Need perfect AM Transmitter circuit?

[Shyamal]

I want too make an AM transmitter circuit for my college project. I know that AM Tx have three parts -

1. Signal amplifier circuit
2. Oscillator circuit
3. Modulator circuit

For this reason, i choose colpitts oscillator. I don't understand that why my oscillator is not working?

My question are that for AM transmitter circuit ----
1. My total circuit is right?
2. Can i use colpitts oscillator?
3. Which microphone pre-amplifier is perfect for this circuit?
4. Which kind of modulator circuit i use?

I am new so have many basic problem?
Please give me some information about this small project.

 

[Nigel Goodwin]

The emitter of the 'modulator' doesn't have any DC connection, so can't work.

 

[Shyamal]

Thanks,
Now how can i solve this?
Please tell me.

 

[Nigel Goodwin]

Where did you get the circuit from?.

 

[Shyamal]

I'm very much interested on Electronics. This is my hobby. I'm also a 2nd year student (Electronics and Communication Engineering) of my college. Still studying on this subject from 2 month ago. So my basic knowledge is not so good. Not only my own interest but also college project i make it.

I have many collection of this circuit diagram which i collect from internet and trying to understand. But, similarly trying to make a new circuit which come out from my own knowledge.
For this reason i make it and trying to evaluate the problem of the circuit.

This is my circuit that i make on project board ......

 

[4pyros]

RF circuits may not work so well on project boards.

 

[panic mode]

bread board has parasitic capacitances (pF) which can interfere with some circuits. but in this case they are small enough to be neglected as frequency is low enough and used capacitors are significantly larger. one important thing when building RF circuits is to keep the connections short (the shorter the better). this too is increasingly more important with higher frequency. to get optimal performance, i'd probably pack circuit to no more than one quarter of shown breadboard. as mentioned above, you will want to properly bias emitter of transistor used in modulator (connect lower end of R8 to GND, get rid of C10, connect lower end of C6 to collector of audio amp (lower end of R12).

 

[Shyamal]

Where i will connect the output of the amplifier of message signal?

 

[panic mode]

use capacitor (C6) from collector of "message" transistor to emitter of "modulator".

note, i didn't have time to analyse circuit, this is just what modifications seem reasonable to get the modulator to run.
you could bring the signal also to base of that transistor.

what i don't see is a way to set modulation depth. i'd rather be able to do that at least until circuit is proven as stable.
you don't want to over-modulate and distort output. where did you get this circuit?

 

[Shyamal]

Thanks,
I understand that you told. I will try it and also notify you after my work.

I make this circuit which come out from my own small knowledge.

 

[Dragon Tamer]

Bread boards have parasitic capacitance yes, but it should have little effect on a lower frequency device like an AM transmitter. I have mad AM transmitters on bread boards before with a fair amount of success. There is only about 5pF of capacitance between each column.

Try using a smaller value of C6 and move it to the collector-emitter junction of the 2060 transistor. Move the output from the mic preamp and the audio input to the base of the 2060 transistor decoupled with a 100nF capacitor (10nF to 100nF should be sufficient).

 

[Shyamal]

Thanks to all. I am successful to make the circuit.