Amplifier Circuit DiagramsAudio Circuit Diagrams

AM modulator for Intercom Schematic Circuit Diagram

Origin and Purpose: Mains Intercom Circuit

Initially, this circuit was conceived as a basic mains intercom system tailored for home use. To fully implement the setup, the ‘Mains Remote Transmitter’ can be employed in conjunction. However, it’s crucial to acknowledge that our trials with the mains intercom yielded less than satisfactory results, primarily due to an incessant audible mains hum emanating from the speaker. Despite this limitation, the AM modulator’s inherent utility remains intact, making it an invaluable tool for experimentation in the realm of amplitude modulation. The corresponding receiver for this setup is aptly named the ‘AM Demodulator for Intercom.’

AM Modulator: A Valuable Experimentation Tool

Even though our tests with the mains intercom were hampered by a persistent mains hum issue, the AM modulator itself retains its fundamental value. It remains a highly practical circuit for enthusiasts interested in experimenting with this specific modulation technique. The design’s intrinsic capabilities provide an excellent platform for individuals keen on exploring various aspects of amplitude modulation. As a result, despite the challenges faced during intercom applications, the circuit continues to serve as a valuable tool for those engaging in experimental endeavors within the realm of modulation techniques. The complementary receiver designed for this system is aptly named the ‘AM Demodulator for Intercom.’

AM modulator for Intercom Schematic Circuit Diagram

Circuit Components and Functions

The circuit comprises three main sections: a microphone amplifier with adjustable automatic gain control (IC1b/IC2c/IC2d), a speech filter (IC2b), and the core AM modulator (IC1a/IC3b/IC2a/T1). Given the use of an asymmetrical power supply, additional circuitry around IC3a establishes a virtual ground with a potential halfway between the supply voltage. A key component is the dual OTA (Operational Transconductance Amplifier), IC1, where one section is dedicated to the microphone amplifier, and the other to the AM modulator. While describing the intricate operation of an OTA is beyond the scope here, a brief explanation of various components is provided.

Microphone Amplifier and Automatic Gain Control (AGC)

The input potential divider (R7/R8) safeguards IC1b against excessive inputs, and the current output is transformed into a voltage by the buffer stage IC2c. IC1b’s transconductance level is regulated by the bias input (pin 6), with the bias current (IABC) limited to a maximum of 1.5 mA by R13. D1 and C7 rectify the peak level of IC2c’s output and feed it back as a control current to the OTA via the inverting buffer IC2c.

An increase in IC2c’s output voltage results in a smaller bias current and reduced amplification of the microphone signal. This effect is most prominent when preset P1 is at its maximum setting. As P1 is adjusted lower, the amplification of the microphone amplifier becomes more consistent, with P1 fully closed providing a constant 38 dB gain. With P1 at its maximum, the gain can be varied by up to 30 dB, allowing for customized configuration of the microphone amplifier.

Electret Microphone and Bandwidth Adjustment

R6 serves to bias the electret microphone, while R5 and C5 decouple the microphone’s supply. As the microphone’s bandwidth exceeds that of the ‘Mains Remote Transmitter,’ a 3rd order Chebyshev filter (IC2b) is positioned immediately after the microphone amplifier. This filter introduces a 3 dB ripple and a 3.15 kHz bandwidth. The filtered signal is then directed to T1/IC2a, forming a modulatable current source.

This current source generates a ‘constant’ current that varies linearly based on the processed microphone signal and is employed as a bias current for OTA IC1a. This, in turn, causes the signal at pin 1 of K1 to appear at the output of IC3b with its amplitude modulated. IC2a compares the voltage across emitter resistor R22 with its input, leading to linear variation in the current through T1 in response to the voltage at pin 3. Stability and prevention of clipping are ensured by the inclusion of R19 and C12, and a potential divider, R20/R21.

Output and Modulation Amplification

The amplification for the modulator IC1a/IC3b is deliberately maintained slightly below 1, adjustable with P2 between 0.5 and 0.6, as 100% modulation would drive the transmitter to its maximum amplitude, potentially leading to overdriving. Connector K1 matches the transmitter board’s pin-out, drawing its supply from pins 3 and 4. The total current consumption is approximately 25 mA. The output signal from IC3b is connected to pin 2 of K1. Since the circuit was designed for close proximity to the transmitter, no limiting resistor is included at the output. However, when using a longer (shielded) cable between the two, it’s advisable to introduce at least a 47 Ω resistor in series with the output to maintain signal integrity. To accommodate the 143 kHz signal, a fast AD827 op-amp (IC3) was selected for the modulator.


Related Articles

Leave a Reply

Your email address will not be published.

Back to top button