Tuning Circuit of mobile phone signal jammer can be of two types: open-loop and feedback. Open-loop is quite simple and requires just a few op-amps with additional passive components. It is a saw tooth-wave generator which makes VCO to go from lowest to highest frequency. The feedback is using PLL to adjust the VCO’s frequency constantly. For this purpose you need to use microcontroller. It is programmed once and then connected to the VCO, thus it is not adjustable.
RF Amplification Unit is what you need when you wish to increase the area covered by your mobile phone jammer along with its signal blocking power. The more power has your signal blocker, the bigger radius it jams. The cost is, as you may guess, battery working time. More power equals less time for battery to live.
There is a huge selection of gain stage units on the electronics market (7dbm, 17dbm, even 20+dbm). But consider that more powerful amps are bigger and need better power supply.