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posted by  Rescue me on 10/10/2008 3:39:07 PM  |  status: Live  

iA

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Question Details:
The op-amps in the circuit shown are ideal. If Vin = 0.2 V, R1 = 12k Ohm, R2 = 120k Ohm, R3 = 50k Ohm, R4 = 3k Ohm, and R5 = 60k Ohm. Find the value of the current Ia in micro amperes.
 
 
correct answer -28.490
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posted by Shree_vani on 10/11/2008 9:16:35 AM  |  status: Live
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Response Details:
Let Vo1 be the voltage at the output of the first opamp circuit.
Let Vo be the voltage at the output of the second opamp circuit
 
Considering the opamp to be ideal.
The base currents at the inputs of the opamp is ˜0
The voltages at the inverting input is equal to the voltage at the non-inverting input due to virtual short/ground
 
Applying KCL at the inverting node of the first opamp circuit
 2.4Vo1  + Vo = 2 ----------------- (1)
 
Applying KCL at the inverting node of the second opamp circuit
21Vo1 - Vo = 0 ------------- (2)
 
Solving (1) and (2)
23.4Vo1 = 2
Vo1 = 0.085V
Vo = 1.785V
 
ia = Vo1 / R4 = 0.085 / 3000
ia = 0.0283mA
ia = 28.3μA
 
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