Electronic Instrumentation
Basic Circuits with OA
* In this presentation definitions and examples
from Wikipedia, HowStaffWorks and some other sources
were used
Lecturer: Dr. Samuel Kosolapov
Items to be
defined/refreshed/discussed
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Basic types of Unipolar Amplifiers
Models of Bipolar OA
OA741
Problems with Direct connection
Importance of Feedback
Basic circuits with OA
Limitations of “simple formulae”
Second Order Effects
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Basic Types of Unipolar Amplifiers
Amplifier Type
Voltage
VCVS
(Voltage Controlled
Voltage Source)
Current
ICIS
(Current Controlled
Current Source)
Circuit Gain
Specification
Voltage Gain
AV 
Schematics
EWB
(Sources panel)
Voutput
Vinput
[Dimensionless]
Current Gain
AI 
I output
I input
[Dimensionless]
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Basic Types of Unipolar Amplifiers
Amplifier Type
Transconductance
VCIS
(Voltage Controlled
Current Source)
Transresistance
ICVS
(Current
Controlled Voltage
Source)
Circuit Gain
Specification
Transconductance
Gain
Gm 
Schematics
EWB
(Sources panel)
I output
Vinput
[siemens =mho]
Transresistance
Gain
Rm 
Voutput
I input
[ohms]
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Simple Model of Ideal Bipolar Amplifier (OA)
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Ideal Amplifier  Practical Amplifier
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OA 741
7
1
5
U3
3
6
2
4
Parameter
Name
Voltage
Gain
Input
Resistance
Output
Resistance
Symbol
Av
Rin
Rout
741
Example
(LM741)
[Dimensionless] 50,000-200,000
(Adiff)
[Ohm]
0.4 – 2 M
Units
[Ohm]
100Not
specified
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PSpice Model of OA
Av, Rinput, Routput
And OTHER parameters
Can be edited
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Direct connection of Practical Voltage Amplifier:
(Demonstration of parameters importance)
Signal
Source
(Sensor)
Load
Practical
(Actuator)
Voltage Amplifier
Sensor is characterized by:
Voltage of source of signal
Vsignal(t)
Internal resistance of signal source Rsignal
Practical Voltage Amplifier
is characterized by:
Input resistance Rin,
Output resistance Rout,
Voltage gain
Av.
Actuator is characterized by:
Load resistance Rload
9
Direct connection of Practical Voltage Amplifier:
Replacement step:
Signal
Source
(Sensor)
Load
Practical
(Actuator)
Voltage Amplifier
Goal: Vout(t) > Vsignal(t).
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Manual Calculations
(Two Voltage Dividers)




RIN
RLOAD



 AV VIN ;
VIN  
VSIGNAL ; VOUT  

 RSIGNAL  RIN 
 ROUT  RLOAD 



RIN
RLOAD



 AV VSIGNAL
VOUT  


 RSIGNAL  RIN   ROUT  RLOAD 
System' s Voltage Gain 

VOUT
RIN
 
VSIGNAL  RSIGNAL  RIN


RLOAD
 
 AV
  ROUT  RLOAD 
If RIN >> RSIGNAL and RLOAD>>ROUT then System’s Voltage Gain = AV
Otherwise we do not want to use this system!!!
Use ALGEBRA, instead of numeric calculations to get PROFESSIONAL results
(Explain).
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Problem with Direct Connection
System’s gain depends on:
amplifier’s parameters,
voltage source’s parameters,
and load’s parameters.
 We have Input and Output LOADING Effect
 Bad Design Practice.
because if we’ll change one of the blocks in mass production,
system’s parameters may change in a drastic way.
For example:
Amplifier’s voltage gain of LM741 in mass production may be in range:
50,000 – 200,000! (Explain Why : Highest Semiconductor purity required)
Practically, amplifiers are used with FEEDBACK
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Feedback: Converting Practical Amplifier to Ideal Amplifier
Sensor
Actuator
Feedback
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Important Result
(to be achieved after a hard work)
In the real practical situation, the RESULT will be:
VLOAD
 R2 
R2
 1  VSIGNAL  System' sVoltageGain  1 
R1 
R1

!!! System’s Voltage Gain is defined only by feedback resistors R1 and R2 !!!
(This means that System’s Voltage Gain is INDEPENDENT on
parameters of Source, Amplifier and Load!)
 Art of Electronics: Practical Amplifier Behaves as IDEAL AMPLIFIER
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How to prove this Important Result
To proof this (to solve the problem) we must:
1. Replace sensor, uA741 and actuator to (linear) equivalent circuits
2. Write node’s equations
3. Solve node’s equations (to find VLOAD)
4. Analyze result (and get System’s Voltage Gain)
We must provide SYMBOLIC result.
Numeric result is not good here! (Explain why)
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3
+
V+
U2
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Buffer: Equivalent Circuit (remind usage)
Vin
-
V-
OUT
2
OS1
5
6
Vout
1
Vin
1.000*Vin
Vout==Vin
uA741
4
0
OS2
Buffer Usage:
Convert Practical Voltage amplifier
to Ideal Voltage Amplifier.
Practically: Insert buffer
BETWEEN sensor and amplifier input
and/or
BETWEEN amplifier output and actuator
Rin = infinity
Rout = 0;
Voltage Gain = 1.000
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OA 741. Minimal Set of Parameters
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Basic OA Configurations: Powering
Q: How to connect
Laboratory Power
supply to OA ?
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Basic OA Configurations
Discuss Limitations of this “Fast Evaluation” formulae 
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Simulation usage Example: “Fast Evaluation” is OK
R1 = 1 k; R2 = 10 k;“Expected” Gain = -R2/R1 = -10
Vin = 200 mV pp; Vout = 2 V pp; “Measured” Gain is -10
 “Simple formula” can be applied here
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Simulation usage Example: Problem with Fast Evaluation
R1 = 10 ; R2 = 10 k;“Expected” Gain = -R2/R1” = -1000
Vin = 2 mV pp; Vout is ~1.4 Vpp .  Gain ~ 700 < 1000
+ Some strange phase shift is seen here
 “simple formula” can NOT be applied in this case (Low Accuracy)
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Simulation usage Example: Problem with Fast Evaluation
R1 = 1 ; R2 = 10 ;“Expected” Gain = -R2/R1” = -10
Vin = 200 mV pp; Vout is Heavily Distorted here.
Linear Gain can not be evaluated (Nonlinear case !!!)
 “simple formula” can NOT be applied in this case
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Limitations of Electronic Simulation Software
Q1: What model is used “inside the simulation” ?
A1: Sometimes Beginner/Student cannot know “exactly”
Q2: Simulation result: Gain is OK (== +2). Will You sign that the circuit is OK ?
A2: No !!! Power supply is not connected
(Small letters: Not a bug, Power Supply is connected inside as 741 model default)
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Basic OA Configurations: Example of Usage
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Basic OA Configurations
(Trans-conductance: Voltage to Current Converter)
Here: Gm = 1/R2
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Basic OA Configurations
(Trans-impedance / Trans-resistance) :
Current to Voltage Converter
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Basic OA Configurations
Example of Usage
Explain “optical” wireless communication system
Based on this circuits
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Basic OA Configurations
Example of Usage
Explain need for capacitors
Explain importance for Arduino usage
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Basic OA Configurations
Example of Usage
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Basic OA Configurations
Example of Usage: Analog  Digital ( - Mechanics)
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Basic OA Configurations
And many other Examples of OA Usage:
OA 741 (Amplifier) can be used to build many useful circuits.
This is why most of this introductory course is dedicated to Amplifiers
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Second order Effects  741 Models limitations
Frequency Effects
In order to ensure
stability of OA 741
Its frequency response
changed in such a manner
that
Voltage Gain
is not very high
for the high frequencies
(details: Later + course
Controlled Systems)
Practically: usage of OA741 is
limited to AUDIO signals
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Second order Effects  741 Models limitations
Input Offset Voltage and Current
Q1: How to modify Model / Equivalent circuit to take into account “offset” effects ?
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Second order Effects  741 Models limitations
Power Supply Effects
Important to understand:
In the range of Vcc ~4 - 18 V
Gain is changing,
But still is VERY LARGE
 Gain of the basic circuits
with feedback will not change
The magnitude of the input voltage must never exceed the magnitude of the
supply voltage or 15 V whichever is less
 Use voltage supply 12 V max
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Second order Effects : Input Voltage Limitations
The magnitude of the input voltage must never exceed the magnitude of the
supply voltage or 15 V whichever is less
 Use voltage supply 12 V max
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Second order Effects
Input Voltage Limitations: Output Voltage Clipping
Q1: Output Voltage is distorted (clipped) here. Why ?
Q2: What will happen if SIN is input ?
Beware: Not every EWB model “clips” (restricts) output voltage
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Control Questions
• What have I learned ?
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Literature to read
1. TBD
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