OPERATIONAL AMPLIFIERS

Course P3 CIRCUITS CONTAINING

OPERATIONAL AMPLIFIERS Practical 2

1/3 PRACTICAL WORK PURPOSES :

An operational amplifier is an electronic component used in many applications. During this practical, you will achieve some of them, and you will study its two operating states: the linear state and the saturated state. The applications studied are:

 Amplify an electronic signal by means of an amplifier circuit.  Design a lamplighter circuit.

Operational behaviour of an operational amplifier (O.A.) : An O.A. is a voltage amplifier.

An operating O.A. requires two steady supplies U+

sup = +10 V and

U-sup = -10 V. Once the supplies connected, voltages may be applied to

the non-inverting input (U+) or to the inverting input (U-). The design

of the connecting boards available in the lab achieves a permanent connection of the supplies. You just have to switch the +10 V and -10 V supplies on.

 The O.A. will be assumed ideal. In this model: i+ = i- = 0

 Except for the simple voltage comparator (see II), the O.A. circuits used display a feedback loop. This loop consists of a resistor connected between the output of the O.A. and one of its inputs.

 The input resistance of an ideal O.A. is infinite (input currents are zero) and the output

resistance is zero.

A reminder: the input resistance Re of a device is not a component, but the resistance equivalent to the whole circuit making up the device.

 The operational behaviour below will be assumed :



 = V+-V- = 0. The output voltage U





state.

I AMPLIFIER CIRCUIT The following devices are available :

1 function generator and 1 oscilloscope 1 O.A., assumed ideal, and its DC supplies 1 resistor 2.2 kΩ

1 resistor 1 kΩ 1 resistor 10 kΩ

The purpose in this part is to study an inverting voltage amplifier circuit. The diagram of the circuit is sketched above (the -10 V and +10 V supplies are not shown).

PQ1 Assume that the O.A. is ideal. What are the values of the input currents i+ and i-? Apply the loop rule to loops (M1,E,E-,E+,M1) and (M1,M2,S,E-,E+,M1).

Express then the ratio us/ue in terms of R1, R2. Under what condition on R1 and R2 does this circuit make up an

inverting amplifier?

Three resistances are available. Choose two convenient ones among them and carry out the numerical calculation.

Course P3 Practical 2

2/3

PQ2 Design an inverting amplifier circuit (including a generator), made up with the available devices above. An

oscilloscope is designed to be wired as a Latis-Pro logger. Show then how its channels 1 and 2 should be connected in order to display ue and us.

PQ3 The circuit above is supplied by a voltage ue whose shape is a sinus. Find in terms R1, R2 and Usat the expressions for

ue above which the O.A. operates in its saturated state?

Among the five chronograms below, pick the ones fitting the two following operations of the inverting amplifier: • the magnitude of ue does not reach the saturation limit; • the magnitude of ue exceeds this limiting voltage.

chronogram 1 chronogram 2 chronogram 3

chronogram 4 chronogram 5

EQ1 Wire the circuit above choosing resistors R1 and R2 in order to achieve an inverting amplifier.

Set the generator in order to provide a voltage ue of sinus shape and frequency between 100 Hz and 200 Hz. Adjust the magnitude of the input signal in order to operate the O.A. permanently in its linear mode. Measure ue and us at a given time.

Deduce then the value of amplification us/ue in linear mode. Compare with the theoretical value.

EQ2 Adjust now the magnitude of the input signal in order to operate the O.A. in its saturated mode.

Measure the saturation voltages U+_{sat and U}-_{sat. Is |U}+

sat | = |U-sat| valid?

EQ3 Using two resistors chosen among the three available, design another inverting amplifier.

What will be its amplification?

the limiting input voltage if it is intended to operate the O.A. permanently in its linear mode? Sketch the connections of the oscilloscope used to measure the magnitude of the current through the resistors. Wire the circuit and carry out the measurements required to check both your expectations above.

Course P3 Practical 2

3/3 II A LAMPLIGHTER CIRCUIT

Some lamp posts are equipped with photodetectors which switch them on automatically at twilight or when it is getting dark, and switch them off at dawn or when it is getting clear.

The purpose in this part is to design a model of such a lamp post by means of the various block diagrams shown below:

The following devices are available :

 1 10 V power supply  1 resistor 1 kΩ

 1 LED (light emitting diode)  2 resistors 10 kΩ

 multimeter  1 resistor 3.3 k

 1 O.A. (and its supplies)

 1 light sensor (photoresistor) : roughly 15 k in the dark and 1-1,5 k in the light.

Block diagram 1 : Comparator

PQ4 Briefly explain how the comparator shown in

Figure 1 operates. What are the possible values for Vs?

Operation of the comparator:

Figure 1

Block diagram 2 : LED ignition

It is intended to control the LED operation with this comparator. The features of the LED are shown in figures 2 & 3 below.

Source : Wikipedia

Figure 2 Figure 3 Figure 4

PQ5 Design a wiring diagram downstream of the comparator in order to switch the LED on without damaging it (justify

the numerical values of the components chosen). Under what condition on Vs does the LED light up?

Block diagram 3 : Light sensor and control of the comparator

One single 10 V source is available. The comparator is controlled by a voltage divider bridge (sketched in Figure 4). Dipole D2 is the photoresistor, dipoles D1, D3 and D4 are resistors whose resistances are to be found.

PQ6 Voltages VA and VB are applied to the comparator. Under which conditions on VA and VB does the lamplighter operate

properly? Choose then convenient values for resistors D1, D3 and D4. Sketch a diagram assembling the three blocks.

EQ4 Wire blocks 1, 2 and 3 and assemble the lamplighter. Measure the voltage at each point of the circuit and check if it

operates properly.

INSA Rouen Normandie - STPI 1 - Course P3

Practical 2 - Circuits containing operational

amplifiers

Answer sheet

For every experimental question, write the results obtained and the calculations achieved. Hand in this sheet at the end of the practical session.

I

Amplifier circuit

Choice of resistors: R1= . . . and R2= . . . .

EQ 1 The O.A. is operated in its linear mode.

Show to the teacher the curves displayed on the oscilloscope for validation:  Draw the curves of ue and us (give the horizontal and vertical scales of the oscilloscope).

Measure ue and us at a given time. Show this time on the previous curve.

ue=. . . .and us=. . . ..

Deduce then the value of amplification us/ue in linear mode. Compare with the theoretical value.

. . . . . . . . . . . . . . . .

EQ 2 The O.A. is operated in its saturated mode.

Show to the teacher the curves displayed on the oscilloscope for validation:  Draw the curves of ue and us (give the horizontal and vertical scales of the oscilloscope).

Measure the saturation voltage U_sat+ and U_sat− . Is |U_sat+ | = |U_sat− | valid?

U_sat+ = . . . and U_sat− = . . . . Compare with the theoretical values. . . . . . . . EQ 3 New choice of resistors: R1= . . . and R2= . . . .

Expectations:

amplification: . . . . limiting input voltage if it is intended to operate the O.A. permanently in its linear mode . . . . . . . . Sketch the connections of the oscilloscope used to measure the magnitude of the current through the resistors:

Carry out the measurements required and check your three expectations above. Compare the mag-nitudes of the currents through R1 and R2.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

II

A lamplighter circuit

EQ 4 Wire blocks 1, 2 and 3 and assemble the lamplighter. Measure the voltage at each point of the circuit and check if it operates properly.

Choice of resistors: R1= . . . R2= . . . R3= . . . .

Show to the teacher your circuit wiring for validation: 

in darkness in the light VA (V)

VB (V)

VS (V)