All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > General Electronics Chat

Notices

General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 04-17-2010, 01:57 PM
Mazaag Mazaag is offline
Senior Member
 
Join Date: Oct 2004
Posts: 255
Default Common Mode Gain of Diff Amplifier

Hi Guys,

Could someone please explain to me why we consider the common mode signal going into a differential amplifier (when wanting to calculate the common mode gain) is equal to the average of the inverting and non-inverting signals? I understand what common mode gain is but don't understand why the average is the common mode signal..

Thanks guys!
Reply With Quote
  #2  
Old 04-18-2010, 12:52 AM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
 
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default

Quote:
don't understand why the average is the common mode signal
That isn't correct. A common mode signal is equally present on both inputs. A difference amplifier will not respond to a common mode signal (no difference on either input).

http://www.edn.com/article/CA289961.html
__________________
First comes the hardware, then the software.
Reply With Quote
  #3  
Old 04-18-2010, 02:40 AM
Ron H's Avatar
Ron H Ron H is offline
E-book Developer
 
Join Date: Apr 2005
Location: Idaho, USA (GMT-7)
Posts: 7,050
Default

Quote:
Originally Posted by beenthere View Post
That isn't correct. A common mode signal is equally present on both inputs. A difference amplifier will not respond to a common mode signal (no difference on either input).

http://www.edn.com/article/CA289961.html
That is true if the amplifier is ideal.
Reply With Quote
  #4  
Old 04-18-2010, 04:49 AM
The Electrician The Electrician is offline
Senior Member
 
Join Date: Oct 2007
Posts: 1,722
Default

Quote:
Originally Posted by beenthere View Post
That isn't correct. A common mode signal is equally present on both inputs. A difference amplifier will not respond to a common mode signal (no difference on either input).

http://www.edn.com/article/CA289961.html
The very first paragraph of the article you referenced says that it is correct:

"A common-mode voltage is one-half the vector sum of the voltages between each conductor of a balanced circuit and the local ground."
Reply With Quote
  #5  
Old 04-18-2010, 05:24 AM
The Electrician The Electrician is offline
Senior Member
 
Join Date: Oct 2007
Posts: 1,722
Default

Quote:
Originally Posted by Mazaag View Post
Hi Guys,

Could someone please explain to me why we consider the common mode signal going into a differential amplifier (when wanting to calculate the common mode gain) is equal to the average of the inverting and non-inverting signals? I understand what common mode gain is but don't understand why the average is the common mode signal..

Thanks guys!
Imagine that we construct signals to be applied to the inputs of a differential amplifier.

Let Vc be a common mode signal. This signal will be applied to both inputs.

Let Vd and -Vd be differential signals applied to the inputs.

To one input we apply Vc + Vd and to the other we apply Vc + (-Vd)

The differential signal seen by the amplifier is the difference between these two, namely (Vc + Vd) - (Vc - Vd) = 2Vd

Let's take the average of the two input signals:

((Vc + Vd) + (Vc - Vd))/2 = (2Vc)/2 = Vc

We have recovered the common mode signal.
Reply With Quote
  #6  
Old 04-18-2010, 02:02 PM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
 
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default

Taking the case of an instrumentation amplifier and applying a resistive bridge (let's say strain gauges) to the inputs. With the bridge balanced and 10 volts applied as the excitation voltage, the common mode voltage is 5 volts. That is, + 5 volts present on the inverting and non-inverting inputs.

The output of the IA is not 5 volts, but 0 volts. The classical three op amp IA appears to be a buffered difference amplifier.
__________________
First comes the hardware, then the software.
Reply With Quote
Reply   Post New Thread

Tags
, , , ,


Related Site Pages
Section Title
Worksheet Differential transistor amplifiers
Worksheet Linear computational circuitry
Worksheet Summer and subtractor opamp circuits
Worksheet Class A BJT amplifiers
Textbook Practical considerations : Operational Amplifiers
Textbook Averager and summer circuits : Operational Amplifiers
Textbook Negative feedback : Operational Amplifiers
Textbook Feedback : Bipolar Junction Transistors
Textbook The common-collector amplifier : Bipolar Junction Transistors
Textbook Differential amplifier : Discrete Semiconductor Circuits


Similar Threads
Thread Thread Starter Forum Replies Last Post
Amplifier Design Problem (High gain, high BW) Schniz2 Homework Help 0 04-15-2010 07:41 AM
Understanding interrupt on port state change ke5nnt Embedded Systems and Microcontrollers 12 01-31-2010 03:12 PM
Does an amplifier have a max gain ? simeonz11 The Projects Forum 4 01-09-2010 12:50 PM
Common Emitter for audio amplifier Ryouko Homework Help 5 04-27-2009 08:41 PM
Common Mode Output/OpAmp zbst Homework Help 2 03-25-2008 12:10 AM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 11:37 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.