04 Operational Amplifier Circuits [OAC]

Update: 17/03/2009 Page 1/16

2140202 Electrical and Circuits Laboratory Faculty of Engineering

Chulalongkorn University

Operational Amplifier Circuits [OAC] Ekachai Leelarasmee [March 14, 2010]

Instructor: Date: Name: 1) ID

2) ID 3) ID

An operating amplifer (OPAMP) is an integrated circuit (IC) chip that consists of several components (transistors, resistors and capacitors) connected to operate equivalently as a high gain differential amplifier as shown in Fig. 1.

iR

outvoR

( )IN INA v v+ --

INv +

INv - INv -

INv + (a) (b)

Fig. 1 An operational amplifier symbol (a) and its small signal equivalent circuit (b)

An ideal OPAMP is the one with A= , Ri and R 0o . As a general purpose circuit component, an opamp enables a designer to implement various circuit functions easily in a block fashion. This includes a low gain amplifier, filter, comparator, D/A converter and etc. These circuits rely on feeding the opamps output back to its negative input as shown in the next few pages. Interestingly, if the opamp is ideal, the synthesized function depends only on external component values. To derive the function of an ideal opamp circuit, always assume that +INv = -INv ( called virtual short circuit ) and apply circuit equations such as KCL and KVL to the external components (R or C). The following figures are typical applications of an opamp circuit.

Update: 17/03/2009 Page 2/16

Inverting Amplifier F

R

+

-1R

inv

1

Fout in

Rv vR

-=

Weighted Summer

+

-1R

1v

FR

1 21 2

F Fout

R Rv v vR R

= - +

2R

2v

Adder

1R1v

( )1 21

Fout

Rv v vR

= - +

1R2v

Non-inverting Amplifier

1R

inv 11 Fout in

Rv vR

= +

FR

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Weighted Subtractor

1R1v

FR

32 1

2 3 1 11 F Fout

R R Rv v vR R R R

= + - + 2R

2v

3R

Subtractor

1R1v

FR

2v

FR

( )2 11

Fout

Rv v vR

= -1R

Integrator

1

1out inv v dtR C

-=

1Rinv

c

Low Pass Filter

1/1 2

out F

in F

v R Rv j f R Cp

= -+

+

-1R

invc

FR

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Unfortunately, ideal opamps cannot be realized in practice. The opamp used in our experiment is TL061 (http://focus.ti.com/docs/prod/folders/print/tl061.html). Its main parameters are tabulated along with their ideal values as shown below.

TL061 Ideal Differential Mode Gain A = 6000 Differential Input Resistance 1210iR = W Output Resistance 100oR = W 0 Bandwidth BW = 1MHz

Although being non-ideal, the incurred error in many typical applications is usually negligible, particularly at low gain applications. Its full symbol and pin assignment are depicted in the figure below. Apart from the input (IN+, IN-) and output (OUT) pins, the chip requires one positive ( +CCV ) and one negative ( -CCV ) DC supplies for biasing. These DC supplies limit the maximum and minimum voltage levels at both input and output. It also has OFFSET N1 and N2 pins for minimizing its DC input offset voltage. Throughout this laboratory, we will not need these two OFFSET pins. They should be left unconnected.

Fig. 2 Pin assignment of TL061

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Instructions: a) Connect two 6V batteries to the PCBs and a wire on the proto-boards as shown

below. This will provide +6V, 0V and -6V busses on the proto-board as indicated.

Figure 3: Proto-board with +6V, 0V and -6V DC

b) Components are provided adequately in a plastic box. c) Resistors are marked with four coloured bands as shown in Fig 4. Each color

represents or codes 1 digit as shown in Table 1. The first 3 colors are for resistance calculation according to the formulae

Resistance= (1st colour value*10 + second colour value)*10 third one value.

As an example, a resistor marked with red (2), green (5) and orange (3), has a resistance of 25,000 ohms.

1st No. 2nd No. No. of trailing zero tolerance

Figure 4: Resistor with color codes

Colour black brown red orange yellow green blue purple gray white Value 0 1 2 3 4 5 6 7 8 9

Table 1: Color code table

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d) Introducing a potentiometer A potentiometer is a 3 terminal resistor with a sliding contact in its middle pin as shown below. Use a screw driver to adjust the sliding contact at the top.

6V6V-in

v

10kW inv

Experiment #1. 1.1) Construct the Inverting Amplifier circuit below.

1R outv

200FR k= W

6V+100kW

6V-

6V6V-

inv

10kW inv

1.2) Adjust the potentiometer with a screw driver to set inv according to the table below. Use a DMM (Digital Multi-Meter) to measure inv , outv and fill in the table.

)(Voltvin )(Voltvout V1.0 actual

-0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

-4.0

-4.5

-5.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

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1.3) Plot the measured results in the graph. Then draw a line approximation of the plotted data with a shape similar to the one shown on the right.

inv

outv

1.4) From the graph, determine the following values in the table. maxV minV a Measured

Theory CCV =...

CCV =...

1RRF- =...

minV

maxV

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Experiment #2. Summing Amplifier with Inverted Output. 2.1) Construct the circuit below.

1R outv

200FR k= W

6V+100kW

2 1.0MR = W6V-

6V-

10kWinv

6V+

inv



-0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

-4.0

-4.5

-5.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

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inv

outv

2.4 ) From the graph, determine the following values in the table. maxV minV a b Measured

Theory CCV =...

CCV =...

1RRF- =...

2

FRR

CCV =...

minV

maxV

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Experiment #3. Non-inverting Amplifier 3.1) Construct the circuit below. Note that FR is changed to a new value.

1Rinv

outv

100FR k= W

6V+100k

6V-

10kW inv

6V+

6V-



-0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

-4.0

-4.5

-5.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

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inv

outv

3.4) From the graph, determine the following values in the table. maxV minV a Measured

Theory CCV =...

CCV =...

11 FR

R =...

minV

maxV

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Experiment #4. A Subtracting Circuit. 4.1) Construct the circuit below.

1Rinv

outv

24FR k= W

6V+100k

6V-

10kWinv

6V+

6V-



-0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

-4.0

-4.5

-5.0


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

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inv

outv

4.4) From the graph, determine the following values in the table. maxV minV a b Measured

Theory CCV =.

CCV =..

1

1RRF+ =...

1

FRR

CCV =

Slope = a

minV

maxV

b

0

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Experiment #5. An integrator with inverted output 5.1) Construct the circuit below. Note that an electrolytic capacitor C with polarity

indicators (+/-) is used. Connect it exactly as shown. Otherwise current leaking through the chemical substance within the capacitor will occur. One can find this type of capacitor starting from 1 Fm to 10000 Fm .

R

inv

outv

1C Fm=

6V+1MW

6V-

6V+

5.2) Set inv = 6V. Connect a wire across both terminals of the capacitor. Use a DMM

to measure and record inv = Volt and outv = 0V = Volt (should be around zero). Now, pull the wire out while connecting the DMM at outv . You should notice that

outv starts ramping downward as shown in the figure below.

t d=

0V

0t =

3.5V-

Measure the time at which outv reaches -3.5 Volt as shown in the figure. You may do this step several times and take the average value. Put your data here. d = Sec

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Assignments:

1) From experiment #1, determine the waveform of outv if inv is as shown below. Then plot it in the same graph.

0.5 /V div

2 /mS div 2) Draw an opamp circuit that synthesizes outv = -2 1v - 2v . Specify all resistor

values in unit of .WK

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3) Determine )(tvout as a function of t from the circuit below.

R

inv

outv

1C Fm=

6V+1M W

6V-

5V+

04 Operational Amplifier Circuits [OAC]

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