# How to analysis the loading effect of this kind of circuit?

1. ## How to analysis the loading effect of this kind of circuit?

I am trying to design two amplifiers in this kind of system, and I think I need to test the open loop performance of the amplifiers before applying them to the system.

could anyone tell me how to find the loading of these two amplifiers in this kind of system?   Reply With Quote

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2. ## Re: How to analysis the loading effect of this kind of circuit?

do you mean what load this circuit presents to the 2V-3V input?
do you mean the load this circuit can drive?

do you mean the capacitor on the read device is 100 farad?  Reply With Quote

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3. ## Re: How to analysis the loading effect of this kind of circuit? Originally Posted by wwfeldman do you mean what load this circuit presents to the 2V-3V input?
do you mean the load this circuit can drive?

do you mean the capacitor on the read device is 100 farad?
It is a amperometric sensor readout circuit used in electrochemistry, the readout circuit is a TIA which integrate the current prom previous stage and transfer it in to voltage. The current will be changed if the voltage of the input amplifier change. The huge resistor and capacitor is the equivalent model of chemical solution. 100fF is the capacitor used for integration.

I think the loading effect of the huge capacitor and resistor should be considered when designing the amplifiers. But I do not know how to analysis this loading effect.  Reply With Quote

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4. ## Re: How to analysis the loading effect of this kind of circuit?

the capacitors are 15 nF and 100 fF or 15 x10^-9 and 100 x 10^-15.
hardly what i would call huge.

you're question is a little out of my expertise
i expect someone will help out  Reply With Quote

5. ## Re: How to analysis the loading effect of this kind of circuit?

The 150 ohm resistance is always in the path between the op amps, thus it is the ohmic portion of the load.

As for capacitors, they present zero resistance for a brief time after a change in current flow. The definition of capacitance is that it impedes a change in voltage.

An op amp output has internal resistance. There may be a certain value of resistor at the output however there are also the output transistors. They change their own impedance, so as to create a certain voltage at the output. They need to set a certain current level, either sourcing or sinking current.

The speed at which your capacitors respond depends on the unseen and unknown resistance in your op amps.  Reply With Quote

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6. ## Re: How to analysis the loading effect of this kind of circuit? Originally Posted by BradtheRad The 150 ohm resistance is always in the path between the op amps, thus it is the ohmic portion of the load.

As for capacitors, they present zero resistance for a brief time after a change in current flow. The definition of capacitance is that it impedes a change in voltage.

An op amp output has internal resistance. There may be a certain value of resistor at the output however there are also the output transistors. They change their own impedance, so as to create a certain voltage at the output. They need to set a certain current level, either sourcing or sinking current.

The speed at which your capacitors respond depends on the unseen and unknown resistance in your op amps.
The Input voltage will be a step voltage not a sine wave. I have tried some single stage OTAs (single pole OTA usually don't have stability issue), but the integration voltage is not a straight line, which means I can not calculate the input current through output voltage when the integration time is fixed, I think it is a large signal problem rather than small signal. And I also don't want to spend a lot of time on resetling the capacitor. A multi-stage OTA might work, but they also have the risk of oscillation if I do not model the system correctly.  Reply With Quote

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