OPA695: Pspice model for RF simulation

Hi Wu,

what is the purpose of this circuit?

Kai

dear kai,

this circuit is for generating a negative resistance, and at the output port, maybe there are also some inductor or capacitance in parallel for impedance matching.

sorry, I'm not familiar with the amp theory. But in wiki, this circuit is proved. 

https://en.wikipedia.org/wiki/Negative_impedance_converter

hope this link helps.

dear kai,

sorry for that, I got the wrong annotations between 'simulation' and 'measurement'. 

The red one should be the 'simulation' result, and the blue one is the 'measurement' result.

wu

Hi Wu,

keep in mind that the OPA695 is no usual OPAmp but an ultra-wideband current-feedback OPAmp. So there might be some restrictions on the workable circuit topologies. Also, the value of feedback resistor R2 is critical for stability. You cannot take any desired value.

Can you show a complete schematic of your circuit, so that a transient analysis and frequency response analysis in TINA-TI can be carried out?

Kai

Hi Wu,

seems not to work at higher frequencies:

wu_opa695.TSC

Is there a special reason why you have chosen the OPA695? Could you take another OPAmp?

What is your frequency range of interest? What are the specifications of 1.94µH inductance (series resistance, self resonance frequency)?

Kai

dear kai,

thanks for your kindly reply. Actually I want to use an amplifier circuit to achieve a negative resistance '-35 + j0' at a certain frequency (above 15MHz, i.e. 20MHz).

And I have used OPA691, it is ok for several times. But it is not stable.

and for the inductor (1.94 uH), it is the impedance matching part on my pcb board to achieve negative resistance. In real measurement, this kind of circuit topology indeed works. But it didn't fit well when R3<50 Ohm. This is also my problem.

Hi Wu,

you could try to add a very small capacitance in the feedback loop of OPA691:

wu_opa691.TSC

I know that it is a no good idea to do this with a current-feedback OPAmp but who knows...

You could also play a bit with a load capacitance and/or a load resistance at the output of OPAmp. Just an idea.

Kai

dear Kai,

thank you for your advice! 

Follow your guidance, I have tried diffrent kinds of OP amps based simulation in TINA-TI. (i.e. different feedback type; different unity gain bandwidth; different power supply)

And, here is the result:

I found that OPA 355 (voltage-feedback amplifier) powered by dual voltage seems to behave as ideal below 100MHz when comparing to others. Can I draw this kind of conclusion from the above result?

Another thing is, I found that the model OPA 691 in the TINA-TI (version 9.3.150.328) isn't the same as which one you used. And I downloaded new one from TI website, it suggests only this model in the TINA-TI (version 9.3.150.328) would be an old version.

Wu

Hi Wu,

very interesting!

For a correct simulation I would need the DC resistance and self-resonance frequency (-> winding capacitance) of 1.94µH inductance and the source impedance of VG1.

Kai

Hi, Kai,

The voltage source impedance is 50 ohms.

I chose the Murata inductor. 3.3uh and 4.7uh are parallel to produce 1.94uh. This is the parasitic parameter:

After adding these parameter, here is the simulation result:

Although OPA355 fits the ideal one well below 100MHz, yet the gain parameter isn't stable.

Wu