AM Transmitter using Colpitts Oscillator

[Frank Casey]

I am experimenting with AM transmitters, I found this Colpitts oscillator circuit and have found the right inductor which produces a frequency of about
1,246,458 Herz which is what I want (in the middle of the AM frequency band).

I cannot figure out where to connect the audio input.

I tried connecting audio to the base of the transistor and also to the output, neither works.

Can anyone assist? I am an RF electronics newbie.

 

[MikeMl]

You need to vary +9V node in the circuit above from ~2Vmin to ~ 16Vmax in step with the audio waveform. That is called high-level Amplitude Modulation. It takes an audio amplifier to do that. When used that way, the amplifer is called a "modulator". That's what it takes to make an AM signal.

Playing with the bias on the base of the oscillator transistor might vary the frequency of the oscillator, and at best, produce a crude form of Frequency Modulation...

 

[jpanhalt]

Here's discussion of a circuit on this forum: https://www.electro-tech-online.com/threads/low-power-am-broadcast-band-transmitter.100706/

It is supposed to produce AM.

 

[ChrisEHunter]

I am experimenting with AM transmitters, I found this Colpitts oscillator circuit and have found the right inductor which produces a frequency of about
1,246,458 Herz which is what I want (in the middle of the AM frequency band).

I cannot figure out where to connect the audio input.

I tried connecting audio to the base of the transistor and also to the output, neither works.

Can anyone assist? I am an RF electronics newbie.

View attachment 109647

Any attempt to directly modulate that oscillator will result in frequency (rather than amplitude) modulation. You need to add an amplification stage to the output of your oscillator, and vary the supply voltage to the amplifier according to the audio that you want to impress upon your carrier. Another problem with your Colpitts oscillator is that it will wander in frequency as the battery supply drains, as the components warm or cool, or even according to the relative humidity! A stable, "free-running" oscillator isn't trivial to construct, so you're always better off using a crystal to determine your frequency. This guarantees frequency stability.

Look around the 'net for AM transmitter circuits to get an idea of the topology of a medium wave AM transmitter.

The simplest AM transmitter circuit that I've used successfully uses a crystal oscillator driving a modulated final stage. The AM is achieved by using the output of an LM386 audio amplifier IC as a "modulated supply" for the output transistor. If you want a circuit that works, let me know!

 

[ChrisEHunter]

The attached is possibly the simplest MW transmitter that works properly, which includes the three basic component parts of an AM transmitter - an oscillator, a Class C power amplifier stage and a modulator.

 

[unclejed613]

i've built successful AM modulators using an audio transformer, with the primary connected to the audio source (a speaker-level output), and the secondary connected in series with the power supply for the RF amplifier (operating Class C). on the RF amplifier side of the secondary, there should be a bypass capacitor to ground, which keeps RF from being coupled back to the audio amplifier (audio amplifiers don't like RF on their output devices). the bypass cap is extremely important, and gets left out of most diagrams, and if there's enough RF coupled back to the amplifier, it can cause "common conduction" in the output transistors, meaning that the RF turns both transistors in the audio amplifier on at the same time, resulting in a large current through them, and they short as a result. if you notice in this schematic:

there's a 100nF cap from the audio transformer secondary to ground. the best way to isolate the RF from the modulator would be to have an RF choke between the top of the 100nF cap and the RF coil connected to the drain of the output transistor. and a cap across the RF coil to make that a resonant circuit. essentially what the audio transformer does, is change the power supply voltage on the FET at an audio rate, which results in an AM signal.