I have to design a gain control to control a modulating signal of amplitude modulation.I'm using LM 324 to amplify the carrier signal.
Does anyone have anyideas ?

 

? Do you mean automatic gain control?; otherwise a pot works quite well. Look into analog opto couplers, [ lightbulb, or LED & Cd S LDR ].

 

A lousy old LM324 has trouble above only 2kHz so it can't produce all audio frequencies, certainly not radio frequencies.
What is your radio carrier frequency??

 

carrier frequency is 10khz and modulating should be low frequency.

 

A lousy old LM324 might not be able to produce a carrier of "only" 10kHz.
Most audio opamps work well up to 100kHz and there are many that can go much higher.

The antenna for that very low frequency is many kilometers long.

 

Design a circuit that will affect the gain of the amplifier when the modulating signal switches between zero and the maximum. A 555-timer circuit could be used to generate the modulating
signal. Alternatively, this circuit could affect the output of the amplifier directly, for example by “pulling it down,” when necessary.

I am having an idea of using a variable resistor by a FET but i'm stuck at bias voltage for the FET.
second idea is making a integrator circuit for modulating signal and take it as a fixed value of gain
Does anyone have any smart ideas ?