The circuit shows the diagram of balanced modulator using FETs. There are three transformers $T_1,T_2,T_3$ .

The carrier signal is applied to the center taps of input transformer $T_1$ and output transformer $T_3$ through the transformer $T_2$.

The modulating signal is applied to the input transformer $T_1$.

The carrier signal is applied to primary of transformer $T_2$.

This signal is further applied to two gates of FETs in phase through the secondary of $T_2$ .

The modulating voltage appears $180^0$ out of phase at the gates , since these are the opposite ends of the centre taped transformer.

Consider that there is no modulating signal applied .Then FET currents due to carrier signal are equal in amplitude but opposite in the directions.

These opposite and equal currents at the primary of the output transformer cancel each other .Hence no output is produced at the secondary of $T_3$.

Thus carrier is suppressed . When modulating signal is applied , the currents i_d1 andi_d2flow in the primary of T_3 due to carrier signal as well as the modulating signal.

The FET currents due to carrier are equal and opposite and cancel each other .

Since the modulating signal is applied $180^0$ out of phase at the gates , the FET currents due to modulating signal are equal but not opposite , hence do not cancel each other.

Therefore , output is produced at the secondary of the transformer $T_3$ .Thus DSB output is produced by FET balanced modulator.