Download Electronic Controls

ACADs (08-006) Covered
5.1.12.1.12e 5.4.1.1 5.4.1.2
Keywords
Switching power supply, amplifier, logic, digital, circuit, solid state.
Description
Supporting Material
PVNGS GOALS
Introduction
Plant Status
Class Guidelines
PVNGS Goals
Focus on Five
Lab PPE requirements
Student Participation
Attendance sheet
Breaks
Course Terminal Objective
Using reference material furnished by the
instructor, the Plant Technician will discuss solid
state fundamentals, build, calibrate and test
electronic circuits to support theory, and
demonstrate an understanding of solid state
fundamentals by successful completion of a Lab
Practical Evaluation (LPE).
Enabling Objectives
1.1.1 Describe the HU fundamentals to be used and hazards associated with working on
electronic components.
1.1.2 Describe the purpose for Inspecting and reworking printed circuit boards.
1.1.3 Describe the functions and associated controls of the Tektronix 2445 Oscilloscope.
1.1.4 Describe barrier potential effects on the PN junction of a general purpose diode.
1.1.5 Build/test/analyze a series and series-parallel diode circuit.
1.1.6 Describe special purpose diode design and application.
1.1.7 Build/test/analyze a Zener diode voltage regulator circuit.
1.1.8 Describe Single Phase DC power supply design and application.
1.1.9 Build/test/analyze a Single Phase DC Power supply circuit.
1.1.10 Describe voltage divider and voltage multiplier device design and application.
1.1.11 Describe Light Emitting Diodes (LED's) device design and application.
1.1.12 Build/test/analyze an LED circuit.
1.1.13 Describe Bipolar Junction Transistor (BJT) device design, operation and application.
1.1.14 Build/test/analyze a Bipolar Junction Transistor (BJT) switching circuit.
1.1.15 Describe Silicon Controlled Rectifier (SCR) device design and application.
1.1.16 Build/test/analyze an SCR switching circuit.
1.1.17 Describe Triac, Diac, and Unijunction Transistor (UJT) device design and application.
1.1.18 Build/test/analyze a Triac, Diac, and UJT circuit.
1.1.19 Describe Field Effect Transistor (FET) device design and application.
1.1.20 Build/test/analyze a Field Effect Transistor (FET) circuit.
1.1.21 Describe Integrated Circuit (Op Amp) device design and application.
1.1.22 Build/test/analyze integrated (Op Amp) circuits.
1.1.23 Describe Digital Logic symbols and truth tables.
1.1.24 Describe Fiber Optic circuit device design and application.
Prevent
Prevent Events
Events
Achieving Breakthrough Performance
It’s everyone’s responsibility to work safely
and prevent events.
Ask yourself before beginning a job:
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Prevent
Prevent Events
Events
What are the critical steps of the task?
What document describes it? Do I
understand it?
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Prevent
Prevent Events
Events
What is the worst thing that can happen
and how can I prevent it?
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Prevent
Prevent Events
Events
What else could go wrong?
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Prevent
Prevent Events
Events
What are the safety and/or radiation
protection considerations?
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Prevent
Prevent Events
Events
Is my training, and are my qualifications
up to date?
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Class Experience
INVERTER AC OUTPUT BREAKER TRIP
CIRCUIT DRAWING E054-00164
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E054-00163
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Backplane Drawing E054-00162
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PC Board HU and Safety
Electrostatic discharge:
•
•
•
•
•
•
•
•
•
Electrostatic discharge is the movement of electrons from a source to
an object.
Static electricity is an electrical charge at rest.
The most common way to build static electricity is by friction.
Friction creates an electron buildup or a negative static charge.
When a person contacts a positive charged or grounded object, all
excess electrons flow (jump) to that object.
Electrostatic discharge can be 35,000 volts or more.
People normally do not feel electrostatic discharges until the discharge
reaches 3000 volts.
Solid state devices and circuits may be damaged or destroyed by a 10
volt electrostatic discharge. The effects of static discharge may be
cumulative and not readily obvious.
Technicians and maintenance Electricians should wear a wrist
grounding strap or other type of grounding device to avoid damage to
solid state devices and circuits.
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U2R9: 2EPNDN14 INVERTER BACKPLANE
COLD SOLDER CONNECTIONS ON XFMR.
Cold Solder Joint
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U2R9: 2EPNDN14 INVERTER BACKPLANE
COLD SOLDER CONNECTIONS ON XFMR.
Cold Solder
Joint
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U2R9: 2EPNDN14 INVERTER BACKPLANE
COLD/DIRTY SOLDER CONNECTIONS ON
BACKPLANE
Cold Solder Joint
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U2R9: 2EPNDN14 INVERTER BACKPLANE
COLD/DIRTY SOLDER CONNECTIONS ON
BACKPLANE
Cold Solder Joint
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U2R9: 2EPNDN14 INVERTER BACKPLANE
COLD/DIRTY SOLDER CONNECTIONS ON
BACKPLANE
Cold Solder Joint
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Cold Solder Joint
U2R9: 2EPNDN14 INVERTER BACKPLANE
N
COLD SOLDER CONNECTIONS ON XFMR.
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U1R9: 1EPKDN44 INVERTER BACKPLANE
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U1R9: INVERTER 1EPKDN44 BACKPLANE
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U1R9: INVERTER 1EPKDN44 J2 BOARD FOIL SIDE
DAMAGED PC BOARD + 15 VOLT PIN CONTACT.
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U1R9: INVERTER 1EPKDN44 J2 BOARD FOIL SIDE
DAMAGED PC BOARD + 15 VOLT PIN CONTACT.
U1R9
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U1R9: 1EPNAN11 INVERTER:
MISSING MOLEX CONNECTOR PIN
U1R9
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U1R9: 1EPNAN11 INVERTER: MISSING MOLEX
CONNECTOR
PIN. DOOR CONTROL PANEL:
U1R9: INVERTER
1EPNAN11:
DEFECTIVE MOLEX CONNECTOR PIN.
U1R9
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Tektronix 2445
• Section 3 – Controls, Connectors and
Indicators
Review of semiconductor fundamentals
1.
2.
3.
In Semiconductors production, doping refers to the process
of intentionally introducing impurities into an extremely
pure (intrinsic) semiconductor in order to change its
electrical properties.
Basic semiconductor materials are silicon and germanium.
In there pure state they are poor conductors and have 4
Electrons in the valence shell, by adding impurities to the
intrinsic structure we create an excess of holes or electrons
in the material.
Boron, arsenic, phosphorus and occasionally gallium are
used To dope silicon and are called dopants.
Silicon and germanium have 4 electrons in the outer shell.
Silicon doped with a trivalent material such as boron becomes known
P-type. The trivalent material has 3 electrons leaving the outer valence
Deficient 1 electron or “hole”. The majority current carriers are holes.
Silicon doped with a pentavalent material such as phosphorus
becomes known as N-type. The pentavalent material has 5
electrons .living the outer valence shell 1 free electron. Electrons
are the majority Current carrier.
PN Junction formation:
Combine them to make one piece of semiconductor which is
doped differently on each side of the junction.
Free electrons on the n-side and free holes on the p-side can initially
wander across the junction. When a free electron meets a free hole it
can 'drop into it'. So far as charge movements are concerned this means
the hole and electron cancel each other and vanish.
As a result, the free electrons and holes near the junction tend to eat
each other, producing a region depleted of any moving charges. This
creates what is called the depletion zone.
P N Junction Biasing
Applying a forward or reverse bias to the junction will change the depletion
region width .
Forward bias reduces the region resulting in conduction
Reverse bias enlarges the region resulting in no conduction.
Junction Diode
The Junction diode is the simplest semiconductor. It is formed by
doping on-half of the intrinsic material with a p-type dopant and
the other half with an n-type dopant. The boundary at the p and
n regions is called the pn junction.
Typical junction drop Silicon-- 0.7volts, Germanium --0.3volts
Junction Diode Characteristic
Junction Diode Parameters
• When selecting diodes, two device ratings must be taken into
consideration; Peak Reverse Voltage and Maximum Average Forward
Current Can be classified as:
– Maximum Average Forward Current is usually given at a special
temperature, usually 25°C, (77°F) and refers to the maximum
amount of average current that can be permitted to flow in the
forward direction. If this rating is exceeded, structure breakdown
can occur.
– Peak Reverse Voltage or Peak Inverse Voltage is the maximum
voltage that a diode can withstand in the reverse direction
without breaking down and starting to conduct. If this voltage is
exceeded the diode may be destroyed. Diodes must have a Peak
Inverse Voltage rating that is higher than the maximum voltage
that will be applied to them when reverse biased.
Zener Diodes
A Zener diode is a type of diode that permits current to flow in the
forward direction like a normal diode, but also in the reverse direction
if the voltage is larger (not equal to, but larger) than the rated
breakdown voltage known as "Zener knee voltage" or "Zener voltage".
The breakdown voltage can be controlled quite accurately in the doping
process. Tolerances to within 0.05% are available though the most
widely used tolerances are 5% and 10%.
Zener Diodes
The zener diode uses a p-n junction in reverse-bias to make use of the
zener-effect, which is a breakdown phenomenon which holds the
voltage close to a constant value called the zener “knee” voltage. It is
useful in regulator applications
Various Diodes and there symbols.
Diode symbols: a - regulating and HF diode, b - LED, c, d -Zener,
e - photo, f,g - tunnel, h - Schottky, i - breakdown, j capacitative
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Slide 15
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Slide 18
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Slide 17
Transistors
• Can be classified as:
– BJT – Bipolar Junction Transistor;
• Minority carrier device;
• Bipolar device.
– FET – Field Effect Transistor;
• Majority carrier device;
• Unipolar device;
NPN and PNP Junctions
A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined
back-to-back. This forms a sort of a sandwich where one kind of semiconductor is
placed in between two others. There are therefore two kinds of Bipolar sandwich, the
NPN and PNP varieties. The three layers of the sandwich are conventionally called the
Collector, Base, and Emitter. The reasons for these names will become clear later once
we see how the transistor works.
NPN and PNP Junctions
Figure 1 shows an NPN transistor with no external voltages. We see a back-to-back pair
of PN Diode junctions with a thin P-type filling between two N-type slices of 'bread'. In
each of the N-type layers conduction can take place by the free movement of electrons
in the conduction band. In the P-type (filling) layer conduction can take place by the
movement of the free holes in the valence band. However, in the absence of any
externally applied electric field, we find that depletion zones form at both PN-Junctions,
so no charge wants to move from one layer to another.
NPN and PNP Junctions
Applying voltage between the Collector and Base parts of the transistor. The
polarity of the applied voltage is chosen to increase the force pulling the Ntype electrons and P-type holes apart. (Collector positive with respect to the
Base.) This widens the depletion zone between the Collector and base and so
no current will flow. In effect we have reverse-biased the Base-Collector diode
junction. The precise value of the Base-Collector voltage can vary here we see
10 v.
Applying a relatively small Emitter-Base voltage designed to forward-bias the EmitterBase junction. This 'pushes' electrons from the Emitter into the Base region and sets up
a current flow across the Emitter-Base boundary. Once the electrons have managed to
get into the Base region they can respond to the attractive force from the positivelybiased Collector region. As a result the electrons which get into the Base move swiftly
towards the Collector and cross into the Collector region. We now see an EmitterCollector current whose magnitude is set by the chosen Emitter-Base voltage we have
applied. To maintain the flow through the transistor we have to keep on putting 'fresh'
electrons into the emitter and removing the new arrivals from the Collector. Hence we
see an external current flowing in the circuit.
Some of the free electrons crossing the Base encounter a hole and 'drop into it'. As a
result, the Base region loses one of its positive charges (holes) each time this happens.
If we didn't do anything about this we'd find that the Base potential would become
more negative (i.e. 'less positive' because of the removal of the holes) until it was
negative enough to repel any more electrons from crossing the Emitter-Base junction.
The current flow would then stop.
To prevent this happening we use the applied Emitter-Base voltage to remove the
captured electrons from the Base and maintain the number of holes it contains. This
have the overall effect that we see some of the electrons which enter the transistor via
the Emitter emerging again from the Base rather than the Collector.
Bipolar junction transistor
The Bipolar junction transistors (BJT) have three terminals named emitter,
base and collector. Two p-n junctions exist inside a BJT: the base/emitter
junction and base/collector junction.
An easy way to identify a specific transistor configuration is to follow
three simple steps:
1. Identify the element (emitter, base, or collector) to which the input signal is
applied.
2. Identify the element (emitter, base, or collector) from which the output
signal is taken.
3. The remaining element is the common element, and gives the configuration
its name.
Common Emitter
Common Base
Common Collector
Bipolar junction transistor
Using Kirchoff's current law and the sign convention shown above we find that the
base current equals the difference between the emitter and collector current.
Bipolar junction transistor
The Base current: Ib = Ie – Ic
The ratio of the collector current to the base current of a
bipolar transistor, commonly referred to as the current
amplification factor or :
The transistor beta : β
β = Ic/Ib
Bipolar junction transistor
hie :dynamic input resistance in the common
emitter
configuration.
hfe: forward current transfer ratio or gain of the
transistor.
1/hoe:the output conductance .
hre: represents a small input voltage developed as a
result
of reverse feedback from the output circuit.
Bipolar junction transistor
Current Gain varies with the Collector Current level, IC. From this graph
we can see that the proportion of electrons 'caught' by a hole while trying
to cross the Base region does vary a bit depending on the current level.
Note that the graph doesn't show the transistor's beta value, it shows a
related figure called the transistor's Small Signal current gain, hfe. This is
similar to the beta value, but is defined in terms of small changes in the
current levels. This parameter is more useful than the beta value when
considering the transistor's use in signal amplifiers where we're interested
in how the device responds to changes in the applied voltages and
currents.
Bipolar junction transistor
The second way we can characterize the behavior of a
Bipolar Transistor is by relating the Base-Emitter voltage, VBE,
we apply to the Base current, IB, it produces. As can expect
from the diode-like nature of the Base-Emitter junction this
voltage/current characteristic curve has an exponential-like
shape similar to that of a normal PN Junction diode.
Bipolar junction transistor
Characteristic curve of a typical transistor.
Bipolar junction transistor
Characteristic curve of a typical transistor.
Bipolar junction transistor
Operating Curve examples:
The transistor Q point or operating point is demonstrated by the
curves below. As long as the input swing is on the linear portion of
the operating curve the output will be linear.
Bipolar junction transistor
When (negative) feedback is introduced, most of these problems
diminish or disappear, resulting in improved performance and
reliability. There are several ways to introduce feedback to this
simple amplifier, the easiest and most reliable of which is
accomplished by introducing a small value resistor in the emitter
circuit. The amount of feedback is dependent on the relative signal
level dropped across this resistor.If RL = Re the gain is a
approximately unity.
The Silicon Controlled Rectifier or SCR
The Silicon Controlled Rectifier (SCR) is simply a conventional rectifier
controlled by a gate signal. The main circuit is a rectifier, however the
application of a forward voltage is not enough for conduction. A gate signal
controls the rectifier conduction.
The schematic representation is:
The Silicon Controlled Rectifier or SCR
The rectifier circuit (anode-cathode) has a low forward resistance and a high
reverse resistance. It is controlled from an off state (high resistance) to the on
state (low resistance) by a signal applied to the third terminal, the gate. Once
it is turned on it remains on even after removal of the gate signal, as long as a
minimum current, the holding current, Ih, is maintained in the main or
rectifier circuit. To turn off an SCR the anode-cathode current must be
reduced to less than the holding current, Ih.
The Silicon Controlled Rectifier or SCR
The reverse characteristics are the same as the diode,
having a breakover voltage with its attending avalanche
current; and a leakage current for voltages less than the
breakover voltage. In the forward direction with open
gate, the SCR remains essentially in an off condition
(notice though that there is a small forward leakage) up
until the forward breakover voltage is reached. At that
point the curve snaps back to a typical forward rectifier
characteristic. The application of a small forward gate
voltage switches the SCR onto its standard diode
forward characteristic for voltages less than the forward
breakover voltage.
The Silicon Controlled Rectifier or SCR
The Silicon Controlled Rectifier or SCR
Obviously, the SCR can also be switched by exceeding
the forward breakover voltage, however this is usually
considered a design limitation and switching is normally
controlled with a gate voltage. One serious limitation of
the SCR is the rate of rise of voltage with respect to
time, dV/dt. A large rate of rise of circuit voltage can
trigger an SCR into conduction. This is a circuit design
concern. Most SCR applications are in power switching,
phase control, chopper, and inverter circuits.
The Silicon Controlled Rectifier or SCR
Major considerations when ordering SCR’s:
(a) Peak forward and reverse breakdown voltages
(b) Maximum forward current
(c) Gate trigger voltage and current
(d) Minimum holding current,Ih
(e) Power dissipation
(f) Maximum dV/dt
The DIAC, or diode for alternating current, is a
bidirectional trigger diode that conducts current
only after its breakdown voltage has been
exceeded momentarily. When this occurs, the
resistance of the diode abruptly decreases, leading
to a sharp decrease in the voltage drop across the
diode and, usually, a sharp increase in current flow
through the diode.
The diode remains "in conduction" until the
current flow through it drops below a value
characteristic for the device, called the holding
current. Below this value, the diode switches back
to its high-resistance (non-conducting) state. When
used in AC applications this automatically happens
when the current reverses polarity.
The DIAC
Circuit symbol and device cross-section of a) a Diode AC switch
(DIAC) and a Triode AC switch (TRIAC
FET: Field-effect transistors
•
Two primary types:
– JFET, Junction FET
– . n or p channel depletion mode.
– MOSFET, Metal-Oxide-Semiconductor FET. Also known as IGFET –
Insulated Gate FET;
•
MOS transistors can be:
– n-Channel;
• Enhancement mode;
• Depletion mode;
– p-Channel;
• Enhancement mode;
• Depletion mode;
The JFET is a class of Unipolar semiconductor device which comes in
two basic varieties, n or p channel. The device in general `runs in the
depletion mode with three regions operation: Pinchoff, Breakdown
and Triode.
FET: Field-effect transistors
The JFET is a class of Unipolar semiconductor device which comes in
two basic varieties, n or p channel. The device in general `runs in the
depletion mode with three regions operation: Pinchoff, Breakdown and
Triode.
In the n-channel JFET the device is generally
operated with the gate to channel pn junction in a
reverse bias, the level of the reverse bias determines
the thickness of the depletion region in the channel
to control the Drain to Source resistance.
With a steady gate-source voltage of 1 V there is always 1 volt across the
wall of the channel at the source end. A drain-source voltage of 1V means
that there will be 2 volts across the wall at the drain end. (The drain is ‘up’
1V from the source potential and the gate is 1V ‘down’, hence the total
difference is 2V.) The higher voltage difference at the drain end means
that the electron channel is squeezed down a bit more at this end.
When the drain-source voltage is increased to 10V the
voltage across the channel walls at the drain end increases to
11V, but remains just 1V at the source end. The field across
the walls near the drain end is now a lot larger than at the
source end. As a result the channel near the drain is squeezed
down quite a lot .
Increasing the source-drain voltage to 20V squeezes down this end of the
channel still more. As we increase the drain-source voltage we increase
the electric field which drives electrons along the open part of the
channel. However, we can now see that increasing the drain-source
voltage also squeezes down the channel near the drain end. This reduction
in the open channel width makes it harder for electrons to pass. The two
effects of greater push along the channel and a tighter squeeze tend to
cancel out. As a result the drain-source current tends to remain constant
when we increase the drain-source voltage.
FET: Characteristic Curve
FET: Characteristic Curve
Pinchoff: Vp is that value of V d-s at which the knee of the V g-s =0
curve occurs. Vp is defined as the gate –source voltage required to reduce
the drain current to 1 micro amp at a Vd-s of 15v. It can range from -.4 to 8.0v
Breakdown: At some positive V d-s the device will have breakdown,
at which point the current will increase very rapidly (unless limited by the
external circuit)
Triode mode is usually used to turn something on as in the circuit
below. It may not be obvious from this circuit but you want the voltage
drop across the FET to be as small as possible when the relay is switched
on. Usually when you turn something off you want the switching element,
FET, to have infinite impedance. Cutoff mode is great for that. When you
turn something on you want the switch to have 0 impedance. This is so
the device being turned on gets all of the power and no power is wasted
or dissipated in the FET.
Triode Mode example
MOSFET SYMBOLS
MOSFET
NMOS cross section
Biasing:
Creating a channel for
current flow.
Placing an insulating layer between the gate and the channel allows for
a wider range of control (gate) voltages and further decreases the gate
current (and thus increases the device input resistance). The insulator is
typically made of an oxide (such as silicon dioxide, SiO2), This type of
device is called a metal-oxide-semiconductor FET (MOSFET) or insulatedgate FET (IGFET).
Cross-section and circuit symbol of an n-type MetalOxide-Semiconductor-Field-Effect-Transistor (MOSFET
Since there is an insulator between the gate and channel,
the gate is not restricted in polarity. There are 2 modes of
operation for the MOSFET, they are enhancement and
depletion.
Figure 1. Construction of a MOSFET. (n-channel, enhancement),
Figure 2. The MOSFET with a positive gate voltage.
MOSFET under operation in the triode region.(V d-s small),
Pinchoff is simply an equilibrium point with constant
current for a fixed gate-source voltage. Figure below shows
the channel in pinchoff. Pinchoff begins when V g-d < VT.
The enhancement mode device is also referred to as the
“normally off” MOSFET.
this is because with zero gate bias voltage the source and
drain contacts are separated by two pn junctions connected
back to back.
The depletion mode device is also referred to as the
“normally on” MOSFET.this is because with zero gate bias
voltage the source and drain islands, which are of n type
material are connected by an n channel.
A large enough negative bias to the gate will be sufficient to
convert the n material in the channel to p type. In this way the
2 n islands become separated by a p region.
MOSFET Characteristic Curve
MOSFET Characteristic Curve
MOSFET:
Id vs V g-s curves
VT Design Trade-Off
(Important consideration for digital-circuit applications)
• Low VT is desirable for high ON current
– where VDD is the power-supply voltage
• …but high VT is needed for low OFF current
log IDS
Low VT
High VT
IOFF,low VT
IOFF,high VT
0
VGS
Typical Operational Amplifiers
An inside look at the uA 741 Op Amp
•
The Operational Amplifier is a solid state Integrated circuit that consists of
a direct coupled high gain amplifier with differential inputs and usually a
single ended output that uses external feedback to control it’s functions.
• Differential amplifier
– Input stage — provides low noise amplification, high input impedance
usually a differential output.
• Voltage amplifier
– Provides high voltage gain, a single-pole frequency roll-off, usually singleended output .
• Output amplifier
– Output stage — provides high current driving capability, low output
impeadence, current limiting and short circuit protection circuitry.
The Op- amp in sections
An inside look at the uA 741 Op Amp
A. The section outlined in blue is the Differential input stage.
The blue outlined section is a differential amplifier. Q1 and Q2 are input emitter
followers and together with the common base pair Q3 and Q4 form the differential
input stage. In addition, Q3 and Q4 also act as level shifters and provide voltage
gain to drive the class A amplifier.
The differential amplifier formed by Q1 - Q4 drives a current mirror active load
formed by transistors Q5 - Q7. Q7 increases the accuracy of the current mirror by
decreasing the amount of signal current required from Q3 to drive the bases of Q5
and Q6. This current mirror provides differential to single ended conversion as
follows:
The signal current of Q3 is the input to the current mirror while the output of the
mirror (the collector of Q6) is connected to the collector of Q4. Here, the signal
currents of Q3 and Q4 are summed. For differential input signals, the signal currents
of Q3 and Q4 are equal and opposite. Thus, the sum is twice the individual signal
currents. This completes the differential to single ended conversion.
The open circuit signal voltage appearing at this point is given by the product of the
summed signal currents and the paralleled collector resistances of Q4 and Q6. Since
the collectors of Q4 and Q6 appear as high resistances to the signal current, the
open circuit voltage gain of this stage is very high.
B. The sections outlined in red are Current mirrors.
The input stage DC conditions are controlled by the two current mirrors on the left.
The current mirror formed by Q8/Q9 allows for large common mode voltages on the
inputs without exceeding the active range of any transistor in the circuit.
The current mirror Q10/Q11 is used, indirectly, to set the input stage current. This
current is set by the 5 k Ω resistor. The input stage bias control acts in the following
manner. The outputs of current mirrors, Q8/Q9 and Q10/Q11 together form a high
impedance current differencing circuit.
If the input stage current tends to deviate (as detected by Q8) from that set by Q10,
this is mirrored in Q9 and any change in this current is corrected by altering the
voltage at the bases of Q3 and Q4. Thus the input stage DC conditions are stabilized
by a high gain negative feedback system.
The top-right current mirror Q12/Q13 provides a constant current load for the class A
gain stage, via the collector of Q13, that is largely independent of the output voltage.
C. The section outlined in magenta is the Class A gain stage.
The stage consists of two NPN transistors in a Darlington configuration and
uses the output side of a current mirror as its collector load to achieve high gain.
The 30 pF capacitor provides frequency selective negative feedback around the
class A gain stage as a means of frequency compensation to stabilize the
amplifier in feedback configurations. This technique is called Miller
compensation.
Darlington transistor is a semiconductor device which combines two bipolar
transistors in tandem (often called a "Darlington pair") in a single device so that the
current amplified by the first is amplified further by the second transistor. This gives
it high current gain (written β or Hfe), and takes up less space than using two
discrete transistors in the same configuration. The use of two separate transistors in
an actual circuit is still very common, even though integrated packaged devices are
available.
D. The section outlined in green is the Output bias circuitry.
The green outlined section (based around Q16) is a voltage level shifter or Vbe
multiplier; a type of voltage source. In the circuit as shown, Q16 provides a
constant voltage drop between its collector and emitter regardless of the current
passing through the circuit. If the base current to the transistor is assumed to be
zero, and the voltage between base and emitter (and across the 7.5 kΩ resistor) is
0.625 V (a typical value for a BJT in the active region), then the current flowing
through the 4.5 kΩ resistor will be the same as that through the 7.5 kΩ, and will
produce a voltage of 0.375 V across it.
This keeps the voltage across the transistor, and the two resistors at 0.625 +
0.375 = 1 V. This serves to bias the two output transistors slightly into conduction
reducing crossover distortion. In some discrete component amplifiers this function
is achieved with (usually 2) silicon
E. The section outlined in cyan is the Output stage amplifier.
The output stage (outlined in cyan) is a Class AB push-pull emitter follower (Q14,
Q20) amplifier with the bias set by the Vbe multiplier voltage source Q16 and its
base resistors. This stage is effectively driven by the collectors of Q13 and Q19.
The output range of the amplifier is about 1 volt less than the supply voltage, owing
in part to Vce(sat) of the output transistors.
The 25 Ω resistor in the output stage acts as a current sense to provide the output
current limiting function which limits the current flow in the emitter follower Q14 to
about 25 mA for the 741.
Current limiting for the negative output is done by sensing the voltage across
Q19's emitter resistor and using this to reduce the drive into Q15's base. Later
versions of this amplifier schematic may show a slightly different method of output
current limiting. The output resistance is not zero as it would be in an ideal op-amp
but with negative feedback it approaches zero.
Logic Symbols
Equivalent circuit
Ideal characteristics of the Op Amp
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The ideal operational amplifier would have the following characteristics
:
1. Infinite gain
2. Infinite Bandwidth
3. Infinite input impendence: Zin
4. Zero Output impendence: Zo
5. Zero input offset voltage
6. Infinite Io drive capability
7. Zero input current
8. True differential amplification
9. 1-8 true at all operating temperatures.
EO1
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List the ideal values of Op Amp parameters
Typical vs. Ideal operational amplifier values:
Typical 741 pin out showing the P/S connections.
EO2 Define the following terms:
Common mode rejection ratio (CMRR)
The measure of the ability of the Op Amp to reject signals that are
simultaneously present at both input signal terminals.
CMRR = 20 Log Ad/Ac
Ad = Differential Gain
Ac = Common mode gain
Expressed in db
Showing Differential Mode Input
Showing Common Mode Input
The Op Amp is a current device in, and a voltage device out. The Op Amp's output
is dependent on the current difference between the input pins. If you tied both pins
together and applied a very large signal to this connection: the output would be
unchanged, or at most, a weak replica of the input. This is known as common mode
rejection, (CMR).
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B. Gain-bandwidth-product (GBP)
The product of the closed-loop gain of the Op amp and it’s
corresponding closed-loop bandwidth.
C. Input offset voltage (Voi)- the voltage that must be applied to one of
the input terminals to give zero output voltage under no signal
conditions.
D. Inverting input – The input terminal to which the signal is applied and
produces a 180 degree phase shift at the output.
E. Non-inverting input – The input terminal to which the signal is
applied and produces no phase shift at the output.
F. Slew rate (SR) – The maximum rate of change of the output voltage
under large signal input conditions.
Slew rate (SR) – The maximum rate of change of the output
voltage under large signal input conditions.
One of the practical op-amp limitations is the rate at which the output voltage
can change. The limiting rate of change for a device is called its "slew rate".
The slew rate for the 741 is typically 0.5V/microsecond.
The indicated Slew rate amounts to 693.4 V/µs
EO3 Given a circuit diagram explain the basic operation of the
following Op-Amp configurations.
a. Inverting amplifier
b. Non-Inverting amplifier
c. Voltage follower
d. Summing Amplifier
e. Difference amplifier
f. Integrating amplifier
g. Differentiating amplifier
h. Comparator amplifier
i. Schmidtt trigger
j. Potentiometric amplifier
B. Basic Operation of the Inverting Amplifier
Circuit has Zin = Ra, Low Zo, and phase inversion.
Inverting amplifier
Since the resistance at the inverting terminal is very high, no current can
flow through the inverting input.
At the Rf / R1 junction the current in is the same as the current
out. Therefore the current is the same through R1 and Rf.
Kirchhoff I:
Substituting:
Rearranging:
Therefore:
I1 + I2 = 0
Vin/R1 + Vout/Rf = 0
Vin /R1 = - Vout/Rf
Vout /Vin = - Rf/R1
Inverting Amplifier
Input and Output Relationships
A. Basic Operation of the Non-Inverting Amplifier
Circuit has Hi Zin, Low Zo, and no phase inversion.
“Golden Rule” : In a feedback circuit the summing junction
Value is driven to equal the reference junction value.
1. By assuming that the two input terminals are at the same
potential and using Kirchhoff Current law for a series
circuit.
2. We can write the equation for current through Rf and Ra
as follows: Vout -Vin / Ra =Vin- 0/Rf
We can therefore write:
Vin = Vout __R1___
Rf + R1
Rearranging gives:
Vout = Rf + R1
Vin
R1
Therefore:
Vout = 1 + Rf
Vin
R1
C. Basic Operation of the Voltage Follower
Vout / Vin = 1
Circuit has Very Hi Zin, Low Zo, and no phase inversion.
Inverting amplifier
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SinceTypically a buffer amplifier is used to transfer a voltage from the first
circuit, having a high output impedance level, to a second circuit with a low
input impedance level. The interposed buffer amplifier prevents the second
circuit from loading the first circuit unacceptably and interfering with its
desired operation.
•
If the voltage is transferred unchanged (the voltage gain is 1), the amplifier is a
unity gain buffer; also known as a voltage follower.
•
Although the voltage gain of a buffer amplifier may be (approximately) unity, it
usually provides considerable current gain and thus power gain. However, it is
commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to
the voltage gain.
D. Basic Operation of the Summing amplifier
Summer with gain
E. Basic operation of the Difference Amplifier
Eo = -R3/R1 e1 + (1+R3/R1)(R4/R2+R4) e2
Eo = e2 - e1
provided r1=r2=r3=r4
F. Integrating Amplifier
Integrating Amplifier
In its simplest form the Integrating Amplifier has part of its’
input offset current charging the feedback capacitor, this
can result in a constantly changing output even with no
input called “integrator drift”.
R2 is made large compared to Xc over the desired
frequency range. The gain of the circuit to dc is limited to –
R2/R1 thereby preventing drift into saturation which would
occur if R2 were not present.
Integrating Amplifier
R2 limits low frequency gain.
Prevents Dc offset voltage from being integrated.
Prevents Dc offset voltage from eventually saturating the
amplifier.
R3 limits the input bias current.
Input bias current causes dc offset voltage.
R3 = R1R2/R1+R2 If Fo>Fc =1/6.28R2
If Fo<Fc Av = -Vo/Vin=-(R2/R1)
R1*C = 1/f
G. Differentiating Amplifier
Differentiating Amplifier
A differentiator has a linear increase in gain with
frequency, this characteristic can give poor noise
performance.
Since circuit gain increases with frequency, high
frequency noise is amplified more than low frequency
noise. If the output bandwidth is high very high noise levels
may be produced.
By choosing Xc1>> R1 and Xc2>>R2 the op amp will
differentiate over the frequency range of interest, T=C1R2.
At higher frequencies Xc1 and Xc2 are small compared to
R1&R2 the gain becomes C2/r1 approaches 0.
Differentiating Amplifier
Vo = -R2C dVi/dVt
Fo<Fc (vi/vt)
Differrentiates only below the cutoff frequency
Xc=1/6.28FC as FXc untill Xc approaches 0.
R1 decreases high frequency gain by the ratio of R2/R1
differentiates below cutoff freq (Fc)
Fc=6.28R1 C and tc = R2 C =1/Fo
when Fo>Fc Vo/Vin=-(R2/R1)
H. Comparator Amplifier
A comparator circuit compares two voltage signals and determines which
one is greater. The result of this comparison is indicated by the output
voltage: if the op-amp's output is saturated in the positive direction, the
noninverting input (+) is a greater, or more positive, voltage than the inverting
input (-), all voltages measured with respect to ground. If the op-amp's voltage
is near the negative supply voltage (in this case, 0 volts, or ground potential),
it means the inverting input (-) has a greater voltage applied to it than the
noninverting input (+).
H. Comparator Amplifier
The non-inverting input is at 2.5 V -- we'll call this the threshold
voltage. Whenever the input voltage is below the threshold voltage, the
output is high. Since the output is effectively an open circuit, no current flows,
so there is no voltage across R3, and the output voltage is 5 volts. When the
input is above the threshold, the output is low. This is illustrated in the
diagram below.
I. Schmidt trigger
The schematic for a Schmitt trigger is given above. When the output is
high, the threshold voltage will be 2.5 volts, and when it is low, the
threshold voltage is 1.67 (=5/3) volts (figure it out). This creates two
separate thresholds. If we apply the same input to this new circuit, we now
get one transition of the output, because of the changing threshold
voltage. (When the output is high, the threshold is high -- when the output
is low, the threshold is low). Note that the output now only goes up to 2.5
volts.
J. Potentiometric Amplifier
Potentiometric Amplifier
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This is a special configuration of Inverting amplifier.
Used when Rin needs to be Hi and gain needs to be Hi.
Solves problems with noise and frequency response.
Allows use of smaller Rfb.
Voltage gain Av (Rfb>R1) Vo/Vin =Rfb/Rin[(R1/R2+1]
Potentiometer provides variable amount of signal feedback.