8.2 Power Supplies ..................................................................................... 8-2 8.3 Offsetting and Stabilizing........................................................................ 8-3 8.4 Impedance Matching and Phase Compensation .......................................... 8-5
Appendix A. ..Commonly Used Terms ................................................................ A-1 Appendix B. ..Bibliography ............................................................................... B-1
Introduction to Operational Amplifier Design
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List of Circuits Circuit 4-1 Voltage Follower .........................................................................................4-1 Circuit 4-2 Inverting Amplifier or Attenuator...................................................................4-2 Circuit 4-3 Non-inverting Amplifier...............................................................................4-3 Circuit 5-1 Inverting Summation Amplifier .....................................................................5-1 Circuit 5-2 Non-inverting Summation Amplifier ..............................................................5-3 Circuit 6-1 Inverting Integrator ....................................................................................6-1 Circuit 6-2 The Non-inverting Integrator ........................................................................6-3 Circuit 7-1 Inverting Differentiator ................................................................................7-1 Circuit 7-2 Non-inverting Differentiator..........................................................................7-2
Introduction to Operational Amplifier Design
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List of Figures Figure 3.1-1 Loop analysis example ..............................................................................2-1 Figure 3.1-1 Ideal operational amplifier .........................................................................3-1 Figure 3.1-2 Ideal op amp input impedances ..................................................................3-1 Figure 3.2-1 Ideal op amp feedback model 1..................................................................3-3 Figure 3.2-2 Ideal op amp feedback model 2..................................................................3-4 Figure 3.3-1 Non-inverting examples ............................................................................3-5 Figure 3.3-2 Loop equations for Fig 3.3-1 ......................................................................3-5 Figure 3.4-1 Black box diagram....................................................................................3-6 Figure 3.4-2 Cascaded transfer functions .......................................................................3-6 Figure 3.5-1 Zero impedance source .............................................................................3-7 Figure 3.5-2 Source with internal impedance..................................................................3-7 Figure 4.2-1 E
- and E
+ loops for Circuit 4-2 ...................................................................4-2
Figure 4.3-1 E- and E
+ loops for Circuit 4-3 ...................................................................4-3
Figure 5.1-1 E- loop for Circuit 5-1 ...............................................................................5-1
Figure 5.1-2 Thevenized input for Circuit 5-1..................................................................5-2 Figure 5.2-1 E
+ loop for Circuit 5-2...............................................................................5-3
Figure 5.2-2 E- loop for Circuit 5-2 ...............................................................................5-3
Figure 5.2-3 Thevenized input for Circuit 5-2..................................................................5-4 Figure 6.1-1 E
- and E
+ loops for Circuit 6-1 ...................................................................6-1
Figure 6.2-1 E+ loop for Circuit 6-2...............................................................................6-3
Figure 7.1-1 E- and E
+loops for Circuit 7-1....................................................................7-1
Figure 7.2-1 E+
loop for Circuit 7-2 ..............................................................................7-2 Figure 8.2-1 Filtering the Power Supply .........................................................................8-2 Figure 8.3-1 Avoiding common mode noise ....................................................................8-3 Figure 8.3-2 DC baseline shift example..........................................................................8-3 Figure 8.3-3 E
- loop with Vs = 0 and E
+ loop with bias VB ................................................8-4
Figure 8.4-1 Impedance matching ................................................................................8-5 Figure 8.4-2 Canceling reactance..................................................................................8-6
1 Preface
1-1
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1 Preface The scope of this course is the design of basic voltage feedback operational amplifier circuits. Using the ideal op amp model and solving for the currents and voltages at each terminal we get the transfer function as a Laplace Transform. This course provides a practical way of going from paper design to prototyping working circuits. This course is intended for professional electrical engineers. The course-taker should be familiar with the Laplace and inverse Laplace Transforms and basic AC network analysis. After completing the course, there is a quiz consisting of 16 multiple choice questions. On completion, 4 professional development hours will count towards satisfying PE licensure renewal requirements. Navigating the course is facilitated by hyperlinked table of contents on each page or the tags in the bookmark pane.
2 Introduction
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2 Introduction The voltage feedback differential amplifier (“op amp” as it is called) is used in a wide variety of electronic applications such as: linear amplifier/attenuator, signal conditioner, signal synthesizer, computer, or simulator. A practical way to approach designing and implementing an op amp circuit is to start with the ideal model and get an expression that relates the output to the input, regardless of the input. This is accomplished by working with the loop equations in the frequency or s domain [1 ]. In summary, the key to getting the transfer function is that the voltages at the input terminals of a closed-loop op amp circuit mirror each other. Also, no current flows into or out of an input terminal. When a signal is applied to either or both terminals, the output will adjust itself to meet these constraints.
Figure 3.1-1 Loop analysis example
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Transform ; vo(t)=L-1
[ Vi(s)·Av(s) ].
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3 The Ideal Voltage Feedback Op Amp
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3 The Ideal Voltage Feedback Op Amp The voltage feedback op amp is a discrete device that has 2 input terminals and one output terminal. Without feedback, the output is the difference between the input voltages, multiplied by the open-loop gain (transfer function) of the op amp.
Figure 3.1-1 Ideal operational amplifier
In the open loop model, each input terminal has infinite impedance so no current can flow into an input terminal even with a voltage source or a ground applied. The output terminal has zero output impedance.
Figure 3.1-2 Ideal op amp input impedances The ideal characteristics are summarized in Table 3-1 below
Avol is the open loop gain (transfer function) of the op amp itself, without feedback
The key is in maintaining E += E -
otherwise the output will saturate:
If we apply a voltage at E +and ground E -
E o will saturate to positive supply voltage.
If we apply a voltage at E -and ground E +
E o will saturate to negative supply voltage.
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3 The Ideal Voltage Feedback Op Amp 3.1 Ideal Characteristics
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3.1 Ideal Characteristics
Summary Of Ideal Characteristics
Zin = ∞ the input impedance at each terminal is infinite
Zout = 0 the output impedance is zero
0=±I no current flows into either of the input terminals
Avol = ∞ the open loop gain is infinite
Bandwidth = ∞ the bandwidth is infinitely wide
No temperature drift
E o = 0 the output voltage is zero when E += E -
Table 3-1 Summary of ideal characteristics
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3 The Ideal Voltage Feedback Op Amp 3.2 Ideal Model with Feedback
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3.2 Ideal Model with Feedback With feedback, all or a portion of the output is tied to either or both input terminals. The difference between the voltages at the input terminals is still equal to zero and again no current flows into either of the input terminals. The voltage at each input terminal is treated as a node and is determined by the loop equations. The terminal voltages are equated and we get a transfer function or closed loop gain which provides the output over the input as a ratio (Laplace Transform). In the following subsections, we’ll look at the feedback models : the non-zero reference and the zero reference feedback models. As the names imply, the non-zero reference model is characterized by the
voltage at either input terminal, 0≠±
E , and the zero-reference model is where the voltage at either
input terminal is referenced to 0 volts or ground; 0=±E .
3.2.1 The non-zero reference model
Figure 3.2-1 Ideal op amp feedback model 1
3.2.1.1 The input impedance
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3 The Ideal Voltage Feedback Op Amp 3.2 Ideal Model with Feedback
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3.2.2 The zero-reference model
Figure 3.2-2 Ideal op amp feedback model 2
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3 The Ideal Voltage Feedback Op Amp 3.3 Inverting or Non-inverting
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3.3 Inverting or Non-inverting The determining factors for whether the output is inverted or not, are the circuit configuration and the loop equations for the terminal currents and voltages. With voltage feedback op amp circuits, which terminal the signal is connected to, by itself, does not determine whether the output is inverted or not. For example, the 2 circuits below are both inverting amplifier/attenuators circuits. The only difference is the op amp input terminals are reversed but both provide an output that is a linear amplifier or attenuator with 180° phase shift or inversion:
Figure 3.3-1 Non-inverting examples Figure 3.3-2 Loop equations for Fig 3.3-1
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3 The Ideal Voltage Feedback Op Amp 3.4 About the Transfer Function
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3.4 About the Transfer Function The transfer function gives you the output over the input expressed as a ratio. To get the output for a specific input, multiply the Laplace Transform of the input by the transfer function. To convert to the time domain, take the inverse Laplace Transform [2 ]. Using Laplace Transform tables available on the internet or in printed textbooks, is a very useful tool in op amp circuit design. Some online references are listed in Appendix B. Working in the s domain (using Laplace Transforms) is advantageous and readily gives any transients that might exist
Figure 3.4-1 Black box diagram
Figure 3.4-2 Cascaded transfer functions In the time domain, you could not express the transfer function as a ratio. You would have to solve differential equations for the terminal voltages and currents and use the convolution integral to get the output as a function of time, because superposition does not apply.
[2] The time domain representation of Av (that is av(t) = L-1
[Av(s)] ) is actually the unit impulse response of the circuit. The
output vo(t) after applying an arbitrary input vi(t) is found by convolving vi(t) with av(t) { vo(t) ≡ vi(t)*av(t) } .
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3 The Ideal Voltage Feedback Op Amp 3.5 About Input Impedances
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3.5 About Input Impedances An input source can be either a directly connected zero impedance source, or a thevenin equivalent source with an internal impedance. We need to be aware if a source does have internal impedance. This needs to be considered in determining both the input impedance seen by the voltage source and the transfer function of the circuit.
3.5.1 Directly connected source Consider a directly-connected source, with zero output impedance, connecting to an op amp input terminal, through a series impedance Z1 :
Figure 3.5-1 Zero impedance source
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Figure 3.5-2 Source with internal impedance
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4 Linear Amplifiers and Attenuators 4.1 Voltage Follower
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4 Linear Amplifiers and Attenuators The most basic circuit configurations are linear amplifiers and attenuators. Ideally, they provide flat gain or loss across the device-rated bandwidth.
4.1 Voltage Follower The voltage follower circuit provides unity gain with no inversion. This circuit is used to isolate a high impedance source input and provide a buffered output.
Circuit 4-1 Voltage Follower
4.1.1 The Input impedance
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4 Linear Amplifiers and Attenuators 4.2 Inverting Amplifier or Attenuator
4-2
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4.2 Inverting Amplifier or Attenuator The inverting configuration provides flat gain or attenuation with constant 180° phase shift.
Circuit 4-2 Inverting Amplifier or Attenuator
Figure 4.2-1 E- and E
+ loops for Circuit 4-2
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4 Linear Amplifiers and Attenuators 4.3 Non-Inverting Amplifier
4-3
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4.3 Non-Inverting Amplifier The non-inverting configuration provides flat gain with no phase shift.
Circuit 4-3 Non-inverting Amplifier
Figure 4.3-1 E- and E
+ loops for Circuit 4-3
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5 Summation Amplifier 5.1 Inverting Summation Amplifier
5-1
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5 Summation Amplifier The summation amplifier provides flat gain or loss and can be configured in an inverting or non-inverting configuration. Typical uses are signal combiner, voltage comparator or summing junction.
5.1 Inverting Summation Amplifier The inverting summation amplifier provides flat gain or loss across the rated bandwidth with constant 180° phase shift.
Circuit 5-1 Inverting Summation Amplifier
Figure 5.1-1 E- loop for Circuit 5-1
zero to out balance voltages terminal the*
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5 Summation Amplifier 5.1 Inverting Summation Amplifier
5-2
TOP
Figure 5.1-2 Thevenized input for Circuit 5-1
5.1.1 The Input impedance
impedance. input the asRZ sees V sources the of each 1,-5 CircuitFor
source. each with series in is impedancewhatever be to going siZ , With Z
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5 Summation Amplifier 5.2 Non-inverting Summation Amplifier
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5.2 Non-inverting Summation Amplifier The non-inverting summation amplifier provides flat gain across the rated bandwidth with no phase shift.
Circuit 5-2 Non-inverting Summation Amplifier
Figure 5.2-1 E+
loop for Circuit 5-2
Figure 5.2-2 E
- loop for Circuit 5-2
k-
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Course E04-002
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5 Summation Amplifier 5.2 Non-inverting Summation Amplifier
5-4
TOP
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛+⋅++=⎟⎟
⎠
⎞⎜⎜⎝
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5.2.1 The Input impedance
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⎞⎜⎜⎝
⎛+
=++=f
o RRR
k where , VVVk
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5 Summation Amplifier 5.2 Non-inverting Summation Amplifier
5-5
TOP
( )( ) ( )
( )
( )( ) ( )
V2V
V3RZ
input. the at sourcesother the by seen impedances input the getting to apply nscalculatio same The
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6 The Integrator 6.1 Inverting Integrator
6-1
TOP
6 The Integrator Typical uses of the integrator circuit are: noise reduction, simulation of a first order RC low-pass circuit, low pass filter, calculating an integral or just phase compensation.
6.1 Inverting Integrator The inverting integrator provides low-pass filtering with constant +90° phase shift. The roll-off starts at DC. Typical use is direct integration of a time domain function, noise reduction, or low pass filter.
Circuit 6-1 Inverting Integrator
Figure 6.1-1 E- and E
+ loops for Circuit 6-1
6.1.1 The Input impedance
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1
1
1
1
R is 16 Circuitfor V source by seen impedance input The
R
R
V
VZ
R
VI With
R
VII case this in
I
VZ is V by seen Z impedance input The
inin
1inin
inin
−
===
===
zero to out balance voltages terminal the* 0
- 0
oV
:arevoltages terminalthe1,-6CircuitFor
o
==
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E
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A
sCRVoV
R
VVsC
so II , I and III With
sCVI and , R
VI
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sC
VI and ,
R
VI
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f
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1
11
11
1
11
11
1
0
1
−=
−=−=
−===+
==
=
⎟⎠⎞
⎜⎝⎛ −=
−=
−=
∴
−−
−
−−−
−
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E
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6 The Integrator 6.1 Inverting Integrator
6-2
TOP
6.1.2 Bandwidth Considerations
.components DC have that signals input with working are we if this offset to want may We supply. single a using are we if volts 0or voltage, supply
negative the at saturated be will circuit the of output the means which A (DC), 0s at see We
js at (s)vA of value the us gives js ngsubstituti ; sCR
A 1,6 CircuitFor
v
v
∞−==
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1
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0
0
0
0
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11
221
21
21
ω=ω
=ωω
=ω
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ω=ωω
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A
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6 The Integrator 6.2 Non-inverting Low-pass Filter
6-3
TOP
6.2 Non-inverting Low-pass Filter The non-inverting low-pass filter provides filtering with phase shift that varies with the complex transfer function.
Circuit 6-2 The Non-inverting Integrator
2.-6 Circuitfor functiontransfer the is sCR
1RR
VV
A
sCR1
RR
sCR1
k
VV
so VksCR
1V ,
- With
12
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1
⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛+==
⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛+=⎟⎟
⎠
⎞⎜⎜⎝
⎛+
==⎟⎟⎠
⎞⎜⎜⎝
⎛+
=+
11
11
11
1
3
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6.2.1 The input impedance
To calculate the input impedance seen by Vi we analyze the E + loop.
Figure 6.2-1 E+
loop for Circuit 6-2
. sC
R is 2-6 Circuitfor V source by seen impedance input The
sCR
sCsCR
sCRsC
V
VZ so
sCRsC
V to simplifies I in1
1
11
11
1
11
1
1
+
+=+
=
⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅
=⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅
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divider voltage a beingk
RRR
k where Vk -
divider voltage a being sCR
1
sCR
1V
sCR
sC1
V
V
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2
2o
1
11
oo
3
1
11
+==
⎟⎟⎠
⎞⎜⎜⎝
⎛+
⎟⎟⎠
⎞⎜⎜⎝
⎛+
=⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
+=
=
+
E
E
E
i i
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V Recall
R
V I have we 1,-6.2 Figure From
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=+
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=
=
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ECourse E04-002
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6 The Integrator 6.2 Non-inverting Low-pass Filter
6-4
TOP
6.2.2 Bandwidth considerations
( )
( ) ( )
( ) 11
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v
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==+
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==
=⎟⎟⎟
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θ⋅=ω=⎟⎟
⎠
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⋅=
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⎛+⎟⎟
⎠
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⎛+
⋅⎟⎟⎠
⎞⎜⎜⎝
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=
θ
θ
21
1
21
11
21
21
0
1
1
11
1
2
22
2
33
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Course E04-002
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7 The Differentiator 7.1 The Inverting Differentiator
7-1
TOP
7 The Differentiator Typical uses of the differentiator circuit are: simulation of a first order RC high-pass circuit, high pass filter, calculating a derivative or just phase compensation.
7.1 The Inverting Differentiator The inverting differentiator provides high-pass filtering with constant - 90° phase shift. The roll-off starts at DC. Typical use is direct differentiation of a time domain function, or high pass filter.
Circuit 7-1 Inverting Differentiator
Figure 7.1-1 E- and E
+loops for Circuit 7-1
7.1.1 The input impedance
sC is 1-7 ircuitCfor V source by seen impedance input The
sCV
VZ sCVI with
CIIwhereI
VZ is V by seen impedance input The
inin
inin
in
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==
==
0 -
0
V
: are voltages terminal the 1,-7 CircuitFor
oo
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=
=
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E
. Circuit orf
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sCRVoV
RoV
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VCI
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RoV
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sC
VCI
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: have we loop thefor 1,-7.1 Figure From
17
1
11
0
11
0
1
1
−
−==
−==
−=
=
=−
−−
−−
=
−===+
−
∴−
=
=
−−
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E
Course E04-002
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7 The Differentiator 7.1 The Inverting Differentiator
7-1
TOP
7.1.2 Bandwidth considerations
volts 0 be will circuit the of output the means whichA (DC), 0s at see We
js at (s)vA of value the us gives js ngsubstituti ; sCRA 1,7 CircuitFor
v
v
01
==
ω=ω=−=− .
0
0
0
0101
0
11
2
2
2
2
ω=ω
=ωω
=ω
ω
ω⋅=ωω
ω=ω
ωω
°ω=
π
⋅=ω−=−
at occurs bandwidth 3dB-our So
)(A
)(A iprelationsh direct the us gives This
)(A)(A which at the is bandwidth 3dB-Our
A
CR i.e ; at magnitude gain desired the us give to CR select
would we , frequency lfundamenta a at design the start we If of tindependen is shift phase
90 - constant the that noting ; CR A where eA A as CRjA rewrite can We
ov
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vv
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v
j
vvv
.
Course E04-002
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7 The Differentiator 7.2 Non-inverting High-pass Filter
7-2
TOP
7.2 Non-inverting High-pass Filter The non-inverting differentiator provides high-pass filtering with frequency-dependent phase shift. Typical use is direct integration of a time function, or noise reduction. The total phase shift is overall frequency dependent (with an imaginary zero and complex pole).
Circuit 7-2 Non-inverting Differentiator
2.-7 Circuitfor functiontransfer the is sCR
sCRRR
VV
A1
1
2
3ov ⎟⎟
⎠
⎞⎜⎜⎝
⎛+
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛+==
11
i
7.2.1 The input impedance
To calculate the input impedance seen by Vi, we look at the E + loop
Figure 7.2-1 E+
loop for Circuit 7-2
sCR is 7.2 Circuitfor V by seen impedance input The
sCR
sCRCs
V
VZ us gives
I
VZ into I I gubstitutinS
sCRCs
VsCR
sCRVCsI , gSimplifyin
1
1
1
inin
inCin
11
1c
1
1
1
111
+
+=
⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅
===
⎟⎟⎠
⎞⎜⎜⎝
⎛+
=⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛+
−=
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⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛+=⎟⎟
⎠
⎞⎜⎜⎝
⎛+
=
=⎟⎟⎠
⎞⎜⎜⎝
⎛+
=
+==
⎟⎟⎠
⎞⎜⎜⎝
⎛+
=⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
+=
≡
+
+
11
11
1
11
3
1
1
2
3
1
1o
o1
1
2
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1
1
1
1
oo
sCRsCR
RR
sCRsCR
k
VV
VksCR
sCRV so
-
RRR
k where Vk -
sCR
sCRV
sCR
RV
V
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E
E
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛+
−=⎟⎟⎠
⎞⎜⎜⎝
⎛+
⋅=+
⎟⎟⎠
⎞⎜⎜⎝
⎛ +−=
+−
=
=
=
11
1
1
1c
1
1
c
Cin
inin
sCRsCR
VVCsI so sCR
sCRV Recall
VsC
sC
VI have we 1,-7.2 Figure Form
I I 2,-7 CircuitFor
I
VZ is V source by seen impedance input The
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EECourse E04-002
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7 The Differentiator 7.2 Non-inverting High-pass Filter
7-3
TOP
7.2.2 Bandwidth considerations
( )
( ) ⎟⎠⎞
⎜⎝⎛ −⋅
==⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
+⋅=
=+
⎟⎠⎞
⎜⎝⎛ −π
==⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
+⋅=
φ⋅=ω=⎟⎟
⎠
⎞⎜⎜⎝
⎛+
⋅=
⎟⎟⎠
⎞⎜⎜⎝
⎛+⎟⎟
⎠
⎞⎜⎜⎝
⎛+
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛+==
+
∂
θφθ
14
121
2
21
1
11
1
21
2
2
3
kCR
ω at elyapproximat is ωCR
1k1
)ω(A oS
)ω(A where elyapproximat is bandwidth 3dB The
and ωCRtan , ωCR
ωCRk1
A where
je A A us gives js gsubstituin and form,phasor in A Rewriting
sCRsCR
k1
A so k1
with
RR
replace slet' ; sCR
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VV
A be to functiontransfer the found we ,Previously
ed.approximat be will bandwidth 3dB The
0. be cannot and DC beyond just frequency low a is ,ω frequency lfundamenta the So
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Course E04-002
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8 Practical Considerations
8-1
TOP
8 Practical Considerations Device selection, component tolerances and power supply stability should be the priorities when designing and prototyping a circuit. Practical considerations are device and application specific. Various types of op amps ranging from general purpose, low and high frequency, to application specific devices have their own specifications and recommendations for optimal performance.
8.1 Device Parameters Some commonly used terms and their textbook definition are listed in Appendix A. Manufacturers may use the same terms, call them by other names or introduce new parameters to thoroughly characterize their op amp. Manufacturers may also include application notes on how best to make offset or compensation adjustments for voltage, current and phase. Most of these parameters are centered on an open loop mode. So a circuit designer may not need to be concerned with all of the specifications when designing a circuit. It’s important to select a device that exceeds expectations. For example, the common mode rejection ratio is a value based on open loop gain and small, simultaneous signal variations on larger input levels; essentially nolise. A widely accepted minimum for the CMRR is 70dB. This value implies a level of stability that applies to a closed loop circuit and other parameters of the op amp. Less than 70dB indicates a noisier or less stable device. Below are data sheets for 2 op amps that can be used for comparison. [* links open in new browser window ]
Device Description Overview and Specs PDF
99http://www.national.com/mpf/LM/LM741.html LM741 - Operational Amplifier
General purpose low frequency
99http://www.national.com/ds/LM/LM741.pdf
99http://www.national.com/pf/LM/LMH6609.htmlLMH6609 - 900MHz Voltage Feedback Op Amp
High speed high frequency
99http://www.national.com/ds/LM/LMH6609.pdf
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8 Practical Considerations 8.2 Power Supplies
8-2
TOP
8.2 Power Supplies The decision to use single or dual voltage power supplies may be arbitrary or a design constraint. The better choice is to use a dual balanced supply to minimize circuitry required to offset the DC baseline for AC inputs. If there is no choice and a design must be a single voltage supply, each of the circuits above needs to be modified.
8.2.1 Bypassing the power supply Use a well regulated power supply and bypass with electrolytic capacitors, choke coils or a combination of the 2 to further filter the supply voltage to the circuits. Keep lead lengths or printed circuit board layouts as short as possible.
The best case for filtering out power supply connections is a series choke coil shunted by a capacitor (electrolytic or mylar) to minimize if not eliminate any noise and ringing on the supply rails. Choke coils may be expensive and impose additional space requirements in a final product, but they provide excellent filtering for a prototype.
Figure 8.2-1 Filtering the Power Supply Large electrolytic capacitors shunt out high frequency components and help keep the supply voltage regulated by absorbing voltage spikes or instantaneous load changes. Filtering power supplies helps keep the op amp operating in the range of its rated PSRR
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8 Practical Considerations 8.3 Offsetting and Stabilizing
8-3
TOP
8.3 Offsetting and Stabilizing The op amp data sheet may have specifications for offsetting and stabilizing. Below are cases for avoiding common-mode noise and shifting the DC baseline.
8.3.1 Common mode noise When the intention to apply a constant 0 volts to either input terminal, tie the terminal directly to ground rather than through a resistor. This would prevent any voltage appearing due to leakage currents and having to offset or null it with additional biasing. Recommendations for offsetting mentioned in the op amp data sheet should also be followed.
Figure 8.3-1 Avoiding common mode noise
8.3.2 Shifting the DC baseline Working with AC sources that alternate between positive and negative voltages, and being constrained to using single supply op amps, requires shifting the DC baseline to avoid clipping of the output signal.
Figure 8.3-2 DC baseline shift example
Take Circuit 4 2 Inverting amplifier or attenuator as an example. If we are using a single supply and apply a sine wave as an input, the output produced would only be the positive portion of the input signal. Modifying the circuit as in the figure to the left will provide the DC shift that is needed to provide the expected output; an amplified or attenuated sine wave alternating about a DC baseline With no signal but the DC bias applied, V1 = V2 = VB and R is the thevenin equivalent of voltage dividers as shown in Figs 8.3-3.
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8 Practical Considerations 8.3 Offsetting and Stabilizing
8-4
TOP
Figure 8.3-3 E- loop with Vs = 0 and E
+ loop with bias VB
With no signal applied, the output Vo = VB which is now the baseline for this circuit.
( ) ( )
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8 Practical Considerations 8.4 Impedance Matching and Phase Compensation
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8.4 Impedance Matching and Phase Compensation
Figure 8.4-1 Impedance matching
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8 Practical Considerations 8.4 Impedance Matching and Phase Compensation
8-6
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Appendix A Commonly Used Terms
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Appendix A. Commonly Used Terms Common-Mode Voltage Range Typically the range of voltages on the input terminals for which the amplifier’s performance is specified Common-Mode Rejection Ratio The ratio of differential voltage amplification to common-mode voltage amplification. It is measured by determining the ratio of a change in input common-mode voltage to the resulting change in input offset voltage change. Gain Bandwidth Product The product of a given input frequency and the op-amp open loop gain at that frequency (usually specified in MHz, voltage feedback amplifiers only.) Input bias current The Input Current specification is the average of the currents drawn by the two input pins. Input current is also often called "bias current" Input Offset Current The difference of the currents entering the two input terminals of a balanced amplifier Input Offset Voltage The DC error voltage which exists between the input terminals due to non-ideal balancing of the input stage to the output. It is multiplied by the closed loop gain Offset Current Temperature Coefficient The average rate of change in offset current for junction temperature variation over a specified temperature range Offset Voltage Temperature Coefficient The average rate of change in offset voltage for the junction temperature variation over a specified temperature range Output Offset Voltage The output voltage when the 2 input terminals are grounded. Output Voltage Swing The maximum peak-to-peak output voltage swing under specified load and supply voltages Power Supply Rejection Ratio Power Supply Rejection Ratio (PSRR) can be one of two specifications. DC PSRR is the ratio of the change in a specified parameter (e.g., Full Scale Error) that results from a specified change in the power supply voltage. AC PSRR is measured with a signal of specified frequency and amplitude riding upon the power supply and is the ratio of the output amplitude of that signal at the output to its amplitude on the power supply pin. PSRR is usually specified in dB
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Appendix A Commonly Used Terms
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Slew Rate The rate that an amplifier output changes from one voltage level to another, usually given in V/µsec, when a step or square wave input is applied. Typically it is the average rate measured from 10% to 90% of the total output voltage change Unity Gain Bandwidth The frequency where the amplifier open loop gain equals to one. It equals GBW if the op amp has a single pole roll-off in its frequency response
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Appendix B Bibliography
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Appendix B. Bibliography Printed Material Millman, Jacob. Microelectronics Digital and Analog Circuits and Systems. New York: McGraw-Hill
Book Company,1979
Online Reference List [* links open in new browser window ] Table of Laplace Transforms, s.v. 11http://www.vibrationdata.com/Laplace.htm (September 30, 2007)
Laplace transform:” Table of selected Laplace transforms”.
11http://en.wikipedia.org/wiki/Laplace_transform#Table_of_selected_Laplace_transforms (September
30, 2007)
National Semiconductor, High-Performance Analog for Energy-Efficient PowerWise Designs.
11http://www.national.com/ (September 30, 2007)
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