# Differential amp

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Sep 26, 2012.

1. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
What is the input on pin2 and pin3 and what is the vo
thanks AND IF I NEED THE VO TO BE 10 V WHEN THE INPUT VOLTAGE 1000V WHAT IS THE VALUE OF RESISTORS IN THIS CASE

• ###### DIFF.PNG
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Last edited: Sep 26, 2012
2. ### Dodgydave Distinguished Member

Jun 22, 2012
4,782
707
Your drawing is wrong , where is the ground in relation to the battery input?

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3. ### ramancini8 Member

Jul 18, 2012
438
114
Your circuit doesn't make sense. What are you trying to do?

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4. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
sorry i forget the ground ,i attach new drawing

5. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
i change the drawing can you look to new schematic
thanks

6. ### Dodgydave Distinguished Member

Jun 22, 2012
4,782
707
Pin 2 will have 0.6v as the diode d2 is forward biased and the wrong way round, what are the voltages of d1&d2

7. ### crutschow Expert

Mar 14, 2008
12,537
3,071
The circuit is a differential input design so the differential gain is R5 / (R3+R4) for ideal resistors (much less than 1 in this case). Vo thus is equal to the total value of the input voltage (V1-V2) times this gain value. I'll leave the calculations as an exercise for the reader.

Note that for real resistors you will need to take the resistor tolerance into account.

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8. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
v1-v2=0v or 600v and what is the voltage across d1 and d2
thanks

9. ### TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
and how can i make vo =10

10. ### Dodgydave Distinguished Member

Jun 22, 2012
4,782
707
Last edited: Sep 26, 2012
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11. ### ramancini8 Member

Jul 18, 2012
438
114
The sad news is that unless the external components are matched you get an error because the mismatch creates a differential signal. If you use 3% resistors, and the mismatch between input resistors is 3%, you get an 18V error. Differential amps are very hard to make.

12. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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I'll believe 18mV error.

13. ### crutschow Expert

Mar 14, 2008
12,537
3,071
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14. ### Ron H AAC Fanatic!

Apr 14, 2005
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I think Dodgydave forgot about the feedback.

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