# Design AM modulator circuit

I having an assignment that need to design AM Transmitter using Multisim with following specification:
• Audio input (200-14000Hz)
• Double Sideband Full Carrier
• RF carrier frequency of 50 MHz
• Carrier Power 1 mW

I have totally no idea how to design a AM modulator from scratch, so i found this video, and built this circuit and try to modify the carrier signal frequency from 11kHz to 50Mhz it fail and didn't show any signal at the oscillator.

I know some basic concept about how AM can be achieve using transistor,but have no idea how to design it.
Is there any detail guideline i can refer or follow to design a circuit from scratch?
How do all the value of resistor is being calculated, and why is capacitor and inductor being use at the collector of the transistor.

Update: I had revise about the DC biasing of the transistor and made a update about my prototype. Here I will explain what my understanding and how I design.

First, I hope my Q-point is in the middle, so i check the datasheet of BC107B and found that Vce(sat)=200mV while Ic=100mA (there are another value Vce=70mV when Ic=10mA). Then i go though some calculation. There are too many variable, therefore i assign some of it to simplify the calculation(Re and R1). For the tank circuit, I use frequency 50Mhz as my question RF carrier require and remain capacitor unchanged to find out the inductor value. The final circuit result is as follow. The circuit is not working. Please tell me if there any mistake or what i did wrong.

• What do you suppose the capacitor and inductor are there for? What is the resonant frequency of the cap and inductor? What should it be? What's your theory of how the circuit works? – John D Jul 28 at 18:59
• Most of my understanding is from this post, electronics.stackexchange.com/q/199232/227569, what I understand is that if there a current at the emitter site it will change the Vce and therefore the AM can achieve. Other component I really have no idea what to do with. – damonlimys Jul 28 at 19:15
• If you really, truly, sincerely have "no idea how to design a AM modulator from scratch" and "no idea how to design it" then you should probably drop the course and consider a different degree program. If, on the other hand, you do have some idea then tell us everything you understand about the circuit you have provided, explain what you want it to do, and describe what it actually does. – Elliot Alderson Jul 28 at 19:20
• The last time I saw a similar circuit is when I study about transistor and how to set it Q point, but when I study the C1 and L1 is replace by a single resistor, so I not sure what to do with capacitor and inductor. I always having trouble when analysing actual circuit, I like electronic but analogue for me is a huge pain, I willing to learn and want to learn how to analyse and design actual circuit. – damonlimys Jul 28 at 19:29
• The point is that assignments like this don't come out of nowhere - the previous parts of the course were supposed to give you the background to get to the point from where you could do this with whatever style of solution is expected. You should go back and review those. – Chris Stratton Jul 28 at 19:50

My (quick and dirty) AM Tx design for a low power personal use requires you know how to generate the input frequency amplitude exactly or at least close enough for personal use. Using the low end of the AM makes it easier to listen to and use a 1m wire as a loop antenna in series with the 10K resistor to limit power to 1mW with a 9V battery and a low ESR e-cap across the battery with short leads to everything.

Stray capacitance to ground will lower the tuning frequency significantly ( not intended to be a great design resulting in a change in amplitude and bandwidth of the audio.

You can use a XTal or PLL or even a Relaxation Oscillator if stable enough then an R divider to get the correct level.

RF input 1Vpp in the Lowest AM range. ( ~> 520kHz or between channels)
Audio input @Vpp max

9V battery current 250uA idle and 500uA with full modulation.

Every part serves a purpose. The main part for the LC on the collector is to always have Vce avg= Vbat and act as a BPF to input RF or a crude channel filter for harmonics.

The resistors are selected to yield 1mW of RF out with minimal current from a 9V battery and almost 100% AM with 2Vpp audio input such as from a smart phone.

No that with no audio input the RF output goes to 1Vpp steady carrier in this design and thus full modulation goes from0 to 2Vpp. Real AM broadcast Txers are much more complicated with audio compression and a lot more.

For fun, change the antenna wire length or put in series with the inductor instead and keep away from metallic or your body to avoid detuning it a bit. It's very low power and safe.

When in doubt measure your DC voltages everywhere then Vac. Have fun.