Im working on a junior project in which we need to buffer an input signal from a RX antenna, pass the signal through 3 filters in parallel, and pipe the outputted signal into a RTL-SDR. It serves very little practical purpose as i know that the filtering could be accomplished digitally, but for the project we need to design an analog component.
My analog component is essentially going to comprise of: -> Recieve signal from antenna -> sma connector to pcb board -> input buffer op amp(Ive read these are important at buffering our input) -> active parallel filters(1 x band-pass, 1 x high-pass, 1 x low-pass) -> op-amp summer -> output sma connector (will be connected via short sma cable to SDR)
The frequency range we are going to be sampling is 700KHz - 1300Khz (part of the AM radio band).
This is my first exposure into looking at op-amps and researching the right ones to purchase for this project. In our lab i found a few LF357N op-amps and saw on the data sheet that they had a wide band-width of 20MHz. I figured i would try to breadboard a quick circuit to test the feasibility of this design. All i was trying to achieve was unity-gain using a simple non-inverting design. ** I understand that there are huge problems once we start getting into high frequencies in terms of signal distortion and loss on lines, especially bread boarding. This is why i wanted to make a mock up design before eventually trying my hand at creating a pcb for the final design of this project.
After i was having issues with this, i went to Multisim and started simulating the most simple circuit i could with the LF357N.
What ive seen is that while unity gain seems to work at very low frequency(10Khz), once i bumped my frequency up to 1MHz, the gain skyrocketed to tens of thousands. Then i threw in 2 resistors to give myself a gain of 2, which worked very well. But as i increased the frequency in 1MHz increments, i noticed the gain was affected adversely again.
Now, my question is why? Is the LF357N not unity gain stable at high frequencies? Even though the datasheet said the bandwidth goes up to 20MHz? What should i look for in op-amps for buffer amps?
Will the same op-amps work for buffering and summing? Or will i need to find a different style op-amp for both of those operations. My understanding is that i will just need 1 kind, as long as the Unity-Gain-Bandwidth is high enough.
Thank you for any help you can give me! I know this may be fairly trivial information, but Ive been searching the web for a few weeks trying to figure all this out and figured i would finally ask a question on it.