| |
| |

| | | | | |

|

EE TIMES NETWORK

Online Editions
EE TIMES
EE TIMES ASIA
EE TIMES CHINA
EE TIMES FRANCE
EE TIMES GERMANY
EE TIMES INDIA
EE TIMES JAPAN
EE TIMES KOREA
EE TIMES TAIWAN
EE TIMES UK

EE TIMES EUROPE
ANALOG EUROPE
INDUSTRIAL EUROPE
AUTOMOTIVE DL EUROPE

POWER DL EUROPE

Web Sites
• Audio DesignLine
• Automotive DesignLine
• Career Center
• CommsDesign
• Microwave
Engineering
• Deepchip.com
• Design & Reuse
• Digital Home DesignLine
• DSP DesignLine
• EDA DesignLine
• Embedded.com
• Elektronik i Norden
• Green SupplyLine
• Industrial Control
DesignLine
• Planet Analog
• Mobile Handset
DesignLine
• Power Management
DesignLine
• Programmable Logic
DesignLine
• RF DesignLine
• RFID-World
• Techonline
• Video | Imaging
DesignLine
• Wireless Net
DesignLine

ELECTRONICS GROUP SITES

• eeProductCenter
• Electronics Supply &
Manufacturing
• Conferences
and Events
• Electronics Supply &
Manufacturing--China
• Electronics Express
• Webinars

22 September 2009
SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block

This series will look at the basic building blocks of the analog signal chain; this first installment looks at op amps

By Bill Klein, Senior Applications Engineer, Texas Instruments
Planet Analog
November 1, 2007 (5:33 PM EST)

| Rate this article |

WORSE | BETTER
| | | |
1 | 2 | 3 | 4 | 5
| |
| |

Welcome to Signal Chain Basics, a series of articles describing the operation of the analog signal chain. Over the course of this series, topics will include analog signal processing and the devices required to support them. Your comments are always welcome; they could even be the topic of a future discussion. (You can also see a short video introduction here. )

Operational Amplifier—The Basic Building Block

The basic building block of the signal chain is the operational amplifier (op amp) (Figure 1). In the simplest form this is a device with a differential input of infinite input impedance, and a voltage-controlled voltage source with a gain approaching infinity. These features alone would be of little value. However, through the use of various feedback techniques, this becomes a very valuable device.

Figure 1: An ideal op amp

The transfer function of the ideal op amp is seen from the circuit to be:

With a very large value for Aol (open-loop gain) this circuit is of minimal value. A survey of data sheets will reveal that the absolute value of Aol is not tightly controlled in production. Adding negative feedback, as shown in Figure 2 is the solution to the problem.

Figure 2: The ideal op amp with feedback

Since there can be no current flow at the input pins, the current through Ri must equal the current through Rf. This can be expressed as:

Combining these two terms, setting V2=0, and assuming the open-loop gain is very large results in the standard closed-loop gain (Acl) equation:

Notice, from the first equation, that the op amp amplifies the difference between the input voltages. As long as the op amp is operating in a linear mode, the input pins are at the same voltage (Figure 3).

Figure 3: Standard circuit drawing for the inverting op amp configuration.

For the non-inverting configuration, the gain equation results are slightly different (Figure 4).

Fig. 4: The non-inverting op amp configuration

With a very large value of Aol, the gain expression reduces to:

The complete development of the transfer functions can be found in the associated sidebar below. This development also treats the case where Aol is less than infinite.

Starting with this basic building block a large number of analog computing circuits can be configured. Three basic concepts developed here will be used many times in future articles: gain expression for very large Aol, gain expression of restricted Aol, and the concept that the op amp drives the output such as to keep the input pins at the same voltage.

Sidebar: Development of closed-loop gain expression

For the non-inverting configuration:

When Aol is a very large number the expression reduces to:

This is the closed loop gain expression for the ideal condition where Aol is very large. When the open loop gain is less than ideal the actual closed loop gain expression becomes:
.

Since Aol will always be less than infinite there will always be some error in the gain expression. The value of Aol is usually large enough that this error can be ignored.

A similar development for the inverting case:

When Aol is a very large number the expression reduces to:

This is the closed loop gain expression for the ideal condition where Aol is very large. When the open loop gain is less than ideal the actual closed loop gain expression becomes:

Notice that the Acl here is the non-inverting Acl.

About the author

William P. (Bill) Klein is a Senior Applications Engineer with the High Performance Analog group at Texas Instruments. Bill joined TI through its acquisition of Burr-Brown in August 2000. His experience as an analog circuit designer covers over 40 years in fields ranging from mineral exploration to medical nuclear imaging. One current role Bill has is hosting the Analog e-LAB Web Cast, presenting real world solutions to real world problems in analog circuit design. In addition to a BSEE from Arizona State University and registration as a Professional Engineer in the State of Arizona, he has authored numerous magazine articles, application notes and conference papers

Related Links:

SIGNAL CHAIN BASICS Series (Part 2): Op Amp--Basic operations
Welcome to Signal Chain Basics, a series of articles describing the operation of the analog signal chain.

Industry's smallest 16-bit ADC - Get samples and evaluation modules today! Click here to learn more.

EE Times TechCareers

Search Jobs

Enter Keyword(s):

Function:

Engineering & Arch.

Information Tech.

State:

Post Your Resume
-----------------
Employers Area

Most Recent Posts
BAE Systems seeking Director-Architecture in Herndon, VA
SEL seeking Business Development Manager in Alexandria, VA
ITT Systems seeking System Administrator in IQ, Middle East
ITT Systems seeking Supply Clerk/Tech in IQ, Middle East
AWR Corp seeking Microwave Library Development Engr in El Segundo, CA
More career-related news, resources and job postings for technology professionals

Sponsor Links

45V, 50mA Synchronous Buck Converter with Only 12µA Quiescent Current
Linear's LTC®3642, a 45V input capable synchronous buck converter, can deliver up to 50mA of continuous output current from a 3mm x 3mm (or MSOP8-E) package. It operates from an input voltage range of 4.5V to 45V (60V Max), making it ideal for automotive & industrial 4mA to 20mA control loop applications. For more information, visit here

Watch the latest videos from Fairchild covering topics on power supply and portable applications.

Industry's smallest 16-bit ADC - Get samples and evaluation modules today! Click for more information.

<A href="http://as.cmpnet.com/event.ng/Type=click&amp;FlightID=150903&amp;AdID=237064&amp;TargetID=796&amp;Segments=884,909,3108,3448,13831&amp;Targets=785,796,2625,2878&amp;Values=34,46,51,63,77,87,91,102,140,204,304,309,442,452,656,1255,1311,1388,1405,1431,1716,1767,1785,1925,1945,1970,2229,2299,2310,2313,2326,2352,2678,2727,2767,2862,2878,2942,3621,3712,3714,3890,3904,4079,6293,6332,6391,6392,6393,6422,6541,6567,6580,6815,6967,7035&amp;RawValues=&amp;Redirect=http://www.maxim-ic.com/landing.cfm?lp=397&amp;CMP=4586" target=_top><IMG border=0 src="http://i.cmpnet.com/ads/graphics/as5/ac/Maxim/1-Wire-hl_08042009.jpg" width=468 height=60></A>

All materials on this site Copyright © 2009 TechInsights, a Division of United Business Media LLC.
All rights reserved.
Terms and Conditions | Privacy Statement | Your California Privacy Rights

|