Emitter modulator is one of many ways you can construct a modulator. It is classified as an active modulator because a BJT transistor is used in its construction. Passive modulators uses diodes for its operation. Here an emitter modulator design is shown and it is build on a breadboard. You can easily test this circuit at home with PC oscilloscope. Emitter modulator is used to generate AM(Amplitude Modulation) signal which will be shown here.
This is the second emitter modulator design that is shown here. The first emitter modulator design was illustrated in the blog post Amplitude Modulation with Emitter Modulator. The difference between the first and this second modulator is that,
(1) the LC bandpass filter is not placed at the output but between the power supply and the collector. That is this circuit is LC tuned amplifier circuit. Usually, when LC tuned circuit is used with an amplifier(the BJT in this case), the transistor is biased to operate as a class C amplifier. Here the BJT is biased and operated in class A.
(2) an inductive coupling is used at the emitter for the modulating signal(message signal or information signal) which was absent in the first emitter design. In the first design, coupling capacitor was used to inject the modulating signal into the emitter.
(3) a second order RC HPF(High Pass Filter) is used at the output to filter out the modulating signal which appeared strongly in the first emitter modulator design.
Emitter Modulator on Breadboard
The emitter modulator was build on a breadboard. A 5V regulated power supply was used. The transistor used is BC547. Other BJT transistors like BC107(which was used in the first emitter modulator), 2N3904, 2N2222 can also be used.
The message signal is fed using the earphone jack into the emitter of the transistor. That is why it is called emitter modulator. The carrier signal is fed into the base another sine signal using the PC.
The circuit schematic is shown below.
As shown in the circuit diagram above, the modulation signal is applied to the emitter of the BC107 transistor through the coupling capacitor C3 (10 µF). In some emitter modulator designs, transformer coupling is used instead of a capacitor. In this implementation, however, a simple capacitive coupling method is used for simplicity. Likewise, the carrier signal is applied to the base of the transistor through the coupling capacitor C1.
In most emitter modulator circuits, transformer coupling is more commonly used for injecting the modulating signal into the emitter, while the carrier is fed into the base. In this low-frequency experimental setup (1 kHz modulating signal and 5 kHz carrier signal), capacitive coupling works reasonably well. However, at higher frequencies—such as those typically used in practical AM transmission—this approach may introduce issues. This serves as a caution rather than a strict limitation.
Transformer coupling generally provides better port isolation and impedance matching. Without sufficient isolation, the modulating signal applied to the emitter can leak strongly into the output, affecting signal quality. For higher-performance RF modulators, transformer coupling is usually the preferred approach. I may explore a transformer-coupled version in a future article and share the results. In theory, coupling can be achieved using capacitors or inductors (via transformers), but in RF applications, good isolation and impedance matching are critical, making transformers a more suitable choice.
In the schematic shown above, the carrier signal has a peak amplitude of 100 mV, while the modulating signal has an amplitude of 80 mV. This gives a modulation index of 0.8 (or 80%). The modulation index must not exceed 1 (100%), otherwise overmodulation occurs and distortion appears in the output waveform. The concept of modulation index, along with distortion effects in AM signal generation, has been discussed in detail in earlier posts on AM modulation techniques and BJT-based modulators.
The emitter modulator circuit is DC biased using the voltage divider biasing method. This biasing network consists of resistors R1, R2, RC, and RE. The name “voltage divider bias” comes from resistors R1 and R2 forming a voltage divider that sets a stable base voltage at their junction. Capacitors C1, C2, C3, and C5 serve as coupling capacitors, while C4 acts as a bypass capacitor.
If you would like to understand how to calculate the resistor and capacitor values for proper biasing, refer to my detailed tutorial on voltage divider biasing for BJTs, where step-by-step formulas and design equations are explained. Additionally, I have published related articles on BJT amplifier design, differential amplifiers, and AM modulation circuits that complement this topic. For those who prefer a quicker approach, you can also use an online BJT amplifier design calculator to simplify the design process.
