Utilizing Bilateral Switches for Analog Signal Switching
In scenarios where analogue signals require switching, bilateral switches like the 4066 are commonly employed. However, due to this IC’s compatibility with single-ended supply alone, all associated components are typically linked to the same asymmetrical supply, usually set at 8 V. One drawback of this setup is that the operational amplifier operates on merely half of the power supply voltage, leading to a proportional reduction in output voltage. Consequently, this results in diminished signal-to-noise ratios, among other issues.
Innovative Solution to Voltage Imbalance
This circuit offers an ingenious solution to the previously mentioned issue. Its design capitalizes on the fact that current in the bilateral switch (IC1) can flow bidirectionally at zero volts. To prevent negative input voltages, precautions are taken to ensure the input voltage at the switch does not become negative. This safeguard is implemented by utilizing one of the switches in IC1 to momentarily short the input to ground at the appropriate time.
Operational Process and Signal Amplification
Here’s how the operation unfolds: when the switch input is ‘high,’ IC1a is closed, and IC1b is open, preventing any signal from reaching the inverting input. IC1a effectively grounds the signal. Conversely, when the switch input goes ‘low,’ IC1a opens, IC1b closes, and the audio signal is then amplified by the operational amplifier.
Ensuring TTL-Compatible Switching Levels
To maintain TTL-compatible switching levels, a minor buffer stage can be added to each input. This buffer stage can consist of a BC547 or a 7406, as demonstrated in the schematic. Additionally, multiple inputs can be connected to the virtual earth node, creating a mixing circuit if necessary. Duplicating the circuit inside the box enables the implementation of this configuration.
The gain is easily calculated using the standard formula:
Uout = –(Uin·R2)/R1. The input impedance of each individual input is about 10 kΩ. Take note: the circuit inverts!
It speaks for itself that any spare ports and switches can be used for additional inputs.