An amplifier that works in reverse?

[kthouz]

I am making a circuit of a car toy.
Actually, i have already done its first step. The next step i want to make a one which is able to reverse and i think i will need an amplifier which can work in both biases (foward and reverse). So do exist amplifiers that can work in reverse as well as in foward bias?

 

[vk6kro]

Yes, it is a H-Bridge amplifier

Have a look at this:
http://www.bobblick.com/techref/projects/hbridge/hbridge.html

Basically, you have 4 transistors in a bridge circuit with the motor across between the centre points of two pairs of series transistors. You then turn on transistors that are diagonally opposite each other. Doing this causes the current to flow in opposite directions in the motor.

Alternatively, you could just reverse the leads to the motor with a set of relay contacts.