diiferential amp(p spice)

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Mar 5, 2014.

1. TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
I am trying to do simulation for differential amplifier by using P Spice ,but the output is not right ,can please someone check what is the problem
the output should be 10 Vdc

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2. crutschow Expert

Mar 14, 2008
21,146
6,014
What output are you getting? The differential gain of the amp is 100k/3010k = 0.0332 and thus the output for the 600V at the input should be 19.93V, not 10V.

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3. TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
thanks ,I got 14.4 Vdc,i attach the p spice

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Last edited: Mar 5, 2014
4. crutschow Expert

Mar 14, 2008
21,146
6,014
Of course. Your amp is saturated. It's trying to get to 20V but can only get to 14.4V with a 15V supply.

5. LvW Well-Known Member

Jun 13, 2013
778
105
Takymounir, your circuit cannot work as a diff. amplifier because the input signals have no ground reference.

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6. TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
the gain is 100k/6010 =.0166 so vo= 600 * .0166=9.983 is this right or no
I get 14.4 not 9.98

7. LvW Well-Known Member

Jun 13, 2013
778
105
Kilo-Volts, Milli-Volts, Volts or Amperes or something else?

Question: Did you correct the circuit (input ground reference)?

8. crutschow Expert

Mar 14, 2008
21,146
6,014
The input doesn't need a separate ground reference for this circuit. That's provided through R5, R6, and R7.

TAKYMOUNIR, the differential gain is 100k/3010K (look here under Differential Amplifier). Don't know how you arrived at your value.

9. ifixit AAC Fanatic!

Nov 20, 2008
650
118
The resistor values in the second circuit are different than the first circuit. 6 Meg vs 3 Meg for R3 & 6.

10. LvW Well-Known Member

Jun 13, 2013
778
105
Crutschow, thank you. You are right. There was something like an obstacle in my head. Perhaps, because the circuit looks rather uncommon?

11. crutschow Expert

Mar 14, 2008
21,146
6,014
OK, I missed the resistor values being changed in the second circuit. For that the calculated output should be 9.98V, and that's what my simulation also showed, so don't know why your simulation is different. What op amp is that?

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12. TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
sorry I change it it is 6 mega instead of 3 mega

13. TAKYMOUNIR Thread Starter Active Member

Jun 23, 2008
351
1
I use LF411,I WAS USING LETTER M AS MEGA AND THAT IS WHY MY SIMULATION WAS WRONG,SO SORRY FOR THIS

Last edited: Mar 6, 2014