One method to build a modulator is with differential amplifier. A Differential Amplifier constructed with either BJT or FET transistors can be used to build an amplitude modulator. The differential amplifier modulator illustrated here used BJT (Bipolar Junction Transistor).
The following circuit diagram shows an amplitude modulator constructed using differential amplifier.
The BC107 BJT transistors Q1 and Q2 form the differential pair of the differential amplifier. The carrier signal (Vc) is applied to the base of the first transistor Q1 of the differential amplifier. The base of the second transistor Q2 is grounded. Hence we are using a single-ended differential amplifier configuration. The third transistor Q3 acts as the constant current source, that is, Q3 supplies a fixed emitter current Ie, half of which (Ie/2) flows through transistors Q1 and Q2. The modulating signal (Vm) is applied to the base of transistor Q3. The resistors R3, R4 and R5, along with Vee, bias the constant-current source Q3. Without any signal at the base of Q3, a constant current Ie flows into the differential amplifier. But with a modulating signal into the base of the constant current source via Q3, a varying current is produced and therefore an amplitude modulation (AM) signal is produced at the collector output. This modulation action can be observed using the gain equation. The approximate gain of the circuit is given by the following equation:
\[A = \frac{R_{C} I_{E}}{50}\]
This gain is for a single-ended unbalanced differential amplifier (see Types of Differential Amplifier and how they work) where the output is taken from one collector with respect to ground. If output is taken between the collectors, that is if the amplifier is balanced type, then the gain is twice the above gain.
From the above gain equation, we can observe that the gain depends on the values of \(R_{C}\) and \(I_{E}\). For ordinary differential amplifier usage, \(R_{C}\) and \(I_{E}\) are fixed, thereby producing constant gain. However, if the differential amplifier is used as a modulator, the current \(I_{E}\) can be made variable by applying a modulating signal to the base of Q3.
The output is therefore a modulated signal which contains not only the AM signal but also harmonics, as was previously explained in the tutorial How to design AM modulator circuit using transistor. To remove the modulating signal and harmonics (intermodulation products), we can use a band pass filter centered at the carrier frequency. This is shown in the above circuit diagram where an LC parallel resonant circuit is used as a bandpass filter.
The following is a differential amplifier modulator circuit with LC band pass filter with carrier frequency of 10KHz and modulating signal of 1KHz.
The value of the inductor L and capacitor C for the resonant circuit was calculated using the LC Parallel Resonant Circuit Online Calculator.
The following shows the modulating signal, carrier signal and the modulated AM signal waveform on an oscilloscope.
As can be observed from the above waveform amplitude on the oscilloscope, the modulated signal has low amplitude. That is why differential amplifier modulators are also referred to as low level AM modulators. To increase the amplitude and hence power of the modulated signal, an amplifier or amplifier stages are required after the band pass filter. One way to achieve this was illustrated in the earlier AM modulator tutorial Simple Amplitude Modulation (AM) circuit using Single Diode Modulator. The amplifiers are usually class A power amplifier, class B power amplifier or class AB power amplifier.
The following is the frequency spectrum graph of the output signal from the modulator.
In this way we can build a differential amplifier modulator for generating AM signals. Other methods of building amplitude modulators use diodes, which was illustrated in the previous tutorial How does Single Diode Modulator Circuit work?
For testing the above design at home, we have several options. We can test the modulator circuit using a PC oscilloscope and function generator at home. We can use LabVIEW software as illustrated in the tutorial Testing of self-biased BJT amplifier on breadboard with PC oscilloscope, or use MATLAB software as shown in the tutorial LM324 op-amp Integrator testing with MATLAB Simulink oscilloscope, or even use Proteus electronics circuit software as was illustrated in the tutorial How to build LM324 Instrumentation Amplifier & Test It.
that is good list
I think the part about the role of LC tank circuit could be more specific. It's role is not to pass certain band of frequencies but to reject the resonant frequency band (as impedance tends to infinity), thus allowing to pass carrier frequency to the load.
the LC tank is resonant to the carrier frequency and so it resonant with the modulated signal, so this is being passed, why reject resonant frequency? it is exactly the reason why LC tank is there, to resonate with the carrier signal(modulated signal) meaning to pass through and reject other frequencies...