Operational Amplifiers

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```					            Operational Amplifiers

What is an Op amp?
A multistage high-gain amplifier integrated in analysis as a
separate block.
 The input of an op amp is a differential amplifier therefore
has 2 inputs.
 The output is singled ended.
 Typically configured for a dual power supply rails (+/_V)
Op Amps (differential input)

Differential Amplifier considerations :
2 NPN, 2 PNP or 2 FETs (matched pairs)
Inherent symmetry
Current stability
Input impedance
Beta
Op amps (differential input)

D.C. considerations of Diff Amp:
 Emitter type bias
 Total current ( I T )
 Input offset current and voltage
Will both transistors have identical characteristics?
- Differences in junction characteristics will develop
variations in base and collector currents. Because the diff
amp amplifies the difference potential it sees between the
two inputs so too will these unwanted differences be
amplified.
Op Amps (differential input)

The D.C. performance of an op amp can be measured.
 Input offset current (difference between both base
currents)
 Output offset current )difference between two collector
currents)
 Output offset voltage (difference between two collector
voltages)
 The amount of voltage required to offset the base voltage
difference is called the input offset voltage.
 Ideal op amp will have all three at 0.
+Vcc

IC2
IC1          RC1           RC2

Q1                  Q2
RB1                                     RB2

IB1-->                                <--IB2

IBT           RT

-VEE
Op Amps (differential amplifiers)

A.C. CONSIDERATIONS
Four signal circuit configurations:
1. Differential input, differential output
2. Differential input, single-ended output (most common)
3. Single-ended input, differential output
4. Single-ended input, single-ended output
Op Amps (differential amplifier)

A.C. CONSIDERATIONS
Assume a single-ended input and single-ended output:
 The voltage output of the Q2 collector is in phase with the
Q1 input.
 The voltage output of Q1 is out of phase with the Q1 input.
rc
Av 
2re'  rE 
 The voltage seen across the two collectors will be 180
degrees apart. If we take the differential output we observe
a doubling of the signal.
rc
AV 
(re'  rE )
+Vcc

RC1               RC2
Vout 2
Vout 1
Q2
Q1

S                                                S
RB1

RB2
Vin2
G
G
IBT            RT

-VEE
RC1

RC2

re1
S

G                     re2

rE1   rE2

Rt
Op Amps (differential amplifier)

COMMON MODE
In phase signals are common-mode signals
Ideal op amp will suppress common-mode signals entirely
with a common mode gain of 0
A real op amp will not display perfect common mode
rejection
Dominant causes of error are:
1. Resistor tolerance variations.
2. Transistor parameter deviation..
Op Amps (differential amplifier)

A.C. CONSIDERATIONS
Assume a single-ended input and a single ended output:
 Voltages at both inputs that are in phase will develop out
of phase collector outputs that when added will be 0. (see
diagram)
 In summary, a differential amplifier suppresses in phase
input signals and elevates out of phase input signals.
Op Amps

Common applications:
• Inverting amplifier
• Non inverting amplifier
• Comparators
• Voltage level detectors
• Instrumentation amplifiers.
+V

R2
Q3

R5
Q1             Q2
D1
Vout
Vin-                                      D2
C1

Q5
Vin+
Q4
R4

R3

R1

-V
+Vcc

7

1

5
Vin-   2   -
6   Vout

Vin+   3
+
4

-VEE
Op Amps

Attributes of an ideal op amp:
Infinite gain
Unlimited bandwidth
Infinite input impedance
Very little output impedance
Op Amps

Open loop versus closed loop configuration:
=>Open loop has infinite gain (no feedback) therefore it is
suitable for comparator or voltage detection applications.
Why? Output can drive to a desired saturation level
depending on events observed at the input.
=> Closed loop uses negative feedback to control gain at the
cost of bandwidth.
Inverting Amplifier
Rf

VCC

7

1

5
Rin
2
-                   RL
6

S             3
+
G

4
-VEE
Non Inverting Amplifier
Rf

VCC

7

1

5
Rin
2   -                   RL
6

3   +
4

S
-VEE
G

Virtual ground

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