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

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. R1R1 R2R2 R3R3 RFRF RFRF RFRF V in V out

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.

Example Let R F = 470 k , R 1 = 4.3 k , R 2 = 33 k , R 3 = 33 k . Calculate the output voltage for an input of 80  V. The amplifier gain Therefore, the output voltage

Exercise 18 k  V in V out 510 k  22 k  680 k  33 k  750 k  20  V Calculate the output voltage in this circuit.

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.

Voltage-Controlled Voltage Source V in V out + + _ _ kV in R1R1 RFRF V out V in R1R1 RCRC RFRF V out V in

Voltage-Controlled Current Source V in + _ kV in I out R1R1 RLRL V out V in I out I1I1

Current-Controlled Voltage Source V out RLRL I1I1 I out V out I1I1 kI 1 + -

Current-Controlled Current Source I1I1 I out RLRL R1R1 R2R2 I2I2 I1I1 I1I1 kI 1

INSTRUMENTATION AMPLIFIER An instrumentation amplifier 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.

Basic Instrumentation amplifier model V2V2 V1V1 (+) input V out R R R R’ R R aRaR (-) input A1 A2 A3

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

V 1 is applied to the (+) input and V 2 to the (-) input. V out is proportional to the difference between input voltages. Characteristics of IA :  Voltage gain, from differential input (V 1 – V 2 ) 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.  V out does not depend on the voltage common to both V 1 and V 2 (common-mode voltage), only on their difference.

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

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.

V out = V ref + m(V 1 - V 2 )  Assume that V 1 and V 2 are set equal to 0 V.  The outputs of A1 and A2 will equal 0 V. Thus, the inputs of A3 is 0V.  V ref 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 V out equals V ref R = 10 k  R RLRL V 2 =0 V V 1 =0 V V ref = 0 to 10 V V out = V ref for V 1 = V 2 = 0 V ref /2

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