Due to their extremely low static current consumption, varactors based on BST are perfect devices for realization of tunable and re-configurable components. This work presents fully screen-printed MIM thick-film BST varactors that are used to tune the load impedance for GaN HEMTs. The varactor tuning voltage is supplied in discrete steps using a high-speed GaN-based modulator. Modulated measurements with LTE and WCDMA signals show, for the first time, the functionality of a BST-based load modulation system and the power consumption of the load-modulation in dynamic operation. Using discrete dynamic load modulation, an average PAE of 27.3% was measured for the LTE signal with an ACLR below −45 dB.