# Transient response of integrator circuit from t = 0

Discussion in 'Homework Help' started by Muhammad Danyal, Jan 3, 2015.

Jan 3, 2015
2
0
I am new to the forum. I want to ask my first question. What is the output of an integrator during transient when a switch (connected to dc 1.5V) is closed at time t=0?
I guess the input is a ramp function and the output will be a parabolic function.

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2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Welcome. The moderators will likely move your question to its own thread. This looks like homework?

You are guessing wrong. Let me ask you this: What is the current in R1 after the switch closes? Once you know the current, the output voltage is the simple integral of that current.

Jan 3, 2015
2
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yes it is a question from exercise. The current through the resistor is 0A due to infinite input resistance. so what is the output?

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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No it is not. Write a KCL equation for the inverting input of the opamp.

5. ### crutschow Expert

Mar 14, 2008
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What resistance do you think is infinite?

6. ### MrAl Distinguished Member

Jun 17, 2014
2,573
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Hi,

The input resistance to the OP AMP is often considered infinite for basic calculations, but the input resistance to the CIRCUIT is not infinite because the inverting input is considered a 'virtual ground' and so current flows from the battery though the resistor and then to the virtual ground.

The output is the time integral of the input voltage times a constant. If R=1 and C=1 that constant would be equal to 1, but in this case R and C are not 1 so it is a different constant that you have to find.

If the input voltage was really a ramp input and not the constant voltage battery then the output would be parabolic, but the input is not a ramp here it's a constant.

What techniques did you use in the past so far, and what kind of math have you had already?