1 Table of Contents

2 Introduction.........................................................................................................................................2

3 Theory:................................................................................................................................................3

3.1 IC Operational Amplifiers.............................................................................................................3

3.2 Negative Feedback Control..........................................................................................................4

3.3 Op-Amp Connected as a Summer................................................................................................5

3.4 Op-Amp Connected as a comparator..........................................................................................6

4 Task Brief.............................................................................................................................................7

4.1 Negative feedback.......................................................................................................................7

4.2 Comparator (Non Zero Detector)................................................................................................9

4.3 Summing Amplifier....................................................................................................................12

5 Troubleshooting................................................................................................................................14

6 Reference..........................................................................................................................................15

1

2 Introduction

The term operational amplifier, abbreviated op amp, was coined in the 1940s to refer to

a special kind of amplifier that, by proper selection of external components, can be configured to

perform a variety of mathematical operations. Early op amps were made from vacuum tubes

consuming lots of space and energy. Later op amps were made smaller by implementing them

with discrete transistors. Today, op amps are monolithic integrated circuits, highly efficient and

cost effective.

For this assignment I are requires to design and assemble on prototype board of the

following Op-amp circuit (Negative feedback, Comparator, Summing) according to given

specification. And also test their functionality by using appropriate test equipment to confirm that

they meet required specifications.

2

3 Theory:

3.1 IC Operational Amplifiers

Op-amp is a high-gain, direct-coupled differential linear amplifier whose characteristic are

extremely controlled by negative feedback from output to the input. It is widely used in analogue

computers. It also can perform mathematical operations such as summing, integration, and

differentiation and also used as video and audio amplifiers, oscillators, etc, in communication

electronics both in digital and linear circuit.

Figure 2.1 shows the symbol for an op-amp. Note that the operational amplifier, such as

differential amplifier has 2 inputs and marked with (-) and (+). The minus is the inverting input

and a signal applied to it will be shifted in phase 180o at the output. The plus is the non-inverting

input and the signal applied to it will appear in the same phase at the output as the input.

Because of the complexity of the terminal circuitry of an op-amp, the op-amp symbol is used

exclusively in circuit diagram.

Figure 2-1: Symbol for operational amplifier

3

3.2 Negative Feedback ControlThe basic circuit was shown in Figure 2-2, including the negative feedback loop of an op-amp.

Note that the output is feedback to the inverting input terminal in order to provide negative

feedback for the amplifier. The inverting input in Figure 2-2 is applied to the input signal. As a

result, the output will be inverted. It is possible to operate the op-amp as a non-inverting

amplifier by applying the signal to the plus input as in Figure 2-3. In this circuit the feedback

network still connected to the inverting input.

Figure 2-2: Circuit of op-amp showing the negative feedback loop

Figure 2-3: Op-amp operated as non-inverting amplifier

The output of the amplifier is defined by equation (2-1)

Vout=-RF/RR . VIN (2-1)

The minus sign indicates that the sign of the output is inverted as compared to the input.

Equation for the gain of this amplifier is

Gain=-RF/RR (2-2)

For the non-inverting amplifier in Figure 2-3

VOUT= (1+RF/RR)VIN (2-3)

And its gain

Gain= (1+RF/RR) (2-4)

4

3.3 Op-Amp Connected as a Summer

In the circuit of Figure 2-4, the op-amp is connected as an algebraic summer. It can be seen

that if RF=R1=R2, the voltage output

VO=-(V1+V2) (2-5)

That is, the output voltage is the sum of input voltages with the sign inverted. If the input

voltages are opposite sign, the circuit of Figure 2-4 acts to invert and subtract the input voltages.

For example, if V1=+3V and V2=-2.5V, then

VO=-(+3-2.5) =-0.5

The output voltage can be made equal to the sum of a desired ratio of input voltages, depending

on the values of RF, R1 and R2. For example, if RF=2R1=3R2, then

VO=-(RF/RR. V1 + RF/RR . V2) =-(2V1+3V2) (2-6)

Figure 2-4: Operational amplifier connected as a summer

The summer of Figure 2-4 can be modified to provide three or more inputs.

5

3.4 Op-Amp Connected as a comparator

Figure 2-5: Operational amplifier connected as a comparator

The Zero-level detector can be modified using a voltage divider to set the reference voltage. VREF as follow:

Vref= R2R1+R2

(vin)

Circuit Operation

Where +V is the positive op-amp dc supply voltage. As long as Vin is less than VREF, the output remains at the minimum negative level. When the input voltage exceeds the reference voltage, the output goes to its maximum positive voltage.

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4 Task Brief

4.1 Negative feedback

Circuit no. 1

Description Specification Circuit type Negative Feedback Circuit configuration Non InvertingInput voltage (Vin) 1V Vpk-pk ~2V pk-pk at 1khzFeedback Network To be selected appropriatelyClosed loop Gain Acl 5~10

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Result of the circuit

Vin(Vpk- Vpk) Vout(measured) Vout ACL & Vin (calculated)

2.0 23.2 22.02.5 28.4 27.53.0 33.8 33.0

Calculation

VOUT= (1+RF/RR)VIN

V out=1+ 10K1K

(1 )=11 x2=22v

V out=1+ 10K1K

(1.25 )=12.5 x2=25V

V out=1+ 10K1K

(1.5 )=15 X 2=30V

And its gain

Gain= (1+RF/RR)

Gain=(1+ 10k1k )=11

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4.2 Comparator (Non Zero Detector)

Description Specification Circuit type Comparator Circuit configuration Voltage Divider ReferenceInput voltage (Vin) 15Vpk-pk ~20V pk-pk at 1khzReference Voltage (Vref) 3.5 V / 5 V / 1.5 V

Circuit no.2

Result

9

R1 R2 Vout=

R2R1+R2

(vcc)

3.3 k 1 k 3.5 v2 k 1 k 5 v

3.3 k 2 k 5.7 v

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Circuit Operation

Where +V is the positive op-amp dc supply voltage. As long as Vin is less than VREF, the output remains at the minimum negative level. When the input voltage exceeds the reference voltage, the output goes to its maximum positive voltage.

Conclusion

In op-amp comparator, when the input voltage exceeds a specified reference voltage, the output change state. Then the hystereses give op-amp noise immunity. A comparator switches to one state when the input drops below the lower trigger point (LTP)

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4.3 Summing Amplifier

Description Specification Circuit type Summing Circuit configuration Inverting SummingInput voltage (Vin) Vin1 = +1.0V ~ 5V

Vin2 = +3.0V ~ 8VClosed loop Gain Acl To be selected

V1 V2 Theoretical Vout Measured Vout

1v 1v -2V -2.2

1v 2v -3V -3.1

Circuit no. 3

Result

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Calculation

A general expression for a unity-gain summing amplifier with n input where all resistor are equal in value.

Vout = ( Vin1 + Vin2 +Vin3+---------+Vinn)

Vout = -(1v+1v) = -2v

Vout = -(1v+2v)= -3V

Conclusion

-The output voltage of summing amplifier is proportional to the sum of the input voltages.

-An averaging amplifier is a summing amplifier with a closed-loop gain equal to the reciprocal of the number of inputs.

-In a scaling adder, different weight can be assigned to each input, thus making the input contribute more or contribute less to the output.

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5 Troubleshooting

Although integrated op-amp are extremely reliable and trouble-free, failures do occur from time to time. One type of internal failure mode is a condition where the op-amp output is in a saturated state resulting in constant high or constant low level, regardless of the input. Also, external component failures will produce various types of failures modes in op-amp circuit.

Fault in the non inverting amplifier

Open feedback Resistor

If the feedback resistor, Rf opens, the op-amp is operating with its very high open-loop gain, which cause the input signal o drive the device into severely clipped output signal.

Open or Incorrect Adjusted Offset Null Potentiometer

In this situation, the output offset voltage will cause the out signal to begin clipping on only one peak as the input signal is increased to sufficient amplitude.

Faulty Op-Amp

As mentioned, many things can happen to an op-amp. In general, an internal failure will result in a loss or distortion of the output signal. The best approach is to first make sure that there are no external failures or faulty condition.

External component failures in comparator circuit

-In addition to failure of the op-amp itself, a zener diode or one of the resistors could be faulty. For example, suppose one of the zener diodes opens. The effectively eliminates both zeners and the circuit operates as an unbounded comparators. With a shorted diode, the output is limited to the zener voltage(bounded) only in one direction depending on which diode remains operational. In the other direction, the output is held at the forward diode voltage.

-The resistance R2 is open. Essentially all of the output voltage is fed back to the non inverting (+) input and since the input voltage will never exceed the output, the device will remain in one of its saturated state. This symptom can also indicate a faulty op-amp, as mentioned before. Now assume that R1 opens. This leaves the non inverting input near ground potential and causes the circuit to operate as a zero-level detector.

Component failures in summing amplifiers

-One of the input resistors in a unity-gain summing amplifier opens; the output will be less than the normal value by the amount of the voltage applied to the open input. State another way, the output will be the sum of the remaining input voltages.

-An open input resistor cause the output to be less than normal by an amount equal to the gain times the voltage at the open input. In another situation, for example averaging amplifier, an open in input resistor will result in an output voltage that is the average of all the inputs with the

open input averaged in as a zero.

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6 Reference

THOMAS L.FLOYD/electronic device/fith editon

Sergio Franco. Design with Operational Amplifiers and Analog IntegratedCircuits. McGraw-Hill, Inc., 1988.

Lecture notes

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