LM7171: Output Voltage is Ringing and not as per gain

Hi Selva Kumar,

After looking over your circuit you provided, I do have some clarifying questions. I was wondering if you could confirm the signal frequency. The reason I ask this is because at a gain of 5 V/V, the LM7171 is not fast enough for a signal frequency of 100MHz. In the TINA circuit your input was a square wave operating at 10MHz which the LM7171 can manage so clarification on that will be very helpful in finding you the correct amplifier for your application.

I would also like to point out that you are in a gain of 1V/V, which the LM7171 cannot do as it is stable in gains of 2 V/V and above. This is one reason the response is not whats expected. The 5000 at your feedback is also affecting the stability of the device because it is interacting with the input capacitance of the device, but regardless this device is not designed to be in this buffer configuration.

Another consideration when implementing operational amplifiers is their output headroom. In the case of the LM7171 the output can go up to 2V from each supply rail in a load of 1000-ohms. This means from a 0V to 12V supply like your circuit, the max the amplifier can output will be a voltage from 2V to 10V. This is why you were seeing 10V at the output. We recommend giving your design additional headroom, so the amplifier is not railing at the output. This is done by reducing your input signal voltage, increasing supplies, or reducing the amplifiers gain. The input common-mode voltage range is a similar idea, and both of these specs can be found in the electrical characteristics table in the device's datasheet.

For this reason, in a gain of 5V/V with a 3.3V input signal with 12V supplies, you will be violating the output swing of the device and it will not behave as expected. With a lower supply of 0V, you would also need to have a DC offset, so you are not violating the input common-mode of the device.

I broadly explained some of the important topics and design considerations needed for operational amplifiers, but we do have plenty of information available on TI.com and we have a video series that explains the ideas I brought up along with many others. The video series can be found here.

Best Regards,

Ignacio