How To Build A Simple Single Transistor Amplitude Modulation (AM) & Demodulation Circuit (With Explanation Of How It All Works)
I ran into a roadblock. I am systematically going through “The Electronics of Radio” text by David Rutledge and answering the presented problems. Aside from buying the NorCal 40B transceiver kit, I have been able to use only equipment and components I have previously acquired for the first three problems. I hit a wall on the fourth one though. Problem 4, called “Diode Detectors” calls for an amplitude-modulated signal to be produced by a function generator. My Koolertron 60MHz signal generator does not modulate signals. Nor does my Ebay-special Topward 8112 20MHz signal generator. This has now put me in the market for trying to decide on a new (or new-to-me) waveform machine.
While I wrestle with indecision, I did not want to halt my progress with this project. I decided instead to create an amplitude modulation circuit from components I already owned, i.e., a smattering of parts I got many years ago from the EEEEE store on Amazon. In fact, early on in my now 6 1/2 year pursuit of the radio hobby, I bought one of nearly every package on that page. At the time, I had no idea what a rectifier diode or an op-amp was, I just knew I wanted to have a decent chunk of parts around, just in case. The resistor book is one of my favorite products of theirs. Now, for almost any circuit I want to build, I have a suitable array of components on hand. Keep in mind, this blog is not sponsored in any way, I have no relationships or affiliations with anyone at all connected with this site, and this is all my own personal stuff that I paid out of pocket for. As I always say, I just like talking about my gear.
I now needed to find a suitable AM modulation circuit that I could easily build. Like I said, I was only considering circuits that I had components available for, and in particular I wanted to find one that was simple to make and featured a run-of-the-mill BJT transistor. I came across this laboratory exercise from the Indian Institute of Information Technology’s ECE department’s Analog Communication Laboratory Course. I recreated the recommended circuit in LTspice (below).
AMPLIUDE MODULATION CIRCUIT
Let’s go through what this circuit does.
First off, amplitude modulation — or AM — is a process by which a relatively low frequency message signal, also known as a modulating signal, will be combined with a relatively high frequency carrier signal, so that a single resultant waveform emerges. The resultant waveform will have same frequency as the carrier signal but its voltage (i.e., amplitude) will change in proportion to that of the underlying message signal. Amplitude modulated signals, due to their much higher frequency than those frequencies of the messages they carry (often audio waves such as voice or music) are more convenient for over-the-air transmission, and form the basis of many of the voice radio signals found in everyday life (think AM radio). Once the AM signal is received, it can be demodulated back to its original message waveform.
Back to the circuit…
I am using my Topward 8112 signal generator to produce the carrier signal, or the signal that the message signal will be amplitude modulating. Initially, the carrier signal I used was a 3.2V 10KHz sine wave (as instructed in the lab) that travels through what is called a coupling capacitor (C1) whose job it is to only allow alternating current (AC) signal waves to pass through it. Direct current is blocked by C1 from either traveling through the capacitor toward the transistor (Q1) from the carrier signal generator (note, I wouldn’t expect DC to be present on the carrier signal created by the generator anyway), or from VCC (common collector voltage) through C1 back toward the carrier signal generator.
The VCC is +12V DC from one of my power supplies. The VCC provides what is known as “DC bias” to the transistor. DC voltage is an integral part of the operation of a transistor, especially one configured to be a linear amplifier (as this circuit is). DC bias is what sets the “Q-point”, quiescent point or also known as the operating point, needed for proper linear operation. Linear operation, in the case of a transistor amplifier, implies that a signal wave such as a sine wave, can be injected into the amplifier circuit and exit as a proportionally larger (or sometimes smaller) signal wave without distortion.
Resistors R1 and R5 form a voltage divider that essentially splits the DC voltage of the VCC between heading towards the base of the transistor (providing a base bias voltage) and towards the reference ground. This voltage divider configuration allows there to be differing DC bias voltages on the base and the collector of the transistor while using only a single DC voltage source. As suggested, the VCC also is able to — and does — provide a voltage bias to the collector of the transistor.
R4 is the collector resistor, and it serves to provide a load to the transistor so that a voltage drop can be established across it. It is this voltage drop that ultimately establishes the transistor’s output signal. R4 also works in conjunction with the R5/R1 voltage divider to establish the Q-point and plays a significant role in determining the gain of the amplifier.
Lets step back a bit before we wrap this up. This AM modulation circuit described here is built off of what is known as a “common emitter amplifier” utilizing an NPN bipolar junction transistor. I have rebuilt the AM modulator into the common emitter amplifier below. The “common emitter” nomenclature refers to the fact that in this circuit the potential of the emitter provides the reference ground, i.e., common, to the base and collector. The pink 10μF capacitor is called a bypass capacitor and essentially shorts any AC signals on the emitter to ground. The emitter resistor, R2, reduces the amplifier’s voltage gain while helping provide stabilization to the DC bias.
In total, the common emitter amplifier takes a signal in through its base, which is then amplified in the transistor, and outputted through the collector. If all of the biasing voltages are adequate, then the gain of the amplifier is linear, and the outputted signal is larger and phase-shifted 180° from the inputted signal. So if a small sine wave is inputted, an upside-down large sine wave emerges.
But what happens now if we change from the common emitter circuit back to the amplitude modulation circuit? Namely, what happens if you remove the emitter’s bypass capacitor and instead add an AC signal source between the emitter resistor and ground that has a relatively low frequency (much lower than the carrier frequency) and a varying amplitude (i.e.. voltage)? Well, now you have a situation where you have a higher frequency carrier wave that is being injected into the base of the transistor and amplified (and inverted) as would be expected in a common emitter amplifier circuit. BUT! The gain of the amplifier circuit is no longer stable, because the transistor’s emitter voltage is no longer stable. Instead, the emitter voltage is varying by the amplitude of the low frequency modifying waveform that has now been inserted into it, and as the voltage of the emitter goes up and down, so does the gain of the amplifier. The amplification of that carrier wave, which is directly related to the gain of the amplifier, now mirrors the amplitude (again, voltage) of the low frequency message wave. And if you draw a line from the peak of each cycle of the newly modulated carrier wave to the next peak, you would create the AM waveform “envelope” shape: A perfect, but inverted, replica of the message waveform, which could be received and demodulated back into its original form.
Now just to wrap everything up, the newly created amplitude modified signal emerging from the collector of the transistor, now with inverted voltages due to the 180° phase shift characteristic of this type of amplifier circuit, is sent through the output coupling capacitor, C2, which again eliminates any DC from passing along with it. A final load resistor, R3, provide a place for the final AM signal product to be detected. Or be sent on to the next stage of a larger project.
(Keep in mind that this is my understanding of how this works. This may not be a perfect representation of the truth, and I welcome feedback.)
Let’s go ahead and run this amplitude modulation circuit in the LTspice simulation. Below, the modulating signal is the green low frequency sinusoidal wave. The carrier signal is the red higher frequency sinusoidal wave. And the blue waveform is the combined amplitude modulated signal, which I think looks a bit anemic.
So I went ahead and upped the frequency of the carrier wave to 8kHz. The simulation of the circuit now looked like this.
And a little closer up…Keep in mind the blue wave now represent the higher frequency carrier signal. The red wave is now the modulating signal. And the green waveform is the amplitude modulated signal.
I built the circuit in real life on a breadboard as well.
The 8MHz carrier wave looked this on the oscilloscope:
The modulating wave looked like this:
And finally, the AM modulated signal looks like this:
AM DEMODULATION CIRCUIT
Now to actually demodulate the amplitude modulated signal, I followed the circuit recommended in Problem 4 of The Electronics of Radio. Only three components are required: a switching diode (1N4148), and a 3K resistor and 10nF capacitor forming a low pass filter.
Keeping in mind that The Electronics of Radio specifies a 5V 1MHz carrier signal (as opposed to the 8V 14.57kHz signal used in the circuit shown here), I went ahead and ran the above circuit in LTspice. Below is the modulating signal in red, the carrier wave in blue, and the attempt at demodulating the AM signal in green.
Although it’s a bit difficult to see, the 0V line is at the central horizontal aspect of the plot shown below. The effect of the rectifier diode on the AM signal is clear… the resultant “demodulated” (in quotes because this circuit is not actually adequate for demodulating this particular AM circuit) green-trace shows a waveform with only positive voltages retained. As expected, the diode rectified the alternating current of the amplitude modulated signal into a direct current (i.e., positive voltage only) product.
What did not work though was the low pass filter which should have eliminated the high frequency component of the green (“demodulated”) waveform. Which clearly it did not. I went ahead and rebuilt the circuit with somewhat random increases to the values of the resistor and capacitor, 10KΩ and 100nF respectively. Problem 4 of The Electronics of Radio deals with some of the math involved in this calculation and so I will cover the mathematics behind the RC selection more in that post.
This new circuit was simulated again in LTspice. And the demodulated AM signal (green), albeit of much smaller amplitude than the original message wave (red), looks a lot better.
I built the circuit on a breadboard (it’s the headline photo of this blog post) and used all four channels of my analog oscilloscope to produce the image below. Note that the amplitudes for each signal are adjusted so they fit best on the screen rather than to optimize comparison of the voltages. The time domain (x-axis) is the same for all of them however. The results are not perfect, but they demonstrate the concept.
From bottom to top: 8V 14.57kHz carrier signal, 3.2V 1K modulating signal, amplitude-modulated signal, demodulated AM signal.
Well, in summary, I am still on the hunt for a function generator with built in AM modulation. I am sure one will show up soon enough. But when it comes to accomplishing my goal — building an AM modulation circuit that I can use to get through Problem 4 in The Electronics of Radio — I believe I’ve done it. Problem 4 answers will be covered in a separate post.
Always Yours,
KM1NDY