An example apparatus includes: an on-off keying (OOK) modulator including: a first transistor including a first control terminal; a second transistor including a first current terminal, a second current terminal, and a second control terminal, the first current terminal coupled to the first control terminal; a third transistor including a third current terminal, a fourth current terminal, and a third control terminal, the third current terminal coupled to the first control terminal; a fourth transistor including a fifth current terminal, the fifth current terminal coupled to the second current terminal; and a fifth transistor including a sixth current terminal, the sixth current terminal coupled to the fourth current terminal.