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# TINA/Spice/LMH6505: Simulation configuration to determine open loop gain & phase response (stability)

Part Number: LMH6505
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

I need to simulate the open loop gain & phase response of the LMH6505 to see how different Rf and Rg affect stability.  Can someone provide the simulation configuration to do this?  I am trying use a method similar to the one shown here:

Also, in Application Note OA-25 (Stability Analysis of Current Feedback Amplifiers, National Semiconductor May 1995), Figure 3 shows a simulation configuration for a standard CFA.  How do I adapt this configuration for the LMH6505?

Thank you.

• Hi,

FYI, one of our team will take a look at this

Herman
• Hello RM1,

Unfortunately, this part is not a typical voltage feedback or current-feedback amplifier and includes elements of both that is internal to the model and device. This makes getting an open loop response and phase for this part very difficult. As shown in the schematic below, by inducing a current into the feedback pin and measuring the voltage at our output we can achieve a rough estimate for the open-loop trans-impedance of the current-feedback amplifier in the device (Zol).

The blue line represent a slope of 20 dB/dec. We can see here that a second pole is introduced at around 44.92dB, converting this into Ohms we find a resistance of around 176.2 Ohms.

From this, we can then determine that Rf must be greater than 176.2 Ohms to ensure stability of the Current Feedback Amplifier. The data sheet recommends keeping the max gain of the device between 2 to 100 V/V. Using these two parameters you can find the values of Rf and Rg that should keep the LMH6505 stable. Note that the values inferred from the model are rough estimates and should not be taken as guaranteed results for a real case scenario.

For greater detail on the stability analysis of both Voltage and current feedback amplifiers, I would recommend watching the TI precision labs videos regarding these topics. I've linked these below:

https://training.ti.com/ti-precision-labs-op-amps-stability-1?cu=14685

https://training.ti.com/ti-precision-labs-op-amps-current-feedback-amplifiers-overview-and-compensation-techniques?cu=14685

Hope this helped!

-Hasan Babiker

RM1.TSC

• Hasan,

Thank you for your help with this.  It is much appreciated.  In my circuit, Rf is 22.1k and Rg is 1.69k.  Maximum gain is about 12.2.  To test for stability, I applied a 50mV step change to the input of the VGA.  I didn't see any overshoot or ringing, so the amp looks stable.  Can this be proven via simulation?  Application notes on CFAs state that increasing Rf increases stability.  So, with 22.1k, it should be very stable.  Do you agree?

Thank you.

Randy

• Hey Randy,

While it is true that increasing Rf increases the phase margin of the device, it also reduces the devices' bandwidth (More on this on the CFA videos I linked before). Unless there is a reason you would need to greatly increase the phase margin and stability of the device (such as driving capacitive loads), I would recommend you stay within the range of the suggested Rf in the datasheet (1k). This is to ensure best functionality of the LMH6505. In terms of simulations, I provided a TINA sim in my last post that you can modify to see the step response of the part.

Best,

Hasan Babiker