Posted on Feb 6, 2014

This circuit is designed to digitally increase/decrease the volume of an analog audio signal. It uses a single AVR microcontroller to produce the controlling pulse width modulated (PWM) signal. However, the circuit`s innovation is the use of the transconductance amplifier (OTA). Although you can find these amplifiers easily, they are rarely used i

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n home made circuits. Perhaps because most people ignore their existence. The only thing the microcontroller does, is producing a PWM signal with the desired duty cycle. You can replace it with another circuit capable of producing PWM signals. If you are creative enough, you can modify the provided example code, so as to be possible to change the duty cycle, of the PWM signal, wirelessly (for example with an IR remote control, like the ones you use to control the TV). Attention: The 100nF capacitor at the right of the AVR microcontroller must be soldered as close to the power supply pins of the microcontroller as possible (meaning close to the pins 20 and 10). The circuit is based on a transconductance amplifier. The input signal must have a maximum of 5Volts peak to peak voltage. In order to take advantage of the linear portion of the input - output characteristic of the CA3080, we suppress the signal with a voltage divider. We also add a reference dc at Vcc/2 (=2. 5 Volts. Bias voltage). After that, the signal at the input of the CA3080 (pin 2), varies around 2. 5Volts with a maximum of 100mV peak to peak voltage (meaning from 2. 450 to 2. 550 Volts). gm is the amplifier`s transconductance and it can be adjusted by an external bias current IB, according to the equation gm = IB/2Vt, for an area of 4 - 5 decades. Vt is the thermal voltage Vt = KT/q. At room temperature equals to 25 mV. Our goal is change the sound volume....

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