This circuit was originally designed as a simple mains intercom for use in the home. To complete the circuit you could use the ‘Mains Remote Transmitter’. We have to admit that our tests with the mains intercom were somewhat disappointing due to a persistent audible mains hum from the speaker. That doesn’t diminish the usefulness of the AM modulator as such, and it is still a very useful circuit for experimentation with this type of modulation. The accompanying receiver is called ‘AM demodulator for Intercom’.
The circuit consists of a microphone amplifier with a presettable automatic gain control (IC1b/IC2c/IC2d), a speech filter (IC2b) and the actual modulator (IC1a/IC3b/IC2a/T1). Because an asymmetrical power supply is used, the circuitry round IC3a has been added to provide a virtual ground which has its potential at half the supply voltage. The hart of the circuit is formed by a dual OTA (IC1, a dual operational transconductance amplifier), of which one is used for the microphone amplifier and the other for the AM modulator. It would be too much to give a detailed description of the operation of an OTA; it suffices to give a brief explanation of the various parts.
The potential divider (R7/R8) at the input of IC1b protects it against excessive inputs. The current output is converted into a voltage by buffer stage IC2c. The level of the transconductance of IC1b is controlled by the bias input (pin 6). The current fed to this pin (IABC, amplifier bias current) is limited by R13 to a maximum of 1.5 mA. The peak level of the output from IC2c is rectified by D1 and C7 and fed back as a control current to the OTA via inverting buffer IC2c. When the output voltage of IC2c increases, so will the voltage across C7, resulting in a smaller bias current and a reduction in the amplification of the microphone signal. This effect is most pronounced with preset P1 at its maximum setting. As P1 is turned down the amplification level of the microphone amplifier becomes more constant. With P1 fully closed the amplification is a constant 38 dB. With P1 at its maximum, the gain is automatically varied by up to 30 dB. P1 can, therefore, be used to set up the microphone amplifier as required.
R6 is used to bias the electret microphone (in this case an MCE2000 from Monaco). R5 and C5 decouple the supply to the microphone. Since the bandwidth of the microphone is much greater than that available in the ‘Mains Remote Transmitter’, a 3rd order Chebyshev filter (IC2b) has been added directly after the microphone amplifier, which has 3 dB ripple and a bandwidth of just 3.15 kHz. The signal is then fed to current source T1/IC2a. The circuit round T1 functions as a current source that can be modulated: a ‘constant’ current that varies linearly according to the processed microphone signal. This current is then used as a bias current for OTA IC1a, causing the signal that is fed to 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, causing the current through T1 to vary linearly with the voltage at pin 3. R19 and C12 are added for stability and potential divider R20/R21 stops the output of IC2a from clipping. The maximum bias current is about 3.5 mA.
The amplification of modulator IC1a/IC3b has purposely been kept a bit below 1 (it can be varied with P2 between 0.5 and 0.6) since 100% modulation will cause the maximum amplitude to be equal to the input voltage thereby overdriving the transmitter. K1 has the same pin-out as the connector on the transmitter board; the supply is taken from pins 3 and 4. The total current consumption is about 25 mA. The output signal from IC3b is connected to pin 2 of K1. The circuit was designed for use in close proximity with the transmitter, so no limiting resistor was added to the output. When a longer (shielded) cable is used between the two, you should connect at least a 47 Ω resistor in series with the output. A fast AD827 was chosen for opamp IC3 so that the modulator can easily cope with the 143 kHz signal.