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Transistor AM Modulator

There's a general overview under AM Receiver which covers the basic blocks in an Amplitude Modulated [AM] system. However only the definition of Amplitude Modulation addresses this side of an AM system. The dictionary links provide many of the definitions relating to the topic. Although on the receiver side of the system the topic of Diode Detector and Envelope Detector are covered in greater detail.

An Amplitude Modulator [AM] receives an RF sine wave carrier and modulates that signal with information or intelligence, in most cases an audio signal. At least the most common case would be AM radio. This page provides one example of an amplitude modulator designed around a transistor [shown below]. Another example is a Base Injection Modulator, which uses a different configuration.

Collector Injection Modulator Circuit

The circuit is called a collector injection modulator because the modulating signal is injected into the Collector side of the transistor circuit. The transistor configuration used in the circuit is a common-emitter design. The transistor circuit receives its input via the Base-Emitter side of the device and outputs the signal on the collector-emitter side, with the Emitter terminal being common to both input and output circuits. The transistor Q1 happens to be a PNP transistor.

Collector-Injection Transistor Modulator Circuit
AM Modulator

The RF carrier wave is applied to the primary [L1] side of the input transformer [T1]. The frequency of the carrier isn't relevant to the operation of the circuit, but would be selected as the center frequency of the transmitted signal.

The secondary of the transformer couples the incoming RF into the Base of the transistor. Resistors R1 and R2 form a DC voltage divider to bias the Base terminal. Resistor R3 is also a DC bias resistor, for the Emitter circuit. Both R1 and R3 are bypassed by RF capacitors C1 and C2. The transistor functions as an amplifier regardless of an applied modulating signal.

The supply voltage for the Collector [and Base circuit] is generated by Vcc. The Audio Frequency Modulating signal [AF MOD] is applied to the collector circuit via transformer T3. The transformer couples the AF signal in the circuit and isolates one circuit from the other. When an audio signal is applied to the transformer circuit the resultant transformer secondary voltage adds to or subtracts from the collector voltage. So the audio signal is added to the carrier signal.

The output transformer T2 also functions as the tunable tank circuit, in combination of C4. In this case, with the supply voltage applied through the transformer, it constitutes a series fed tank circuit. The variable capacitor C4 is used to adjust the tuned circuit to resonance with the RF carrier frequency.

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