What is the supply voltage?
What transistors do you have?
What is the input signal source (impedance)?
What load does the amplifier have to drive?
Can you tolerate a fixed offset voltage between input and output?
Ok, I see now that what you need is a NPN transitor used as a relay driver (not an "amplifier").
You didn't say what the 1.5V signal comes from? ( I need to know this to see how much current the source can supply to drive the base of the transistor)?
I also need to know the resistance of the 6V relay coil? (Measure it with an Ohmmeter, or look up the datasheet)