7.instrumentation amplifier

OPAMP IC 741 INSTRUMENTATION AMPLIFIER DESIGN

1. Instrumentation Amplifier
Prof. Satheesh MB
Indian Navy

For passive transducer measurement

2. Instrumentation Amplifier...
Prof. Satheesh MB
Indian Navy

For active transducer measurement

3. Instrumentation Amplifier...
Prof. Satheesh MB
Indian Navy

Differential Amplifier:(Single op-amp instrumentation amplifier)
To obtain v
o
in terms of v
1
and v
2
use superposition theorem

4. Instrumentation Amplifier...
Prof. Satheesh MB
Indian Navy

5. Instrumentation Amplifier...
Prof. Satheesh MB
Indian Navy
External Circuit Instrumentation Amplifier

6. Instrumentation Amplifier...
Prof. Satheesh MB
Indian Navy
Three Op Amp Instrumentation Amplifier:
CMRR and Z
in
are very important attributes of an IA
Can increase Z
in
of difference amplifier configuration by adding unity gain buffers
or buffers with gain

7. Instrumentation Amplifier...
Prof. Satheesh MB
Indian Navy
Three Op Amp Instrumentation Amplifier:
CMRR and Z
in
are very important attributes of an IA
Can increase Z
in
of difference amplifier configuration by adding buffers
Common mode signals are not amplified if common R
1
is used and
connection to ground is removed.

8. INSTRUMENTATION AMPLIFIER...
Prof. Satheesh MB
Indian Navy

9. INSTRUMENTATION AMPLIFIER...
Prof. Satheesh MB
Indian Navy

Some applications, such as an oscilloscope input, require
differential amplification with extremely high input resistance.

A3 is a standard difference op-amp with differential gain R2/R1

A1 and A2 are additional op-amps with extremely high input
resistances at v1 and v2 (input currents = 0)
•Differential gain of input section:
–Due to the virtual shorts at the input of A1 and A2, we can write i
A
= (v2 – v1) /R
A
–Also, i
A
flows through the two R
B
resistors, allowing us to write v
02 – v
01 = i
A(R
A + 2 R
B)
–Combining these two equations with the gain of the A3 stage, we can obtain
v
OUT
= (R2/R1)(1 + [2R
B
/R
A
])(v1 – v2)
•By adjusting the resistor R
A
, we can adjust the gain of this instrumentation
amplifier

10. Applications
Prof. Satheesh MB
Indian Navy

Audio amplifiers
Speakers and microphone circuits in cell phones,
computers, mpg players, etc.

Instrumentation amplifiers
Biomedical systems including heart monitors and
oxygen sensors.

Power amplifiers
Analog computers
Combination of integrators, differentiators, summing
amplifiers, and multipliers

11. IC 741 – CASCADED AMPLIFIER
Prof. Satheesh MB
Indian Navy

Connecting several amplifiers in cascade (output of one stage connected
to the input of the next) can meet design specifications not met by a single
amplifier.
 Each amplifier stage is built using an op amp with parameters A, R
id
, R
o
,
called open loop parameters, that describe the op amp with no external
elements.
 A
v
, R
in
, R
out
are closed loop parameters that can be used to describe each
closed-loop op amp stage with its feedback network, as well as the overall
composite (cascaded) amplifier.

12. TRANSFER CHARACTERISTICS
Prof. Satheesh MB
Indian Navy

13. IC 741 – CIRCUIT ARCHITECTURE
Prof. Satheesh MB
Indian Navy
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