Spice circuit to Find Aol and Noise Gain

Hi Eve,

Art is very busy preparing materials for an upcoming trip so I will do my best to assist you. I discussed the problem that you are seeing with the OPA627 I-V simulation with Art. He provided me some information that should help you obtain correct simulation results.

You had set the circuit up correctly in TINA and received simulation results were applicable to what was shown in the original article schematic. I received the same results when I simulated the circuit. It turns out that the voltage and current noise sources were excluded from the published schematic. When included, the circuit simulates correctly providing the correct I_to_V_gain response.

You can see the TINA responses in the diagram shown below.

Regards, Thomas

PA - Linear Applications Engineering

Hi, Thomas

      Thanks very much for your kind help~

      We add en and In of the op amp to the circuit, then we get the right I-V gain . I am wondering how it works? Would u please give me some hints?

Regards!

Eva

Hi， Thomas

     I has a question, in your circuit ,the 'IVgain' curve is from VF3/AM1? Because I recevied a different result when I simulated the circuit you attached , as shown belown, where the 'Gain' curve is from VF3/AM1.

Regards!

Eva

Hi Eva,

After adding the noise generators to the OPA627 I-to-V circuit and seeing that the circuit simulated correctly I wasn't able to reconcile why that was the case. I had been making other changes at the time and may have changed something else unknowingly. So this morning I rebuilt the circuit without the generators and started fresh. Right from the start the circuit simulated correctly with an I_to_V_Gain of 120 dB. That result tells me the generators aren't needed to get a correct simulation.

I went back and re-simulated the circuit using your file and received the faulty I_to_V_Gain of 256 dB. While attempting to make changes to your circuit I observed that when I tried to disconnect, move and remove the current meter that extraneous wires popped back into the schematic. I think something about the drawing is corrupted and was leading to a false result. Below you will find your circuit redrawn and the associated gain vs. frequency plot from simulation. I have attached the TINA file for your use.

Regards, Thomas

PA - Linear Applications Engineering

Eva,

The transimedance gain within the pass-band should be 120 dB. That is the result I received with the OPA627 simulation circuit I sent you today.

The article Fig. 11.22 shows and I_to_V_Gain of 100 dB. That appears to be an error and is the value you would obtain with a 100 k feedback resistor. It should be 120 dB with a 1 Meg feedback resistor.

Regards, Thomas

PA - Linear Applications Engineering

Hi Thomas, 

     Thank u ~

     Yes ,you are right ,with a 1 Meg feedback resistor we should get a 120dB IVgain. Fig.11.22 is not the right simulation plot for Fig.11.20. But in the article , it says

                                                                       I_to_V_Gain=VF3/AM1 .(in the same page of Fig.11.22 ) This is correct ,right?

     So,it's strange that I till received a 299dB Gain when I use VF3/AM1 to generate I_to_Vgain in the post-processing with the circuit you attached., just like I posted  in my last reply. But the IVgain curve you received is right 120dB. How did you define the IVgain in post-processing?

Regards!

Eva

Hi  Thomas,

    In order to make things clear, I post some pictures below. The simulate circuit is you sent me today.

First I add the Gain curve  :

 The simulation plot I got:

Regards!

Eva

Hello,

Here is the expression Thomas has used for I-V gain: "(VF2 - VF1)/AM1". This is nothing but the impedance between the inverting input and VF2 and it is expressed as the voltage / current (= impedance).

With the loop open, the Transimpedance gain can only be done as the impedance between nodes VF1 and VF2.

Regards,

Hooman

Hooman,

Thank you for the assistance, excellent explanation, and TINA illustration of the post processed curves.

Regards, Thomas

PA - Linear Applications Engineering

Hi Hooman,

Thanks  for your clear explaination.

So, in the article there may be some errors.

Thanks again!

Regards!

Eva