# Problem with connecting inverting amplifier with an integrator

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#### firsttimedesigning

##### Full Member level 1
so i tried to connect inverting amplifier with an integrator but the integrator output comes out weird. it goes to 2M Volts...

the first stage is an inverting amplifer...
the second stage is a voltage buffer...
the third stage is an integrator...

the integrator works by itself however, when i connected to the output of voltage buffer, it doesnt work.

R1 = R2 = R3 = 100 ohms
C = 100n F

please tell me what did i do wrong...thanks....

#### Eugen_E

##### Full Member level 6
Re: circuit design

I think the resistor values are too low. Try with at least 1k, better 10kOhms.

Since the (+) input of the inverter and integrator is connected to ground, the input impedance of the inverter and integrator are just 100 Ohms.
So, if the input voltage is 5V, the current from the opamp outputs would be 50mA, well above the maximum current for general purpose opamps. The output current would be limited to something about 20mA, for a general purpose opamp, and distort the signal.

Also, specify what type of opamp you use. With a real opamp, the output voltage would increase to about Vcc or Vee. But for an ideal model the voltage may increase continously. Use a real opamp model.

With an integrator, if you apply a constant voltage at the input, the output voltage will increase in time - since it integrates it over time. The output voltage will be limited to about Vcc or Vee. Usually the integrator is initialized and used periodicaly, since it can integrate the offset and saturate.

Put a high resistor (1-10MOhms) in paralel with the capacitor to limit the gain at DC.

What is the role of the buffer here?

### firsttimedesigning

Points: 2

#### firsttimedesigning

##### Full Member level 1
Re: circuit design

thank you very much for replying...
tried 10k...still doesnt work...it gives me back the same result...
i like to put a high resistor in parallel with the capacitor...
however, eventually i have to put try to put this circuit and other circuits into a chip...
a high resistor 1M ohms probably could not fit into the chip...
i am using a voltage controlled voltage source to model the op-amp,
so the op-amp can be thought as an ideal op-amp...

any other idea why the circuit doesnt work?
i really dont get it...

#### watersky

##### Member level 5
circuit design

I think you can use ideal amp to validate this circuit. I guess it should work normal. so you should check your amp. if dc point of amp'input is not normal, this circuit fails.

#### betty1007

##### Member level 1
circuit design

I think the resistor values are too low

#### rajanarender_suram

##### Full Member level 4
circuit design

try to give VCVS a practical value of gain(not too as as infinity) and try. give around 1000-20000

#### engronger

##### Member level 2
Re: circuit design

1: the wire you connect to ground is a right connect just when the schematic have +/- power supply, so if you have only a positive power, the output must be error

2: when you give the input to integrator by yourself. is it work well? if yes , maybe the output of buffer give a error static work point; if no, please check your integrator

firsttimedesigning said:
so i tried to connect inverting amplifier with an integrator but the integrator output comes out weird. it goes to 2M Volts...

the first stage is an inverting amplifer...
the second stage is a voltage buffer...
the third stage is an integrator...

the integrator works by itself however, when i connected to the output of voltage buffer, it doesnt work.

R1 = R2 = R3 = 100 ohms
C = 100n F

please tell me what did i do wrong...thanks....

#### jjsnail

##### Junior Member level 1
circuit design

~ the resistor values are too low

#### richod

##### Newbie level 1
Re: circuit design

yes, the resister is too small. I use the resister 100K

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