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The miracle of CMOS technology: How to enable electronic switches to achieve bidirectional transmission?
In modern electronic devices, fast, flexible and efficient signal transmission is crucial. Transmission Gate (TG), as a new type of electronic switch, exists like a relay and has the ability to transmit signals in both directions, bringing revolutionary changes to countless applications.
Structure of transmission gate
The transfer gate consists of two field effect transistors (FETs) and is designed with CMOS technology. In this design, PMOS and NMOS transistors are connected in parallel with their respective row and source terminals connected together, while their gate terminals are connected through an inverter (NOT gate). Additionally, the substrate terminals of these transistors are connected to the power supply to ensure that parasitic substrate diodes are reverse biased to prevent interference with signal flow.
PMOS transistors of transmission gates can strongly transmit logic "1", while NMOS transistors focus on transmitting logic "0".
Function of transmission gate
The operation of the transmission gate depends on the control input signal. When the control input is logic zero, neither the NMOS nor PMOS transistors conduct, leaving the transfer gate in a closed state. When the control signal turns to logic one, the NMOS starts to conduct, and the transmission gate starts to work.
This design ensures that the signal flow can be controlled freely in different voltage ranges, significantly improving the flexibility of the electronic system.
Application scenarios
Electronic switch
In the implementation of electronic switches, transmission gates are widely used in analog multiplexers. Take the 4066 type four-way bidirectional switch as an example. This device can handle various analog and digital signals and has been launched by many manufacturers.
Analog multiplexer
In many mixed-signal systems, analog multiplexers are used to route multiple analog input channels to a single analog-to-digital converter, improving overall system efficiency.
Logic circuit
In logic circuit design, transmission gates can also replace traditional CMOS pull-up and pull-down networks. This innovative approach enables circuit designs with advantages in terms of safety and compactness.
Negative voltage application
In addition to conventional digital signals, transmission gates can also switch in the presence of alternating voltages, such as audio signals. In this case, however, the negative supply voltage must be lower than the minimum signal voltage to ensure that the substrate diode does not conduct.
The 4053 standard chip using a transmission gate is commonly used for analog input selection in audio amplifiers, demonstrating its flexibility and functionality.
Looking to the future
With the advancement of technology, the application of CMOS technology and transmission gates will continue to expand to more complex systems and scenarios. Will we see more advanced transmission gate designs in the future to further improve signal processing performance and efficiency?
