single-ended BJT MIXER design help!

[e2]

please help me with my design on metal detector.i need to mix my two
sine inputs(few hundreds of KHz)and extract their difference.i opted
for the bjt.my questions are:

1.biasing
-- using the buil-in transistor of lm389,how do i bias it such that
it operate in "mixing" mode?

2.diode-based
-- can u share any other simple circuit to implement a mixer in the
low frequency range?


thanks!!!!