op-amp applications

OP-AMP applications

UNIT-5

Mohammad Asif IqbalAssistant Professor, Deptt of ECE,JETGI, Barabanki

Basic Op-Amp

The op-amp is a differential amplifier with a

very high open loop gain 25k ≤ AVOL ≤ 500k (much higher for FET inputs) high input impedance 500k ≤ ZIN ≤ 10M

low output impedance 25 ≤ RO ≤ 100

Op-Amp Equivalent Circuit

Op-Amp Specifications – DC Offset Parameters

• Even though the input voltage is 0, there will be an output. This is called offset. The following can cause this offset:• Input Offset Voltage• Output Offset Voltage due to Input Offset Current• Total Offset Voltage Due to Input Offset Voltage and Input Offset Current• Input Bias Current

General Op-Amp Specifications VIO

• Input Offset Voltage VIO

• The voltage that must be applied to the input terminals of an op amp to null the output voltage

• Typical value is 2mV with a max of 6mV• When operated open loop, must be nulled or device may saturate

General Op-Amp Specifications IIO

• Input Offset Current• The algebraic difference between the two input currents• These are base currents and are usually nulled• Typical value IIO 20 nA with a max of 200nA

Technique to Null VO

• Short Input terminals to ground• Connect potentiometer between compensation pins with wiper to VEE

• Potentiometer is usually a 10 turn device

• Connect meter to output and adjust potentiometer for VO = 0

General Op-Amp Specifications CMRR

• Common Mode Rejection Ratio• The ratio of the differential voltage gain (AD) to the common mode gain (ACM)• ACM is the ratio between the differential input voltage (VINCM) applied common mode,

and the common mode output voltage (VOCM)• it can exceed minimum is 70db with a typical value of 90 db• in properly designed circuit, it may exceed 110db

OD

IN

OCMCM

CM

D

CM

VA = V V A = V

A CMRR = 20 log A

General Op-Amp Specifications

• Input Bias Current• The average of the currents that flow into the inverting and noninverting

terminals• Typical values rage from 7nA to 80 nA

• Differential Input Resistance• Also know as the input resistance• Resistance seen looking into the input terminals of the device• Runs from a low of 2M for an LM741 to a high of 1012 for FET input devices

• Output resistance• Resistance between the output terminal ad ground• Typical values are 75 or less

• Input Capacitance• The equivalent capacitance measured at either the inverting or noninverting

terminal with the other terminal connected to ground• May not be on all spec sheets• Typical value for LM741 is 1.4pF

B+ B-B

I + I I = 2

General Op-Amp Specifications

• Power Supply Range• May be differential or single ended• Max is ± 22V

• Output Voltage Swing• Range of output voltage• Depends on power supply voltage used (typically about 85% to 90%)• Usually about ±13.5V for a power supply voltage of ±15V

• Slew Rate• The maximum rate of change in the output voltage in response to an input change• Depends greatly on device, higher is better (output resonds faster to input changes)• For LM741 it is .5V/s while for the LM318 it is 70V /s

• Gain Bandwidth Product• The bandwidth of the device when the open loop voltage gain is 1

Op Amp Equivalent Circuit

Practical Op-Amp Circuits

Typical Op-amp circuit configurations include the:• Unity Gain Buffer (Voltage Follower)• Inverting Amplifier• Noninverting Amplifier• Summing Amplifier• Integrator• Differentiator

Note: the integrator and differentiator are considered active filters

Unity Gain Buffer (Follower)

OV

1

O = 1

V

V A = V

V VA = 1

Inverting Op Amp

The input is applied to the inverting (-) inputthe non-inverting input (+) is groundedRF is the feedback resistor, and is connected from the output to the inverting inputThis is called negative feedback

Inverting Op Amp

We assume that no current enters the inverting terminalII- < 100nAVD 0V

O IN FV

S IN 1

FV

1

V I RA = = - V I RRA = -R

Inverting Op-Amp Gain

Closed Loop Gain is controlled by the external resistors: RF and R1

For Unity Gain: AV is -1 and RF = R1

The minus sign denotes a 180 degree phase shift between input and output

O IN FV

S IN 1

FV

1

V I RA = = - V I RRA = -R

F V

1

RA = - = -1R

Inverting Op Amp Compensated for Ibias

R is used to compensate for difference in IBIAS+ and IBIAS-

FV

1

RA = -R

Inverting Op-Amp

This configuration achieves high gain with a smaller range of resistor values than the basic inverter

A

V-

V+

2 F 2 FV

1 1 3

R + R R R A = - + R R R

Inverting Amplifier with High Zin

Use a Unity Gain Buffer to obtain a very high input resistance with an inverting amplifier

20

Inverting Amplifier for Low RL

Use a Unity Gain Buffer to obtain a very high input resistance to drive a low impedance load

Noninverting Amplifier

V- = V+ = vi

2O in

O 2V

in

R V = V 1 + R1

V R A = = 1 + V R1

Noninverting Op Amp Compensated for IBIAS

Rbias is used to compensate for difference in IBIAS+ and IBIAS-

Differential (Difference) Amplifier

A

AV1V2

O 2V

2 1 1

V R A = - = V - V R

-

Differential Amplifier Output

Instrumentation Amplifier Buffered Input

R1 = R2, RF1 = RF2F

V1

RA = R

-

Instrumentation Amplifier

R1 = R2, RF1 = RF2

F AV

1 B

R R A = 1 + 2 R R

-

Inverting Summing Amplifier

By applying KCL to the multiple inputs, we can consider the contribution of each source individuallyIF + I- = I1 + I2 + I3

but I- 0\IF = I1 + I2 + I3

VO = -IF RF

F F FO 1 2 3

1 2 3

1 2 3 O F

1 2 3

R R RV = - V + V + VR R R

V V VV = - R + + R R R

Non-inverting Summing Amplifier

Perform a source transformation for each inputSum the current sources and find RTH for the resistancesVIN+ = IT RTH

1 2 3IN + TH

1 2 3

TH 1 2 3

IN +O IN F

IN

O F V

IN + IN

V V V V = + + RR R R

where R = R // R // RVV = R + R R

V RA = = 1 + V R

The output is the integral of the inputThis circuit is a low-pass filter circuit, and is used and sensor conditioning circuits

(t)dtvRC1(t)v 1o

Integrator

Differentiator

The differentiator takes the derivative of the inputThis circuit is a high-pass filter circuits

dt(t)dvRC(t)v 1

o

Comparator

• High Gain Op Amp• Operated Open Loop• Designed to compare an input to a reference voltage• Gives output (digital level) to indicate if input is above or below reference

• Circuit designed to give VOSAT and –VOSAT only

Comparator Operation Example

Block Diagram of 555

Astable Multivibrator

555 Used as an Astable Multivibrator

Schmidt Trigger 7414

• A Schmidt trigger (a comparator with Hysteresis) is a bistable digital (two-state) device

• It accepts virtually any analog input and provides a logic 0 or 1 output• A typical use is to take distorted digital signals (due to RC time constant of transmission

line) and provide a used to square-wave output• Can be used to eliminate noise near reference point that would cause problems in analog

comparators

Real world (lab) is analog

V

t t

Computer (binary) is digital

D/A Conversion Computer DAC

Computer DACA/D Conversion

V

Digital to Analog Conversion (DAC or D/A)

8 bits

ComputerA/D

Digital to Analog conversion involves transforming the

computer’s binary output in 0’s and 1’s (1’s typically = 5.0

volts) into an analog representation of the binary

data

D/A conversion can be as simple as a weighted resistor network

4 - bit DAC Converter

Resistor values correspond to binary weights of the number D3 D2 D1 D0 , i.e. 1/8, 1/4, 1/2, and 1

Using EWB we can model this device

1. This setup requires a wide range of precision resistors

A 10 bit DAC needs resistors ranging from R to R/1024.

2. The circuit driving the DAC (usually a computer) must supply a wide range of currents for a constant Vout

Difficulties:

For a 8-bit DAC

Smallest step in output voltage is v/256

8 bits corresponds to 256 different values

For a 5.0 volt DAC this step size is ~ 19.5 mV

As was seen in the Workbench example, the output voltage from a DAC can change by only discrete amounts, corresponding to the level associated with a 1 bit binary change.

A modification of the weighted resistor DAC is the so called R-2R LADDER DAC, that uses only 2 different resistances

An actual R-2R DAC showing input 1 0 1 1

Voltmeter reading is determined by the binary number ABCD and the resistor weights

MSB = 1/2 of Vref

= 1/4 of Vref

= 1/8 of Vref

LSB = 1/16 of Vref

1 0 1 1 = 1/2 (5) + 1/4 (0) + 1/8 (5) + 1/16 (5) 3.4 volts

In actual DACs, the converters will drive amplifier circuits in most cases

Amplified DAC with bipolar ( ± Vout ) output

If one wants only positive or negative output, one can use a BASELINE ADJ. for the Op Amp

Analog-to Digital Conversion (ADC or A/D)

A/D

8 bits

Computer

An ideal A/D converter takes an input analog voltage and converts it to a perfectly linear digital representation of the analog signal

If you are using an 8-bit converter, the binary representation is 8-bit binary number which can take on 28 or 256 different values. If your voltage range were 0 - 5 volts, then

0 VOLTS 0000 0000

5 VOLTS 1111 1111

An 8-bit converter can represent a voltage to within one part in 256, or about 0.25 %. This corresponds to an inherent uncertainty of ± ½ LSB (least significant bit).

Decimal 128 = 0 1 1 1 1 1 1 1

Bit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7 Bit 0

LSBMSB

Notice the bits are designated B7 - B0. Bit B7 is the Most Significant Bit while B0 is the Least Significant Bit

0000

0000

0000

0001

0000

0010

0000

0011

1111

1111

1111

1110

1111

1101

. . . . . . . . .

Volt

age

(Vol

ts)

Analog Voltage

1111

1100

1 LSB

Number of Bits (N) Resolution (1/2N) Increment (mV) for 5 volts

6 1/64 78.1

8 1/256 19.6

10 1/1024 4.9

12 1/4096 1.2

14 1/16384 0.3

16 1/65536 0.07

Types of Analog to Digital Converters

1. Counter Type

2. Integrating or Dual Slope

3. Parallel or Flash

4. Successive Approximation

Counter Type

Control Logic

D A C Counter

START

Vin

Comparator

Digital Output

clock

• When START is received,

• control logic initializes the system, (sets counter to 0), and

• turns on Clock sending regular pulses to the counter.

As the Clock sends regular pulses to the counter, the counter outputs a digital signal to the Digital-to-Analog converter

D A C Counter

START

Vin

Comparator

Digital Output

clock

Control Logic

Control Logic

D A C Counter

START

Vin

Comparator

Digital Output

clock

As the counter counts, its output to the D A C generates a staircase ramp to the comparator.

Control Logic

D A C Counter

START

Vin

Comparator

Digital Output

clock

As the ramp voltage increases to the comparator, it rises closer and closer to Vin at which point the comparator shifts states

When the ramp voltage exceeds Vin , the comparator output shifts which signals the control logic to turn off the clock

Vin

Conversion time

V’in

Conv.time

With the clock off, the counter reading is proportional to Vin

Note that the conversion time depends on the size of the input signal

Vin

Comparator

Control Logic

D A C Counter

START

Vin

Comparator

Digital Output

clock

Once the digital output has been read by the associated circuitry, a new start signal is sent, repeating the cycle.

With a counter type A/D, if the signal is varying rapidly, the counter must count up and reset before each cycle can begin, making it difficult to follow the signal.

Track & Hold Logic

D A C

Up/Down Counter

Vin

Comparator

Digital Output

clock

Tracking ADC - similar to the counter type except it uses an up/down counter and can track a varying signal more quickly

Vin

-Vref

Control logic

Counter

clock

comparator

integrator

Digital Output

Integrating or Dual Slope A/D

Vin

-Vref

Control logic

Counter

clock

comparator

integrator

Digital Output

When conversion is initialized, the switch is connected to Vin which is applied to the op amp integrator. The integrator output (>0) is applied to the comparator

Vin

-Vref

Control logic

Counter

clock

comparator

integrator

Digital Output

As conversion is initiated, the control logic enables the clock which then sends pulses to the counter until the counter fills (9999)

Vin

-Vref

Control logic

Counter

clock

comparator

integrator

Digital Output

overflow

As the counter resets (9999 0000), an overflow signal is sent to the control logic

this activates the input switch from Vin to -Vref , applying a negative reference voltage to the integrator

The negative reference voltage removes the charge stored in the integrator until the charge becomes zero.

The total number of counts on the counter (determined by the time it took the fixed voltage Vref to cancel Vin ) is proportional to the input voltage, and thus is a measure of the unknown input voltage.

At this point, the comparator switches states producing a signal that disables the clock and freezes the counter reading.

fixed timemeasured time

Inte

grat

or O

utpu

t Vol

tage full scale conversion

quarter scale conversion

half scale conversion

The operation of this A/D requires 2 voltage slopes, hence the common name DUAL-SLOPE.

charging up the capacitor discharging the

capacitor

Since this A/D integrates the input as part of the measuring process, any random noise present in the signal will tend to integrate to zero, resulting in a reduction in noise.

These type of A/D s are used in almost all digital meters. Such meters usually are not used to read rapidly changing values in the lab. Consequently the major disadvantage of such converters (very low speeds) is not a problem when the readout update rate is only a few times per second.

Successive-Approximation A/D

Successive Approximation

Register

D/A ConverterVref

clock

analog input

Digital Output Data

At initialization, all bits from the SAR are set to zero, and conversion begins by taking STRT line low.

comparator

STRT

Successive Approximation

Register

D/A ConverterVref

clock

analog input

Digital Output Data

comparator

STRT

Successive-Approximation A/D

First the logic in the SAR sets the MSB bit equal to 1 (+5 V). Remember that a 1 in bit 7 will be half of full scale.

Successive Approximation

Register

D/A ConverterVref

clock

analog input

Digital Output Data

comparator

STRT

Successive-Approximation A/D

The output of the SAR feeds the D/A converter producing an output compared to the analog input voltage. If the D/A output is < Vin then the MSB is left at 1 and the next bit is then tested.

Successive Approximation

Register

D/A ConverterVref

clock

analog input

Digital Output Data

comparator

STRT

Successive-Approximation A/D

If the D/A output is > Vin then the MSB is set to 0 and the next bit is set equal to 1.

Successive bits are set and tested by comparing the DAC output to the input Vin in an 8 step process (for an 8-bit converter) that results in a valid 8-bit binary output that represents the input voltage.

CLOCK PERIOD

¼FS

½FS

¾FS

1 2 3 4 5 6 7 8

analog input voltage

D/A output for 8-bit conversion with output code 1011 0111

Successive approximation search tree for a 4-bit A/D

1111

1110

1101

1100

1011

1010

1001

1000

0110

D/A output compared with Vin to see if larger or smaller

0111

0101

0011

0010

0100

0001

Note that the successive approximation process takes a fixed time - 8 clock cycles for the 8-bit example.

For greater accuracy, one must use a higher bit converter, i.e. 10-bit, 12-bit, etc. However, the depth of the search and the time required increases with the bit count.

THANK YOU!