How does Differential Amplifier Modulator work?

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 show an amplitude modulator constructed using differential amplifier.

Differential Amplifier Modulator

The BC107 BJT transistors Q1 and Q2 forms 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 act as the constant current source, that is, Q3 supplies a fixed emitter current Ie, half of which Ie/2, flows through the transistors Q1 and Q2. The modulating signal(Vm) is applied to the base of the transistor Q3. The resistors R3, R4 and R5, along with Vee will bias the constant-current source Q3. Without any signal at the base of Q3, a constant current Ie will flow into the differential amplifier. But with 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 single ended unbalanced differential amplifier(see Types of Differential Amplifier and how they work) where the output is taken from one of the collector w.r.t ground. If output is taken between the collectors, that is if the amplifier is balanced type, then the gain is twice of the above gain.

From the above gain equation, we can observe that the gain is dependent on the value of \(R_{C}\) and \( I_{E}\). For ordinary differential amplifier usage, \(R_{C}\) and \( I_{E}\) are fixed and thereby producing constant gain. However if the differential amplifier is used as modulator, the current \( I_{E}\) can be made variable by applying modulating signal into 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 which is centered at the carrier frequency. This is as shown in the above circuit diagram where LC parallel resonant circuit is used as 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.

circuit diagram of differential amplitude modulator

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 oscilloscope.

As can be observed from the above waveform amplitude on oscilloscope, the modulated signal has low amplitude. That is why differential amplifier modulator are also referred to as low level AM modulator. 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 amplifier 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 signal. Other methods of building amplitude modulator is using diode 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 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.

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