DiodesBIOE 3300 – Biomedical Electronics(Chapter 10).docx

Diodes
BIOE 3300 – Biomedical Electronics
(Chapter 10)
1
Applications
Signal Processing
(e.g.Rectification)
Power Supplies
AC to DC Conversion
Voltage supply regulation
Electronic Device Protection
Surge Protection (e.g. ECG/ Defibrillator)
Light Sources
Light Emitting Diodes
Laser Diodes
2

What are diodes?
Semiconductor based electronic components that only permit
current to flow in a given direction
More specifically, diodes allow for the manipulation of normal
circuit behavior
Cause currents to deviate from expected path
3
What Are Diodes Made Out Of?
Semiconductor material: normally silicon (Si) or Germanium
(Ge) doped with select impurities
n-type material: electrons move freely
p-type material: impurities result in positive charged particles
known as holes
A natural barrier exists across the pn junction which holds free
electrons on n-side and holes on p-side
4
What Are Diodes Made Out Of?
Si and Ge are both group 4 elements, meaning they have 4
valence electrons.
Their structure allows them to grow in a shape called the
diamond lattice.

In the diamond lattice, each atom shares its valence electrons
with its four closest neighbors.
This sharing of electrons is what ultimately allows diodes to be
built.
When dopants from groups 3 or 5 (in most cases) are added to
Si or Ge. It changes the properties of the material so we are
able to make the P- and N-type materials that become the diode.
The diagram above shows the 2D structure of the Si crystal.
The light lines represent the electronic bonds made when the
valence electrons are shared. Each Si atom shares one electron
with each of its four closest neighbors so that its valence band
will have a full 8 electrons.
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4
Si
+4

5
The Periodic Table of Elements
6
N Type Material
N-Type Material:
When extra valence electrons are introduced into a material
such as silicon, an n-type material is produced.
The extra valence electrons are introduced by putting impurities
or dopants into the silicon.
The dopants used to create an n-type material are Group V
elements. The most commonly used dopants from Group V are
arsenic, antimony and phosphorus.
The 2D diagram to the left shows the extra electron that will be
present when a Group V dopant is introduced to a material such
as silicon. This extra electron is very mobile.

7
P Type Material
P-Type Material:
P-type material is produced when the dopant introduced is from
Group III.
Group III elements have only 3 valence electrons and therefore
there is an electron missing.
This creates a hole (h+), or a positive charge that can move
around in the material.

Commonly used Group III dopants are aluminum, boron, and
gallium.
The 2D diagram to the left shows the hole that will be present
when a Group III dopant is introduced to a material such as
silicon. This hole is quite mobile in the same way the extra
electron is mobile in a n-type material.
+4
+4
+3
+4
+4
+4
+4
+4
+4

8
Physical Fabrication
Thermal Diffusion

Ion Implantation
Diffusion
High temperature diffusion has historically been one of the most
important processing steps used in the fabrication of
semiconductor material
Diffusion is the redistribution of atoms from regions of high
concentration to low concentration
It occurs at all temperatures, but is promoted at higher
temperatures.
Ion Implantation
As of today, ion implantation has become the more dominant
doping method
Very controllable process capable of producing high quality and
uniform semiconductor material.
The PN Junction
Steady State

P
n
- - - - -
- - - - -
- - - - -
- - - - -
+ + + + +
+ + + + +
+ + + + +
+ + + + +
Metallurgical Junction
Space Charge Region
ionized acceptors
ionized donors
E-Field
+
+
_
_

h+ drift
h+ diffusion
e- diffusion
e- drift
=
=
=
=
When no external source is connected to the pn junction, a
natural barrier exists across the pn junction which holds free
electrons on n-side and holes on p-side
Metallurgical Junction: The interface where the p- and n-type
materials meet.
Space Charge Region: Also called the depletion region. This
region includes the net positively and negatively charged areas.
The space charge region does not have any free carriers. The
width of the space charge region is denoted by W in pn junction
formula’s.
(+/-): Represent the amount of negative and positive doping in
number of carriers per centimeter cubed. Usually in the range
of 1015 to 1020.
12
Ideal Diodes
The ideal diode acts as a short circuit for forward currents and
as an open circuit with reverse voltage applied.

+ VD -
13
Circuit Analysis Using the Ideal Diode Model
14
Generalized Analysis of Ideal-Diode Circuits
Assume a state for each diode, either on (i.e., a short circuit) or
off (i.e., an open circuit). For n diodes there are 2n possible
combinations of diode states.
2. Analyze the circuit to determine the current through the
diodes assumed to be on and the voltage across the diodes
assumed to be off.
15
Generalized Analysis of Ideal-Diode Circuits
3. Check to see if the result is consistent with the assumed state
for each diode. Current must flow in the forward direction (i.e.,
iD>0) for diodes assumed to be on. Furthermore, the voltage
must be more positive at the cathode than the anode (i.e.,

reverse bias vD<0) for diodes assumed to be off.
4. If the results are consistent with all the assumed diode states,
the analysis is finished. Otherwise, return to step 1 and choose a
different combination of diode states.
16
A more realistic diode model !
Transconductance Curve
Ideal Actual
This is known as the transconductance curve
17
A more realistic diode model!
Forward Bias Region

Forward Bias Voltage (Vf): In practical diodes, current will
only begin to flow from the anode to cathode once a barrier
potential is overcome. Therefore, as the applied voltage
increases, current starts to flow across the junction once Vf has
been reached. The barrier potential varies for different
materials.
+ -
Vf
18
A more realistic diode model !
Forward Bias Region
Another difference between an ideal diode and actual diode is
how current flows in the forward bias direction
Vf
Vf
i=-Vf/Ra

Vf
19
Reverse Bias Region
Under reverse bias in the ideal case, the diode acts as an “open
circuit” model when VD<0.
In an actual sense, the ideal model is close to the actual
behavior
The only difference is reverse bias occurs when VBR<VD < VF
and there is a small leakage current, Is (saturation current)
which flows under reverse bias conditions.
This can often be ignored as it is in the nA to pA range
This saturation current is made up of electron-hole pairs being
produced in the depletion region. Saturation current is
sometimes referred to as scale current because of it’s
relationship to junction temperature.
Ideal
Actual
Is
VBR VF

20
Reverse-Breakdown Region
In the ideal model, the Reverse-Breakdown Region is ignored
Ideal Actual
21
VBR
|VBR|
22

Diodes normally enter the reverse-breakdown region when VD
<< 0
At this point, the diode current reverses (i.e. the normal or ideal
diode characteristics breakdown.)
For most applications that use standard/normal diodes, the
reverse breakdown region does not need to be considered as the
reverse-breakdown voltage is on the order of -100 to -1000V
However, there are some diodes meant to operate in this region
Zener Diodes
23
Practical Diode Models
VBR
VF
|Is|
or

|VBR|
24
Example: Diode Modeling
Generalized Analysis of Practical-Diode Circuits
1. Assume a state for each diode, either on Forward Bias
Model), off (i.e., Reverse Bias Circuit Model), or Reverse-
Breakdown Model. For n diodes there are 3n possible
combinations of diode states.
2. Analyze the circuit to determine the current through and/or
voltage across the diodes for the assumed state
26
3. Check to see if the result is consistent with the assumed state
for each diode.
- Forward Bias: Current must flow in the forward direction (+)
for diodes assumed to be in this region (ID>0) and VD>VF
-Reverse Bias (diode off): VBR< VD <VF
-Reverse Breakdown: Current must flow in the reverse
direction (-) for diodes assumed to be in this region (ID<Is) and
VD<VBR
4. If the results are consistent with all the assumed diode states,

the analysis is finished. Otherwise, return to step 1 and choose a
different combination of diode states.
27
For example, in a circuit that contains 2 diodes, there are 9
possible scenarios with only 1 being correct (static)
Forward Bias (FB)
Reverse Bias (RB)
Reverse Breakdown (RBR)Diode 1Diode
2FBFBRBFBRBRFBFBRBRBRBRBRRB
FBRBRRBRBRRBRRBR
28
Diodes from a Physics Standpoint
The transconductance curve is characterized by the following
equation:
iD is the current through the diode
IS is the saturation current
vD is the applied biasing voltage
VT is the thermal voltage and is approximately 26 mV at room
temperature.
n is the emission coefficient (takes a value between 1 and 2
depending on device structure)
For a silicon diode, n is around 2 for low currents and goes

down to about 1 at higher currents
Shockley Equation
Transconductance curve
29
Types of Diodes
Normal/Standard PN-Junction Diodes
Zener Diodes
Schottky Diodes
Shockley Diodes
Light Emitting Diodes (LEDs)
Laser Diodes (LDs)
Photodiodes
30
Used to allow current to flow in one direction while blocking
current flow in the opposite direction. The pn junction diode is
the typical diode used in many applications

A
C
Schematic Symbol for a PN Junction Diode
p
n
Representative Structure for a PN Junction Diode
31
Transconductance Curve
Will vary depending on diode model
32
Sample Specification Sheet
Component

Illustration
(2)
(3)
(4)
(5)
33
Zener Diodes
Are specifically designed to operate under reverse breakdown
conditions. These diodes have a very accurate and specific
reverse breakdown voltage.
Often more positive than standard PN diodes (but still negative)
Zener diodes are available for a range of breakdown voltages
Useful in applications for which a constant voltage in the
breakdown region is desired
Application(s): Voltage Regulators, Clipper and Clamp circuits
A
C
Schematic Symbol for a Zener Diode

34
Common Zener Diode Transconductance Response Curve
6V
35
Zener Diode Specification Sheet
36
37
Schottky Diodes:
These diodes are designed to have a very fast switching time
which makes them a great diode for digital circuit applications.
They are very common in computers because of their ability to
be switched on and off so quickly.
A

C
Schematic Symbol for a Schottky Diode
38
Shockley Diodes:
The Shockley diode is a four-layer diode while other diodes are
normally made with only two layers. These types of diodes are
generally used to control the average power delivered to a load.
A
C
Schematic Symbol for a four-layer Shockley Diode
39

Light-emitting diodes are designed with a very large electronic
bandgap so movement of carriers across their depletion region
emits photons of light energy.
(Ultraviolet (UV) – Visible - Near Infrared (NIR)
This band gap is an energy range in a solid where no electron
states exist. In a graph of the electronic band structure of a
solid, the band gap generally refers to the energy difference (in
electron volts) between the top of the valence band and the
bottom of the conduction band.
A
C
Schematic Symbol for a Light-Emitting Diode
The arrows in the LED representation indicate emitted light.
Light-Emitting Diodes:
Wavelength (nm)
40

Atomic Structure Electron Orbitals
Lower bandgap LEDs (Light-Emitting Diodes) emit infrared
radiation, while LEDs with higher bandgap energy emit visible
light.
E: bandgap in J
h: Planck’s Constant 6.626e-34 Js
c: Speed of Light 3e8 m/s
1 J=6.24e18 eV
Reduced Equation
E: bandgap in eV
Many stop lights and illumination lights are now starting to use
LEDs because they are extremely bright, have very long
lifetimes, and are more energy efficient.

Medical LED and Laser Diode Applications
Chemical Analysis
Spectrometry
Blood Gas Sensors
pulse oximeters
Therapeutic Devices (Infrared)
pain management/blood flow
Tissue Ablation/Cauterization
Bloodless surgery
Noninvasive Optical Biopsy
Optical Coherence Tomography (OCT)
Tattoo Removal
Photodiodes:
While LEDs emit light, Photodiodes are sensitive to received
light. They are constructed so their pn junction can be exposed
to the outside through a clear window or lens.
In Photoconductive mode the saturation current increases in
proportion to the intensity of the received light. This type of
diode is used in CD players.
In Photovoltaic mode, when the pn junction is exposed to a
certain wavelength of light, the diode generates a
voltage/current and can be used as an energy source and/or light
detector. This type of configuration is used in the production of
solar power and many optical biosensors.

A
C
A
C
Schematic Symbols for Photodiodes
45
Diode Applications
Rectifiers
Half-Wave (+/-): For use in selectively clipping or removing
either positive or negative components from a signal
Full-Wave (+/-): For use in inverting the polarity of either the
positive (-) or negative (+) components in a signal.
Positive Half-Wave

vin(t)
46
Realistic Diode Half-Wave Rectifier
Applications: Power Supplies, Battery Chargers, Signal
Processing
How would a negative half wave rectifier be constructed?
48
Rectifiers: Power Supplies, Battery Charging Circuits,
Demodulation, etc….
Rectifiers Circuits and Batteries
A battery is a device consisting of one or more electrochemical
cells that convert stored chemical energy into electrical energy.
There are two main types of batteries: primary batteries
(disposable batteries), which are designed to be used once and
discarded, and secondary batteries (rechargeable batteries),
which are designed to be recharged and used multiple times

Rechargeable batteries are very common in portable
instrumentation and medical devices
Prefer to use a convenient power source to recharge (e.g. AC)
Rectifiers Circuits and Batteries
At tR (previous slide), we want to recharge the battery.
What would happen if we connect an AC source to the battery
using a resistor to control the current?
The battery would eventually drain to 0V, as t2 > t1
Obviously, we do not want this to happen. How can it be
prevented?
Half-Wave Rectifier Application:
Battery Recharger
Current flows into the battery whenever the instantaneous ac
source voltage is higher
than the battery voltage.
The resistor limits the current magnitude so not to damage the
rechargeable battery
On the negative cycles (or when vs(t)<Vb), the diode is off
(open circuit) and current is zero.
Therefore, current only flows in the direction that will charge
the battery and not cause a drain.

51
Current flows whenever the instantaneous ac source voltage
is higher than the battery voltage.
A resistor is added to limit the current magnitude so not to
damage the rechargeable battery
On the negative cycles, the diode is off and current is zero.
Therefore current only flows in direction that will charge the
battery and not cause a drain.
AC to DC Adapters/Converter
An AC/DC adapter or AC/DC converter is a type of external
power supply, often enclosed in a case similar to an AC plug.
These adapters are used with electrical devices that require DC
power but do not contain internal components to derive the
required voltage and power from the main AC power delivered
to our homes or businesses.
Use of an external power supply allows portability similar to
battery-powered equipment without the added bulk of internal
power components
Half-
Conversion)

53
Full Wave Rectifier (Positive)
EXAMPLE
54
Smoothing Capacitor
Full Wave
Rectifier
(Concept)
POSITIVE CYCLE OF vin(t) (i.e., vin(t) >0)
Diode C: “on” or “off”?
“off” because VC+ < VC-
Diode A: “on” or “off”?
“on” because iA>0
Diode D: “on” or “off”?
“off”? because VD+ < VD-
Therefore, current will
flow through RL toward ground
Diode B: “on” or “off”?
“on” because iB>0

During the positive cycle of the input (i.e., vin(t) > 0), the
0 (i.e., vo(t) tracks vin(t) for vin(t) > 0)
Full Wave
Rectifier
(Concept)
NEGATIVE CYCLE OF vin(t) (i.e., vin(t) < 0)
Diode B: “on” or “off”?
“off” because VB+ < VB-
Diode D: “on” or “off”?
“on” because iD>0
Diode A: “on” or “off”?
“off”? because VA+ < VA-
Therefore, current will
flow through RL toward ground
Diode C: “on” or “off”?
“on” because iC>0
During the negative cycle of the input (i.e., vin(t) < 0), the
0 (i.e., vo(t) tracks -vin(t) for vin(t) < 0)
Other questions…

What modification would be necessary to create a negative full
wave rectifier?
Could an AC/DC converter be built using a full wave rectifier?
What modification would be necessary for AC/DC conversion?
Positive Full Wave Rectifier
Half-
Conversion)
58
Voltage Regulators
Application: Zener Diodes
Many types of instrumentation require stable DC sources for
proper operation
Voltage Regulators: Create stable DC voltage sources from
either noisy DC sources or DC sources which drift over time
+
VDC or VSS
-
POSITIVE
VOLTAGE
REGULATOR

VDC_Regulated
Voltage Regulators Convert
59
Voltage Regulators
How do they work???
They exploit the reverse breakdown region (RBR)
Consider a voltage regulator constructed out of a 6V Zener
diode with the following transconductance curve
In the RBR, this voltage regulator can be modeled as:
Voltage Regulators
So, under what conditions would this RBR model hold???
From this transconductance curve, in RBR, iD < 0.
How about in terms of Vss(t)?
We see iD=(6-Vss(t))/R < 0 (from model), therefore, we will be
in RBR when Vss(t) > 6V.
Thus, when operating in RBR, we see that VDC_reg(t)=6V
(Stable Source) if Vss(t) remains above 6V

6V Voltage Regulator
Voltage Regulators
So what would happen if Vss(t) does not remain above 6V?
Would the diode remain in RBR???
No, it would transition to reverse-bias (RB) and voltage drop-
out would occur (i.e., VDC_REG(t) would track Vss(t))
For what conditions would RB exist?
-6 < VD < 0.6 where VD=0-VSS(t) (from RB model)
Thus, we will remain in RB as long as -6 < -Vss(t)
<0.6 or restated as 6 > Vss(t) >-0.6
Under this condition (drop-out) VDC_reg(t)=Vss(t) and the
regulator will cease to regulate as desired
Regulator Design Problem
10.29 (a) Design a voltage regulator circuit to provide a
constant voltage of 5V to a load from a variable supply. The
load current varies from 0 to 100 mA and the source voltage
varies from 8 to 10 V. You may assume that you are using an
(b) Find the worst case maximum power dissipated in each
component of the regulator.

VBR
VF
Open Circuit
Relevant to homework problems P10.26,P10.28, P10.30, &
P10.31
Voltage Regulators
Voltage regulators are circuits that produce constant output
voltage while operating from a variable supply voltage.
-To achieve a constant
output voltage, Vss>|Vz|
-For this case Vo=|Vz|
(i.e. the reverse breakdown voltage)
e.g. |VBR|=|Vz |=6V
Note: The breakdown voltage (VBR) is commonly referred to as
the Zener voltage (Vz)
POSITIVE VOLTAGE REGULATOR
64

65
66
Other questions…
What modification would be necessary to create a negative
voltage regulator?
How could a voltage regulator be combined with an AC/DC
converter to create a very stable DC source (i.e., get rid of the
ripple)?
Clipper Circuits
Often used for Electronic Device Protection (EDP)
It purpose is to limit the maximum and minimum voltage of a
signal
In other words, a “CLIPPER” clips off both positive and
negative portions of an input signal at a designed limit
Useful for transmitters and receivers and input protection for
instrumentation (especially, for medical devices used in

intensive care units of hospitals (e.g. ECG)
Ideal
Diode
68
General Clipper Implementation
(not practiced; for concept illustration)
This is a two port network
i.e., vin(t) and vo(t)
Its purpose is to limit vin(t) between Vmax and Vmin
The limited signal appears at vo(t)
Exploits the capability of the diodes to control the flow of
current under certain conditions
Ideal
Diode
e.g. -9V/+6V Clipper Circuit

71
72

73
For practical implementation, Zener diodes are used instead of a
DC power sources (A BETTER CLIPPER CIRCUIT)
VBR
VF
Open Circuit
74
DC power sources (BETTER CLIPPER CIRCUITS)
Assumes VF=0.6V and diode resistance is negligible (i.e. Rd=0
in both RBR and FB)
R
75

UPPER CLIP: Occurs when the diode BZ is in the forward bias
region (iDB>0)and diode AZ is in the reverse breakdown region
(iDA<0).
Occurs if vin(t)>6 then vo(t)=6V
76
LOWER CLIP: Occurs when the diode BZ is in the reverse
breakdown region (iDB<0) and diode AZ is forward bias region
(iDA>0).
Occurs if vin(t)< -9 then vo(t)=-9V
77

TRACKING: Occurs when the diode BZ is in the reverse bias
region (-8.4<vDB<0.6) and diode AZ is reverse bias region (-
5.4<vDA<0.6).
Occurs if -9<vin(t)<6 then vo(t)=vin(t)
78
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Biosignal Amplifiers
BIOE 3300 – Biomedical Electronics
(Chapters 11 & 14)
1
Biomedical Instrumentation
General Architecture
Biomedical
Amplifiers
Human Interface Subsystem

Signal
Processing
User Interface
Human
Interaction
Main Classes of Ideal Amplifiers
Voltage Amplifier
Amplifies an input voltage by a given gain factor (AV)
Vout=AVVin
Current Amplifier
Amplifies an input current by a given gain factor (AI)
Iout=AIIin
Transconductance Amplifier
Senses the input voltage and forces an output current
proportional (G) to this voltage to flow through the load.
Iout=GVin
Transresistance Amplifier
Senses the input current and forces an output voltage
proportional (R) to this current to appear across the load.
Vout=RIin
3

Class Impedance Characteristics
Regardless of the amplifier class, a core objective of an
amplifier system is to ensure that the output signal is a properly
scaled version of the source signal. This can be achieved, in
part, through the impedance characteristics of the amplifier.
4
Input Voltage Amplifiers desire to have a high input impedance
Output Voltage Amplifiers desire to have a low output
impedance
Input Current Amplifiers desire to have a low input impedance
Output Current Amplifiers desire to have a high output
impedance
Ideal Voltage Amplifier Model
5
Understanding Input Resistance (Impedance)
Ri is the input resistance of the amplifier

Rs is the source resistance
vs is the source voltage
vi is the input voltage to the amplifier
6
Voltage Amplifier
Preferred Input Characteristics
In this problem,
vs is the source signal (i.e., heart) that we wish to amplify.
vi is the input voltage to the amplifier.
We have no control over Rs as this is the inherent resistance of
the body.
Therefore, we can only design around Ri (amplifier input
resistance).
The objective is to ensure that vi=vs (no distortion; the signal
we want to amplify makes it to the input of the amplifier.

Voltage Amplifier
Preferred Input Characteristics
The objective is to ensure that vi=vs (0% distortion; the signal
we want to amplify makes it to the input of the amplifier.
Lets assume we design the amplifier with a very low input
In this case, what would vi be equal to?
vi=0; we lost all of our signal vs
(100% Distortion); (not desired)
Lets assume the other extreme such that we design the amplifier
with a ve
In this case, what would vi be equal to?
Since ii=0A then vi=vs (i.e., there can’t be a voltage
difference over RBODY as 0 current is flowing).
(Thus, 0% distortion; objective achieved)

9
Voltage Amplifier
Preferred Output Characteristics
RO is the output resistance of the amplifier
RL is the load resistance
Avo is the gain of the amplifier
iO is the current leaving the amplifier
vo is the amplifier’s output voltage
The objective is to ensure that
vo=Avovi =Avovs (0% distortion; the output is a properly
amplified version of the source signal.)
Lets assume we design the amplifier with a very high output
In this case, what would vo be equal to?
vo=0 as io=0 we lost all of our signal. Avovi does not make it
out of the amplifier
In this case, what would vo be equal to?

=Avovs
(0% distortion; objective achieved)
Inpu
As shown, to minimize distortion, voltage amplifiers must be
designed with a high input resistance and low output resistance
11
Ideal Current Amplifier Model
Ai is the current gain of the amplifier

12
Understanding Input Resistance (Impedance)
is is the source current
ii is the input current to the amplifier
The objective is to ensure that ii=is (0% distortion; the source
signal we want to amplify makes it into the input of the
amplifier.
Lets assume we design the amplifier with a very high input
In this case, what would ii be equal to?
ii=0; we lost all of our signal is
In this case, what would ii be equal to?
therefore, ii=is
(Thus, 0% distortion; objective achieved)

13
Ai is the current gain of the amplifier
14
Current Amplifier
Understanding output impedance
RO is the output resistance of the amplifier
RL is the load resistance
is is the source current
ii is the input current into the amplifier
Ai is the gain of the amplifier
iO is the current leaving the amplifier
The objective is to ensure that
io=Aiii =Aiis (0% distortion; the output is a properly amplified
version of the source signal.)
Lets assume we design the amplifier with a very low output
In this case, what would io be equal to?
io=0 we lost all of our signal. Aiii does not make it out of the
amplifier (all of this current flows through Ro)

In this case, what would io be equal to?
As can be seen, io=Aiii & as
=Aiis
(0% distortion; objective achieved)
As shown, to minimize distortion, current amplifiers must be
designed with a low input resistance and high output resistance
16
Ideal Transconductance Amplifier Model
Senses the input voltage and forces an output current
proportional (G) to this voltage to flow through the load.

Iout=GVin
These amplifiers behave as voltage amplifiers with respect to
the input and current amplifiers with respect to the output.
Therefore, using the same reasons as before, to minimize
distortion, transconductance amplifiers must be designed with a
17
Ideal Transresistance Amplifier Model
Senses the input current and forces an output voltage
proportional (R) to this current to appear across the load.
Vout=RIin
These amplifiers behave as current amplifiers with respect to
the input and voltage amplifiers with respect to the output.
Therefore, using the same reasons as before, to minimize
distortion, transresistance amplifiers must be designed with a

18
Class Impedance Characteristics
Regardless of the amplifier class, a core objective of an
amplifier system is to ensure that the output signal is a properly
scaled version of the source signal. This can be achieved, in
part, through the impedance characteristics of the amplifier.
19
Input Voltage Amplifiers desire to have a high input impedance
Output Voltage Amplifiers desire to have a low output
impedance
Input Current Amplifiers desire to have a low input impedance
Output Current Amplifiers desire to have a high output
impedance
Biopotential Amplifiers: Ideal Impedance Characteristics
To amplify a voltage potential from the
body it is desired to have an amplifier with a high input
resistance
Voltage Amplifier

20
Power relationship
Biocurrent Amplifiers: Ideal Impedance Characteristics
To amplify a current from the body, it is desired to have an
amplifier with a low input resistance
Current Amplifier
21
Basic Amplifier Concepts
Regardless of the class of amplifier, certain basic concepts hold
Input/Output Relationship
Output=(Gain Factor)*Input
Cascade Effect
22
Input/Output Relationship

Positive Gain
Non-inverting
Negative Gain
Inverting
23
Operational Amplifiers
The most common integrated circuit used for voltage
amplification is the operational amplifier (op-amp).
The op-amp is a high-gain differential amplifier
These are active components, therefore they require a external
power supply to operate/function
There are many different versions of op-amps available with
varying specifications (LM741, OP07, OP27, LM747, OP37,
….)
Price ranges vary from $0.15 to $10’s
Price mainly depends on performance specifications
e.g. noise, speed, input resistance, etc…
24

Common Voltage Amplifier Configurations
Inverting
Non-Inverting
Summing
Differential including Instrumentation Amplifier
Buffer or Voltage Follower
Differentiating
Integrating
Bridge
25
Simplified Schematic
MODEL
26

27
Schematic of Common Op-amp
Common IC packages
28
Characteristics of Ideal Op-Amps
Zero gain for the common-mode input signal
vo=0 when v1=v2 (no offset voltage)
Infinite input impedance
Zero output impedance
Infinite bandwidth
(no frequency response limitations or phase shift)

29
Engineering design with op-amps is usually facilitated by
treating these devices as “ideal”.
After the initial design, then the “non-ideal” or “real-world”
properties are usually considered.
At this point, the “brand” or “model” of op-amp is determined
to achieve desired response or the design configuration may
need to altered if the “non-ideal” properties cause a problem
30
Ideal Op-Amp Circuit Analysis
Based on Two Basic Rules
Rule 1: When the op-amp output is in its linear range, the two
input terminals are at the same voltage
Rule 2: No current flows into either input terminal of the op-
amp
31
Circuit Analysis for Op-amp Networks
Operational amplifiers are almost always used with negative
feedback, in which part of the output signal is returned to the
negative input in opposition to the source signal.

This provides a stabilized output through an ability to control
or set the gain of the configuration.
32
Summing Point Constraint
In a negative feedback system, the ideal op-amp output voltage
attains the value needed to force the differential input voltage
and input current to zero. We call this fact the summing-point
constraint.
In other words, if negative feedback is present, you can assume
V+ = V-
I+ = I-=0
33
Circuit Analysis Approach for Op-Amp Networks
(1) Verify that negative feedback is present.
(2) Assume that the differential input voltage and the input
current of the op amp are forced to zero. (This is the summing-
point constraint.)
(3) Apply standard circuit-analysis principles, such as
Kirchhoff’s laws and Ohm’s law, to solve for the quantities of
interest.

34
Standard Voltage Amplifier Configurations
Inverting
Non-Inverting
Summing
Differential including Instrumentation Amplifier
Buffer or Voltage Follower
Differentiating
Integrating
Bridge
35
Inverting Amplifier
36
Resistors

37
Choosing resistors for amplifier design
Prefer Val
Helps avoid unnecessarily large currents from flowing in the
circuit
Avoids damage to op-amp
Human/Animal subject considerations
Power issues
Avoids unnecessarily small currents
Results in noise issues
Very large resistances lead to instability due to leakage currents
over the surface of the resistors and circuit board. Stray pickup
of undesired signals
38
Be aware of tolerance
Tolerance provides an indication of how the actual resistance
value may deviate from the stated value
This gold band indicates a 5% tolerance

Tolerance may lead to unexpected behavior in a design
Error in Amplifier Gain
Unusually low CMRR in differential amplifiers
CMG)
39
Tolerance and Amplifier Gain
(a) Design an inverting amplifier with a K= -150 V/V using 5%
tolerance resistors
(b) Determine the minimum and maximum possible gain
40
Summing Amplifier
A summing amplifier accepts multiple input voltage signals
which are amplified in an inverting manner then summed
together into a single output.
41

Example (Amplifier System Analysis)
Find vo in terms of v1 and v2
Answer: vo=4v1-2v2
vo1
A
B
42
Non-Inverting Amplifier
vin(t)
vout(t)
43
Voltage Follower or Buffer
vin(t)
vout(t)
What is the purpose of a buffer?
What is the simplest configuration that has the same input-
output relationship?
Does a buffer behave in the same manner?

Why is an amplifier needed?
Often in design, we need to cascade multiple configurations to
achieve a desired outcome. In certain cases, however,
subsequent stages can affect the behavior of prior stages
(LOADING EFFECTS)
Used for stage isolation (minimize loading effects)
44
High Pass Filter to Inverting Amplifier Configuration Example
Types of Biopotential Measurements
Two Types
Monopolar Recordings – Single site biopotential measurements
(i.e., made with 1-channel/single input type amplifiers)
Bipolar Recordings – Dual site differencing biopotential
measurements (i.e., made with 2-channel/dual input type
amplifiers)
Most common (e.g., utilized for ECG, EMG, EEG, EOG,etc…)
Regardless, whether monopolar or bipolar biopotential
measurements, the amplifier used to interface to the body must
have an extremely high input impedance to avoid distortion
Standard
Differential Amplifier (Dual-Input)

46
Instrumentation Amplifier
Bipolar Biopotential Amplifier
Excellent Bio-potential Amp
-High input impedance
-High Gain Possible
-Able to distribute over two stages
-Good Noise Rejection
1
2
3
47
Good Biopotential Amp
High input impedance
Good Noise Rejection
High Gain Possible
High Bandwidth
Example: EEG Amplifier Design
Design a bipolar EEG voltage amplifier with a gain of 2000
V/V.
(EEG signals can range from .1mV to 1 mV in amplitude)
For this application, a differential amplifier would be needed

(i.e. bipolar measurements) with a suitable input impedance
(need high impedance).
48
Instrumentation Amplifier
For this design, we need to determine R1, R2, R3, & R4 to
realize an overall gain of K=2000 V/V.
49
Design
It is best to distribute the overall gain between the two stages
Note: too much gain on stage 1 may cause saturation after the
initial stage due to slight DC biopotentials and/or biasing issues
inherent in the fabrication of the op-amp
Too much gain on any single stage may also cause problems
meeting the 1kΩ to 100kΩ preferred resistive component design
requirement
One possible design
K1=50 K2=40
K1*K2=50*40=2000 V/V

50
Design
To design K1=50, choose an R2
Compute an R1
R1=2R2/(K1- -1)
To design K2=40, choose an R4
Compute an R3
51
Realistic Design
Note: resistor tolerance will cause ideal gain to deviate
Another consideration, in regards to gain, is that resistors are
only commercially available in certain values
K=51*40=2040 V/V
If a more precise gain is desired, a potentiometer (i.e. variable
resistor) could be used for R1

52
Noise Rejection and Differential Amplifiers
One main problem in the measurement of bio-potentials is the
presence of noise (especially, 60 Hz power line noise -see
figure next slide)
(1) direct coupling to surface of body
(2) Poorly shielded electrode leads
For a differential amplifier, this noise is present at both inputs.
Signals of this type are known as common-mode signals.
Common to both inputs
These signals can mask the signal of interest, however, through
proper design practices common-mode signals can be attenuated
considerably.
53
The ability of an amplifier to attenuate common-mode signals is
known as Common-Mode Rejection Ratio (CMRR)
Units: decibels (dB)
A more accurate expression for vo

54
Example
Find the minimum CMRR for an ECG amplifier if:
the differential gain is 1000
the desired differential input signal has a peak amplitude of 1
mV
the common-mode signal is a 100 V peak 60Hz sine wave
And it is desired that the output contains a peak common-mode
contribution that is 1% or less of the peak output caused by the
differential signal
55
Example
Solution
: to compute CMRR we need DMG and CMG
Desired peak ECG signal is: (1mV)(1000)=1V
To meet the stated specification, the common-mode signal must
have a peak value less than 1V*1%=0.01 V
Based on this, the CMG=Vocm/Vicm=0.01V/100V=10-4
DMG=1000

CMRR=20*log10(DMG/CMG)=20*log10(1000/10-4)=140dB
56
For ECG amplifiers needed CMRR is between 90-130dB
Common Mode Noise & CMRR
ECG (K=1000 V/V) CMRR 65dB
ECG (K=1000 V/V) CMRR 125 dB
What determines the CMRR for a Differential Amplifier?
(1) The inherent CMRR for the model of op-amp being used.
741: 90dB
OP07: 100-120dB
OP27/37: 126dB
(2) imbalance in resistances. For maximum CMRR R1=R3 and

R2=R4
slight differences between resistances can affect CMRR
significantly)
(3) Frequency of differential and common-mode signal
The model of op-amp determines the maximum attainable
CMRR
58
59

60
CMRR and the Instrumentation Amplifier
For instrumentation amplifiers, the overall CMRR is solely
determined by the second stage alone.
CMG for first stage is unity
Vo1
Vo2
61
Practical Measurement of CMRR
Measure the Differential mode gain (DMG)
Ground one input and apply a sinusoidal input of known
amplitude to the second input

DMG=Vo/Vin
Measure the Common mode gain (CMG)
Apply a high amplitude sinusoid signal of known amplitude to
both inputs
CMG=Vo/Vin
Compute CMRR
62
Practical Considerations
(i.e. non-ideal characteristics)
In practice, op-amps can behave quite differently from ideal
under certain operating conditions
Known as Nonlinear effects
Clipping
Related to the physical achievable output voltage for an op-amp
based amplifier. In practice, it is bound between an upper and
lower limit determined in part by the DC power supply rails.

Slew Rate
Related to the rate of change in output voltage with respect to
time. In practice, this is bound by an upper limit.
Bandwidth
Related to the range of frequencies in which the gain of the
amplifier will behave as ideal.
63
Practical (i.e. non-ideal)
Operation Amplifiers
Clipping
The output voltage of a real op-amp is limited to an operational
range. This range is dependent on the internal design of the op-
amp and the external DC power supply.
When the output voltage tries to exceed these limits, clipping
occurs.

64
Example: Clipping
Listed in Specification Sheet
For a given op-amp model
65

66
Nonlinear Effects
Slew Rate (SR) Units: Voltage/time
Describes another limitation of practical op-amps where the
magnitude of the rate of change of the output voltage is limited.
In other words, SR is how fast an op-amp can change its output
voltage
Very important consideration for amplifiers with time varying
input signals
If the slew rate is exceeded, the output voltage will be distorted
-
For sinusoidal signals, the minimal slew rate needed such that
no output distortion occurs is:

67
68
Example: Slew rate
--Maximum rate of change is
0.5 V/us for a 741 op-amp

Ideal
Actual
(741 op-amp)
If we want to achieve the ideal response for this configuration,
what could be
done?
Use a different op-amp (e.g. OP37 SR=17 V/us)
What is the minimum slew rate needed to avoid distortion?
69
Example 14.6
Nonlinear Effects
Gain Bandwidth Product (GBWP)
Sometimes referred to unity gain (Gain =1) small signal
bandwidth
Determined by the model of the op-amp
For a single op-amp configuration, the GBWP is constant

Allows determination of the range of input frequencies for
which the designed gain of the amplifier will behave as ideal
This is referred to as the amplifier bandwidth
A input signal which has a frequency that exceeds the
bandwidth will not be amplified to the full extent.
Gain
Frequency
Ideal
Non-Ideal
Units: (Hz)
70

71
Example #1
Determine the bandwidth of a non-inverting amplifier designed
for a gain of 50 that is built with a
741 op-amp GBWP = 1 MHz
OP-27 op-amp GBWP = 8MHz
741
Bandwidth=GBWP(741) /50=1MHz/50=20kHz
OP-27
Bandwidth=GBWP(OP-27) /50=8MHz/50=160kHz
72
Example #2
A standard differential amplifier is designed using a 741 op-
amp (GBWP=1MHz) with an overall gain K=1000 V/V

What is the bandwidth?
Bandwidth=GBWP/1000=1MHz/1000=1KHz
73
Example #3
An instrumentation amplifier is designed using 741 op-amps
(GBWP=1MHz) with a 1st stage gain of 50 and a 2nd stage gain
of 20 for an overall gain K=(50)(20)=1000 V/V
Would the bandwidth=GBWP/1000=1kHz?
NO!
Why?
The gain is distributed across multiple op-amp stages
Stage 1: Bandwidth=GBWP/50=1MHz/50=20KHz
Stage 2: Bandwidth=GBWP/20=1MHz/20=50KHz
Therefore Stage 1 limits the bandwidth of the instrumentation
amplifier to 20KHz
However the bandwidth is 20 times larger compared to the
standard differential amplifier case seen in Example #2!
This is a benefit of distributing large gains over multiple stages

74
Other ‘Op-Amp’ Configurations of Interest
Integrators
Differentiators
Modified Bridge Circuits
75
Integrators
Integrators produce an output voltage signal that is proportional
to the running time integral of the input voltage signal.
Note: In a running time integral, the upper limit of integration

is t .
76
Integrator
Purpose: In a general sense, integrators are used to determine
the area under the curve, which can be used to estimate signal
energy.
77
Example
If the input into an integrator is given by:

And R=10kΩ and C=0.1uF, sketch the output. (Assume an ideal
op-amp)

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