It's hard enough finding the right tuning capacitor for the AM BC band. The AM band goes from 550 to 1710 kHz, a ratio of just over 3:1. The capacitor needs to have a range of 9:1 as the capacitance varies as the square of the frequency, so the common value of 50 to 450pF just doesn't quite cut it. It gets worse as you go down to the longer wavelengths and adding fixed capacitance reduces the tuning range even more.
To follow up on ZTZ's comment;properly building such an amp - there are plenty of plans on the net for this - takes some doing. If this isn't up your alley, there are a couple of commercial wideband loops - quite aside from the pricey Wellbrook and Pixel - one of which has been attracting a good bit of attention lately; this would be the W6LVP active loop. MFJ also has one, although I haven't seen nearly the attention paid to it. Links for both can be found in our loops wiki, viz. (links are always blue)
To explain this properly, one needs a good understanding of resonance and loop theory - which in a forum with a 5000 character or so limit, simply can't be done. Get the books that ZTZ recommended in message 3 of this thread. They'll do a far better job than any explanation we can give here.
That's why I said "It's hard enough finding the right tuning capacitor". You can easily add capacitance but it's the lowest that's the problem. The bigger airspaced variables as I said are usually around 50 to 450pF because that is the "right' value for old tube radios to cover the AM BC band - the more modern polyvaricons must have a very low minimum if the maximum is 365pF. Another problem is reducing the self-capacitance of the loop coil.
The 365 uuF variable is ubiquitous because it is commonly used in AM broadcast band radios. (btw, it is also available in 2-gang and even 3-gang versions).
Otherwise, lots to consider here. You have to be able to resonate (tune) the loop at the desired frequency as well as match its impedance to the receiver. The Loop Antenna Handbook by Carr mentioned in my post (#3) devotes an entire chapter each to large loop and small loop antenna theory.
This process can be fascinating or frustrating depending upon how you approach it. My advice is to do some reading and perhaps for your first project, design and build a small loop for just the AM broadcast band. You will probably find that it will take some doing and experimentation to get things just right. Good luck and let us know how it works.