Here a emitter modulator for Amplitude modulation(AM) is build and illustrated on a breadboard. Emitter modulator is a circuit which performs mixing operation of a carrier signal(RF signal or local oscillator signal) with modulating signal(message or information signal) wherein the modulating signal is applied at the emitter. The carrier signal is applied at the base. The modulated AM signal signal appears at the collector which is filtered using usually LC tuned band pass filter.
The following video shows the emitter modulator in action.
The following shows the circuit build on a breadboard.
Here I have used BC107 which is an NPN transistor. I had never used BC107 transistor and heard that it is old but still popular. I had few of these in my stock so it was a chance to experiment with it. Instead of this transistor you can also use 2N3904, 2N2222, BC547 or BC549. I have checked it with 2N3904, 2N2222 and BC547 and they work equally well. I have used 5V regulated power supply from power supply breakout board which you can see on the video posted in this blog post.
The above populated circuit on breadboard may be not easily understandable so the following is the schematic diagram of the above breadboard circuit.
As you can see in the above circuit drawing, the modulation signal Vm is applied to the emitter of the BC107 transistor via the coupling capacitor C3 or 10uF. I have found circuits that uses transformer coupling instead of the coupling capacitor. But here we have used simple coupling capacitor. Similarly, the carrier signal Vc is applied to the base of the transistor via the coupling capacitor C1. I have not seen emitter modulator circuit where the carrier signal is coupled into the base with transformer coupling unlike the modulating signal into the emitter. There could be another reason why the coupling capacitor it worked here which is that I have used low frequency signal. I have used 1kHz modulating signal and 5kHz carrier signal. At higher frequency which is usually the case in AM there could be trouble. This is but only a warning. Using transformer we could have better port isolation. The modulating signal entering into the emitter could become a problem and appear at the output strongly. Hence transformer coupling should be better choice for higher quality modulator. I may look into this in the future and post the result. Theoretically coupling can be done using capacitor or inductor(using transformer) but RF modulator needs high port isolation and impedance matching so transformer should be the first choice.
In the above circuit schematic, the carrier signal has peak amplitude of 100mV and the modulating signal has amplitude of 80mV. The modulation index is 0.8 or 80%. The modulation index cannot be higher than 1 or 100% otherwise distortion results. This modulation index, distortion for AM signal generation was explained in previous blog post AM modulator using BJT transistor.
The above emitter modulator circuit is DC biased using voltage divider biasing. That is done using the four resistors R1, R2, RC and RE. The name of this bias method comes from the fact that the resistors R1 and R2 forms a voltage divider which sets up a fixed base voltage at the junction between them. The capacitor C1,C2, C3, C5 are coupling capacitors and C4 is bypass capacitor. If you want to know how to calculate the resistors and capacitor values then see the tutorial How to bias a BJT using voltage divider biasing. That tutorial has detailed calculation with the voltage divider biasing formula and equations. But for lazy engineers there is even better way- you can use the online BJT amplifier design calculator.
In above emitter modulator circuit after the modulator there is a LC band pass filter. It is used to pass a narrow band of frequency with center of the carrier signal to pass through it and block other signal frequencies. You can calculate the capacitor or inductor or vice versa for desired resonant frequency using another online LC resonant frequency calculator. In the above circuit the resonant frequency was chosen to be that of the carrier signal frequency which is 5kHz.
The next thing about the emitter modulator circuit for AM is that often you may find an RF choke shown in the circuit diagrams in place of the collector resistor Rc. In my case there has been not significant change if the RFC is placed or not so that above circuit drawing has not shown the RFC. RFC function is for DC and AC signal operation. For DC, it is short circuit and for AC it is open circuit.
The following shows the AM signal waveform at the output of the bandpass filter after the emitter modulator.
The following shows the frequency spectrum of the DSB AM signal generated by the emitter modulator.
As you can see from the frequency spectrum it is clearly a double side band(DSB) AM signal.
The following video shows animation of the emitter modulator works.
The above emitter AM modulator circuit was tested with PC oscilloscope. How you can test the circuit home easily is explained in the tutorials How to build base biased BJT amplifier on breadboard and test with PC soundcard based oscilloscope, How to use Matlab Simulink as Oscilloscope or LM741 non-inverting amplifier with dual supply from single supply.
If you are interested in modulators and mixers you may be interested in the the following tutorials.
- How Two Diode Single Balanced Mixer Design Works
- How does Differential Amplifier Modulator work?
- Simple Amplitude Modulation (AM) circuit using Single Diode Modulator
