Hello, I am trying to do a Howland Current Source for a project and I am not getting what I am expecting.
(I am sorry if I made any English mistakes, I am from Brazil and I am not so fluent yet.)

The input voltage of my system is 3V (but it may be necessary to change it), AC, and it has to work for frequencies between 50 kHz and 3 MHz (this is very important!). I want 250 uA in the output, so R3 needs to be 12 kohms. I also used all others resistors with the same value. I'm using the amp-op LM6132.

I tested the circuit in the protoboard. For 50 kHz it worked very well, the current stayed between 260 uA and 280 uA (I think it is already good) , for impedances until 10 kohms. When I tested for 400 kHz the current started to vary a little, and as I increased the frequency the current varied more as I varied the impedance. I would like suggestions about why it did not worked, or what I could do, I tried the Howland current source but it could be another circuit. I attached this image of the simulation I did in Multisim:

 

The problem you're seeing is due to the limited gain-bandwidth product (GBW) of the LM6132, which is only 10 MHz. If you want operation up to 3 MHz you'll need a much faster op amp; I'd suggest something with a GBW of at least 100 MHz, such as an LM6171.

 

A gain-bandwidth of 10MHz means the op amp only has a gain of 3.3 at your desired 3 MHz operating frequency.
You circuit needs much more gain than that for proper operation of the feedback loop in the Howland circuit.
That's why you need a significantly larger gain-bandwidth op amp as OBW0549 suggested.

At 3MHz you may have a problem with the stray capacitances affecting the response when using 12kΩ resistors, so you may have to use lower values also.

 

At 3MHz you may have a problem with the stray capacitances affecting the response when using 12kΩ resistors, so you may have to use lower values also.

Good point; I missed that.