Dear TI :
I am trying to implement a circuit as shown
the following Bode plots
I made a PCB, and choose the component as simulated circuit, but the oscillations phenomena appears without input.
this is my PCB layout and screenshot of spectrum analyzer
I want to know why the oscillations phenomena appeared and how can get the result as Bode plots of the simulation .
thank you very much
Hi Xin,
The circuit you have designed using the OPA847 is inherently unstable.
The OPA847 is stable for gains greater than or equal to 12V/V. Since, the circuit you have developed is unity gain, you will have excess phase shift (>180') at the frequency where the open loop gain intersects with the noise gain of the circuit leading to oscillations. If you look at the intersection of noise gain (1/Beta) and the open loop curve of your circuit, there are three potential points of oscillation (as shown below) where their slope of intersection is greater than or equal to 40dB/decade (or exhibits > 180' phase shift).
Are you trying to achieve some sort of band-pass filter with the circuit? There should be a simpler circuit than this for what you are trying to do. It would be better if you could describe in detail what you are trying to achieve in-order for me to help you further.
Best Regards,
Rohit
Dear Rohit,
Thank you very much for your reply. I really appreciate it.
As you see from the Bode plots, I want to implement a circuit with the function: In the range of 1MHz to 200MHz, with the frequency increased, the gain increased, too. And the gain of low frequency still above 0dB.
I fund the circuit on "Op Amps for Everyone,4E"(written by Bruce Carter)，on the charter6, This meets my requirements
So I made a simulation on TINA, and set the values of R and C to make a peak gain on 200MHz. then make a pcb, and Oscillations.
if you can give me a another circuit which can achieve it. I would be very grateful to you.
