1. CONSTANT-GAIN MULTIPLIER CONTROLLED SOURCES INSTRUMENTATION AMPLIFIER
OP-AMP APPLICATIONS
CONSTANT-GAIN MULTIPLIER
CONTROLLED SOURCES
INSTRUMENTATION AMPLIFIER

2. Constant-Gain Multiplier
Multiple-stage gains
When a number of stages are connected in series the overall gain is the product of the individual stage gains. R1 R2 R3 RF
Vin
Vout

3. Constant-Gain Multiplier
The first stage is connected to provide non-inverting gain.
The next two stages provide an inverting gain.
Therefore, the overall circuit gain is then non-inverting.

4. Constant-Gain Multiplier
Example
Let RF = 470 k, R1 = 4.3 k, R2 = 33 k, R3 = 33 k. Calculate the output voltage for an input of 80V.
The amplifier gain
Therefore, the output voltage

5. Constant-Gain Multiplier
Exercise
Calculate the output voltage in this circuit?
10 k
Vin
Vout
500 k
20 k
200 k
30 k
600 k
100 V

6. Controlled Sources
Op-amp can be used to form various types of controlled sources.
An input voltage can be used to control an output voltage or current.
An input current can be used to control an output voltage or current.

7. Voltage-Controlled Voltage SourceRF
Vout
Vin R1 RC RF
Vout
Vin
Vin
Vout + _
kVin

8. Voltage-Controlled Current SourceRL
Vout
Vin
Iout I1
Vin + _
kVin
Iout

9. Current-Controlled Voltage Source
Vout RL I1
Iout
Vout I1
kI1 + -

10. Current-Controlled Current SourceI1
Iout RL R1 R2 I2
Iout I1
kI1

11. INSTRUMENTATION AMPLIFIER
An instrumentation amplifier (IA) is a differential voltage-gain device that amplifies the difference between two voltages existing at its two input terminals.
It has a large voltage gain, a high CMRR, low input offsets, low temperature drift, and high input impedance.
Found in at least one in every data acquisition unit.
Basic Instrumentation Amplifier (IA) is made from three op-amps and seven resistors.
Simply by connecting a buffered amplifier to a basic differential amplifier.

12. Basic Instrumentation amplifier modelV2 V1
(+) input
Vout R R’ aR
(-) input A1 A2 A3
Vout= V1 - V2

13. INSTRUMENTATION AMPLIFIER
How it works?
Op-amp A3 and its four equal resistors, R, form a differential amplifier with a gain of 1. A3 resistors have to be matched.
R’ : variable value to balance out any common-mode voltage.
aR : to set the gain according to this equation
where

14. INSTRUMENTATION AMPLIFIER
How it works? (cont.)
V1 is applied to the (+) input and V2 to the (-) input.
Vout is proportional to the difference between input voltages.
Characteristics of IA :
Voltage gain, from differential input (V1 – V2) to single-ended output, is set by one resistor.
The input resistance of both inputs is very high and does not change as the gain is varied.
Vout does not depend on the voltage common to both V1 and V2 (common-mode voltage), only on their difference.

15. Example INSTRUMENTATION AMPLIFIER
By referring to figure IA, calculate the voltage gain, if R = 25 k and aR = 50 .

16. Referencing Output Voltage
To offset the output voltage to a reference level other than 0V, can be achieved by adding a reference voltage in series with one resistor of the basic differential amplifier.

17. Referencing Output Voltage
R = 10 k R RL
V2=0 V
V1=0 V
Vref= 0 to 10 V
Vout = Vref for V1 = V2 = 0
Vref/2
Vout= Vref + m(V1 - V2)
Assume that V1 and V2 are set equal to 0 V.
The outputs of A1 and A2 will equal 0 V. Thus, the inputs of A3 is 0V.
Vref is divided by 2 and applied to the A3 op-amp’s (+) input. Then the non-inverting amplifier gives a gain of 2 so that Vout equals Vref

18. Referencing Output Voltage
Vout can be set to any desired offset value by adjusting Vref.
In practice, Vref is the output of a voltage-follower circuit.
5 k +V
10 k pot
Vref
To reference terminal