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Elektronika Dasar

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OPERASIONAL AMPLIFIEROP-AMP

Jurusan Teknik Elektro2012

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Op-amp : suatu IC analogOp-amp : suatu IC analog

+

_

Input 1

Input 2 output

+ VCC

- VEE

SIMBOL

SIFAT IDEAL

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Ideally,

1.No current can enter terminals V+ or V-. Called infinite input impedance.

2.Vout=A(V+ - V-) with A → ∞

3. In a circuit

V+ is forced equal to V-

4.An opamp needs two voltages to power it Vcc and -Vee.

A

VOUT = (AV - AV ) = A (V - V )

+

+

--

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Operational Amplifier (Op Amp)Operational Amplifier (Op Amp)

An An operational amplifieroperational amplifier (Op Amp) (Op Amp)

is an integrated circuit of a is an integrated circuit of a

complete amplifier circuit.complete amplifier circuit.

Op ampsOp amps have an extremely high have an extremely high

gain (gain (A=10A=1055 typically typically).).

Op ampsOp amps also have a high input also have a high input

impedance (impedance (R=4 MR=4 MΩΩ , typically , typically) )

and a low output impedanceand a low output impedance

((in order of 100 in order of 100 ΩΩ , typically , typically) .) .

-

+

Vi1 VoutA

B

Vi2

AVVV iiout 12

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Characters of Operational Characters of Operational Amplifiers Amplifiers

high open loop gainhigh open loop gain

high input impedancehigh input impedance

low output impedancelow output impedance

low input offset voltagelow input offset voltage

low temperature coefficient low temperature coefficient of input offset voltageof input offset voltage

low input bias currentlow input bias current

wide bandwidthwide bandwidth

large common mode large common mode rejection ratio (CMRR)rejection ratio (CMRR)

1 2 3 4

8 7 6 5

Offset null

Offset nullNot used

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Q1 Q2D1

D2

Q3

R2

R1

R3

R4

Q5

C1

R5

Q4

7

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Voltage Output from an AmplifierVoltage Output from an AmplifierThe linear range of an

amplifier is finite, and limited

by the supply voltage and the

characteristics of the

amplifier.A

Linearregion

Non-linearregion

Vout

Vin

Daerah Linier ini sangat Kecil

If an amplifier is driven

beyond the linear range

(overdriven), serious errors

can result if the gain is

treated as a constant.

Kalau A = 106 dan VCC = 12 Volt maka daerah linier = 24 μV

Vin= V2-V1

OPAMP: COMPARATOR(bekerja di daerah jenuh)

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Vout=A(Vin – Vref)

If Vin>Vref, Vout = +∞ but practically hits +ve power supply = Vcc

If Vin<Vref, Vout = -∞ but practically hits –ve power supply = -Vee

Vcc

-Vee VIN

VREF

Application: detection of a complex signal in ECG

A (gain) very high

Vout

A

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OPAMP: ANALYSISOPAMP: ANALYSIS

The key to op amp analysis is simple

1.No current can enter op amp input terminals.

=> Because of infinite input impedance

2.The +ve and –ve (non-inverting and inverting) inputs are forced to be at the same potential.

=> Because of infinite open loop gain

3.Use the ideal op amp property in all your analysis

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Inverting AmplifierInverting Amplifier((bekerja di daerah linierbekerja di daerah linier))

Point Point BB is grounded, point is grounded, point A A

is called Virtual is called Virtual

Grounded.Grounded.

Voltage across Voltage across RR11 is is VVinin, and , and

across across RRFF is is VVout.out.

The output node voltage The output node voltage

determined by determined by Kirchhoff'sKirchhoff's

Current Law (KCL).Current Law (KCL).

Circuit voltage gainCircuit voltage gain

determined by the determined by the ratio of ratio of

RR11 and and RRF.F.

1R

R

V

VG F

in

out

-

+

Vin

Vout

R1

RF

A

B

F

F

RRRR

R

1

13

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PENGUAT INVERTINGPENGUAT INVERTING

((bekerja di daerah linierbekerja di daerah linier))

Kondisi fisikKondisi fisik

1R2

R1

2

3

4

8

7

6

5

input

outputR3

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OPAMP: INVERTING OPAMP: INVERTING AMPLIFIERAMPLIFIER

1. V- = V+

2. As V+ = 0, V- = 0 (VG)

3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2.

4. I1 = (VIN - V-)/R1 = VIN/R1

5. I2 = (0 - VOUT)/R2 = -VOUT/R2 => VOUT = -I2R2

6. From 3 and 6, VOUT = -I2R2 = -I1R2 = -VINR2/R1 (NEG)

7. Therefore VOUT = (-R2/R1)VIN

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Analysis of Inverting AmplifierAnalysis of Inverting Amplifier

Ideal transfer characteristics:

-

+

Vin

Vout

R1

RF

A

BR

ii++

iiFF

ii11

ii--

--

0 ii

VV

FF iiii 1

F

outF

IN

R

VViand

R

VVi

11

000 VVi

F

outIN

R

V

R

V

1

oror1R

R

V

V F

in

out

VIN

KCL at A:

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OPAMP: NON – INVERTING OPAMP: NON – INVERTING AMPLIFIERAMPLIFIER

((bekerja di daerah linierbekerja di daerah linier))

1. V- = V+

2. As V+ = VIN, V- = VIN

3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2.

4. I1 = Vx/R1=VIN/R1

5. I2 = (VOUT - VIN)/R2 VOUT = VIN + I2R2

6. VOUT = I1R1 + I2R2 = (R1+R2)I1 = (R1+R2)VIN/R1

7. Therefore VOUT = (1 + R2/R1)VIN (tak berlawanan)

Vx

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Op-amp circuit is a voltage Op-amp circuit is a voltage

divider.divider.

Noninverting AmplifierNoninverting Amplifier

-

+Vin

Vout

R1

RF

A

BF

outA RR

RVV

1

1

1

1R

R

V

VG F

in

out

Circuit voltage gain determined by the ratio of R1 and RF.

Point VA equals to Vin .

OPAMP : VOLTAGE FOLLOWER (BUFER) (bekerja di daerah linier)

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V+ = VIN.

V- = V+

Thus Vout = V- = V+ = VIN !!!!

So what’s the point ?

The point is, due to the infinite input impedance of an op amp, no current at all can be drawn from the circuit before VIN. Thus this part is effectively isolated.

Very useful for interfacing to high impedance sensors such as microelectrode, microphone…

i = 0

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Differential AmplifierDifferential Amplifier

Point Point BB is grounded, so does is grounded, so does

point point A A (very small).(very small).

Voltage across Voltage across RR11 is is VV11, and , and

across across RR22 is is VV2.2.

Normally: Normally: RR11 = = RR22, , andand R RFF

= = RR3.3.

Commonly used as a single Commonly used as a single

op-amp instrumentation op-amp instrumentation

amplifier.amplifier.)( 12

1

VVR

RV F

out

RF

-

+

V1

Vout

R1

A

BR3

V2

R2

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Analysis of an Instrumentation Analysis of an Instrumentation AmplifierAmplifier

Design a single op-amp Design a single op-amp instrumentation amplifier. instrumentation amplifier.

RR11 = R = R22, R, RFF = R = R33

Determine the instrumentation gain.Determine the instrumentation gain.-

+

V1

Vout

R1

RF

A

B

R3

V2

R2A

F

OUTAA iR

VV

R

VV

1

1

32

2

R

Vi

R

VV BB

B

0 BA ii

2

2

31

1

R

VV

R

V

R

VV

R

VV BB

F

OUTAA

1

12

R

VVVV

R

VVV BA

F

BAOUT

)( 121

VVR

RV F

out

BA VV

SUMMING AMPLIFIER

3

32

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1

VR

RV

R

RV

R

RV FFF

out

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VOUT = -Rf (V1/R1 + V2/R2 + … + Vn/Rn)

If

Recall inverting amplifier and If = I1 + I2 + … + In

Summing amplifier is a good example of analog circuits serving as analog computing amplifiers (analog computers)!

Note: analog circuits can add, subtract, multiply/divide (using logarithmic components, differentiat and integrate – in real time and continuously.

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For the following circuit, calculate the input resistance.

R1Rf

R2

Vin

Vout

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INSTRUMENTATIINSTRUMENTATION AMPLIFIERON AMPLIFIER

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INSTRUMENTATION INSTRUMENTATION AMPLIFIERAMPLIFIER Inverting

amplifier

Non-inverting amplifier

very high input impedance

- So, you can connect to sensors

Differential amplifier -> it rejects common-mode interference -> so you can reject noise

Gain in the multiple stages: i.e. High Gain – so, you can amplify small signals

INSTRUMENTATION AMPLIFIER: STAGE 1

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I1

Recall virtual ground of opamps

I1 = (V1 – V2)/R1

Recall no current can enter opamps and Kirchoff’s current law

I2 = I3 = I1

Recall Kirchoff’s voltage law

VOUT = (R1 + 2R2)(V1 – V2)/R1

= (V1 – V2)(1+2R2/R1)

I2

I3I1

INSTRUMENTATION AMPLIFIER: STAGE 2

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I1

Recall virtual ground of opamps and voltage divider

V- = V+ = VBR4/(R3 + R4)

Recall no current can enter opamps

(VA – V-)/R3 = (V- – VOUT)/R4

Solving,

VOUT = – (VA – VB)R4/R3

I2

I3

VA

VB

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INSTRUMENTATION INSTRUMENTATION AMPLIFIER: COMPLETEAMPLIFIER: COMPLETE

VOUT = – (V1 – V2)(1 + 2R2/R1)(R4/R3)

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As force is applied on the sensor, the value of the variable resistor changes which results in a specific voltage output.

Gain = Vout/Vin = 1 Resistor Values for the Inverting OP-Amp can be changed to modify gain of converter or to amplify the signal of interest.

Analog Signal Conditioner Analog Signal Conditioner (Current to Voltage Converter, (Current to Voltage Converter, LM-324)LM-324)

+5 VDC

Sensor – Variable Resistor

4.7k

10k

10k

5k

Vout

0-5 VDC

I1I2

I3

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KOMPARATORKOMPARATOR

Rangkaian komparator digunakan untuk membandingkan tegangan masukan dan tegangan referensi.

Tegangan keluaran hanya ada dua kondisi yaitu tegangan tinggi atau rendah (negatif). Kondisi ini ditentukan oleh besarnya tegangan masukan apakah lebih tinggi terhadap tegangan referensi atau lebih rendah.

Persoalan dalam komparator sederhana adalah stabilitas. Bila tegangan masukan bervariasi sekitar tegangan referensi maka tegangan keluaran akan berubah-ubah tidak stabil.

Hal tersebut dapat dihilangkan dengan rangkaian schmitt.

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KOMPARATOR KOMPARATOR SEDERHANASEDERHANA

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KURVA HUBUNGAN TEGANGANKURVA HUBUNGAN TEGANGAN

Vi < Vr

Vo = +Vsat

Vi > Vr

Vo = -Vsat

Vr>0

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KURVA HUBUNGAN TEGANGANKURVA HUBUNGAN TEGANGAN

Vi < Vr

Vo = +Vsat

Vi > Vr

Vo = -Vsat

Vi

Vo

Vr=0

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KURVA HUBUNGAN TEGANGANKURVA HUBUNGAN TEGANGAN

Vi < Vr

Vo = +Vsat

Vi > Vr

Vo = -Vsat

Vi

Vo

Vr<0

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STABILITAS KOMPARATOR STABILITAS KOMPARATOR SEDERHANASEDERHANA

Vcc/2

Vin

R

Vcc

R

Tegangan

masukan

Tegangan keluaran

VCC/2

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RANGKAIAN SCHMITT Positive Feedback Positive Feedback

Rangkaian ini disebut komparator Schmitt trigger.

Rangkaian resistor membuat positive feedback.

R

Vout

R

Vout /2

Vin

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CARA KERJA Schmitt triggerCARA KERJA Schmitt trigger

Anggap tegangan masukan kecil, tegangan keluaran menjadi tinggi.

Bila Vout is 4 V, maka masukan non-inverting V+ adalah 2 Volt.

Kondisi keluaran tetap selama Vin kurang dari 2 Volt.

Bila Vin diperbesar sehingga lebih besar dari 2 V, maka Vout akan nol, dan V+ akan nol juga. Kondisi output ini akan tetap, selama Vin lebih besar 2 V.

R

Vout

R

Vout /2

Vin

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TAK STABILTAK STABIL

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STABILSTABILhisterisis

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STABILSTABIL

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STABILSTABIL

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RANGKAIAN dan RANGKAIAN dan OUTPUTOUTPUT

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KETERANGAN SCHMITTKETERANGAN SCHMITT

Schmitt trigger adalah sebuah aplikasi Schmitt trigger adalah sebuah aplikasi comparator yang mengubah tagangan yang mengubah tagangan keluaran menjadi negatif bila mtegangan keluaran menjadi negatif bila mtegangan masukan lebih besar tegangan referensi. masukan lebih besar tegangan referensi.

Kemudian menggunakan Kemudian menggunakan negative feedback untuk mencegah agar untuk mencegah agar tegangan keluaran tdk kembali ke kondisi tegangan keluaran tdk kembali ke kondisi semula saat tegangan kembali kurang semula saat tegangan kembali kurang dari tegangan referensi, sampai nanti dari tegangan referensi, sampai nanti masukan lebih kecil dari yang ditentukan. masukan lebih kecil dari yang ditentukan.

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AAPPLLIIKKAASSII

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PERHITUNGAN PERHITUNGAN Kerja Kerja Schmitt trigger merupakan proses merupakan proses

komparasi dengan threshold ganda. komparasi dengan threshold ganda. Persamaan arus di titik A:Persamaan arus di titik A:

Karena hanya 2 pers, maka harus ada satu R Karena hanya 2 pers, maka harus ada satu R yang ditentukan dulu. yang ditentukan dulu.

Ingat : Vout = VCC saat Vin diatas batas atas (V2)Vout = -VEE saat Vin dibawah batas bawah (V2’).

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uu

LM741

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NOMOR KAKI

46

47

48

49

50

51

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KESIMPULANOp-amp dapat digunakan sebagai :

1. Penguat INVERTING2. Penguat NON INVERTING3. BUFER4. Penguat PENJUMLAH5. Penguat INSTRUMENTASI6. Pengubah ARUS KE TEGANGAN atau

sebaliknya7. KOMPARATOR

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PR

• Buktikan rumus untuk menghitung R1, R2 dan R3 pada komparator Schmitt (slide 41), bila batas atas dan batas bawah diketahui.

• Rencanakanlah sebuah komparator Schmitt dengan menggunakan sebuah op-amp, yang menggunakan single supply 5 Volt. Batas tegangan yang dideteksi adalah diatas 3 Volt memberikan tegangan output tinggi, dan dibawah 1 Volt menghasilkan tegangan output rendah. Tentukan nilai R yang diperlukan.

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