Introduction to Op Amp Circuits ELEC 121. April 2004ELEC 121 Op Amps2 Basic Op-Amp The op-amp is a...

Introduction to Op Amp Circuits

ELEC 121

April 2004 ELEC 121 Op Amps 2

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

• See lm301.pdf or mc1741c.pdf for sample specification sheets


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


Op-Amp Gain

• Op-Amps have a very high gain. They can be connected open- or closed loop.• Open-loop (AVOL) refers to a configuration where

there is no feedback from output back to the input• AVOL may exceed 10,000

• Closed-loop (AVCL) configuration reduces the gain In order to control the gain of an op-amp it must have negative feedback• Negative feedback will reduce the gain and

improve many characteristics of the op-amp


Typical Op Amp Frequency Response


Change in AV with Feedback


Virtual Ground

Since ZIN is very high, we assume no current can flow into any lead of the op ampWhen the non-inverting input pin is at ground, the inverting input pin is at 0V

The 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 V

A = 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 RR

A = -R


Inverting Op-Amp GainClosed 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 RR

A = -R

F V

1

RA = - = -1

R


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


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

A

V1

V2

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 individually

IF + I- = I1 + I2 + I3

but I- 0IF = 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 + V

R 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 = + + R

R R R

where R = R // R // R

VV = R + R

R

V RA = = 1 +

V R

Introduction to Op Amp Circuits ELEC 121. April 2004ELEC 121 Op Amps2 Basic Op-Amp The op-amp is a...

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