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The wisdom hidden in the chip: Why can the transmission gate switch freely between analog and digital signals?
In modern electronic devices, the transmission gate (TG) plays a key role, allowing the device to smoothly switch between analog and digital signals. This is similar to the function of a relay, which can effectively conduct or block current under the drive of a control signal. Therefore, it is very important to have a deep understanding of the operation principle of the transmission gate and its application.
Transmission Gate Structure
The basic structure of a transmission gate consists of two field effect transistors (FETs), which are n-channel and p-channel MOSFETs. This is different from a conventional field effect transistor because in a transmission gate the base terminal (bulk) is not connected to the source terminal. This configuration ensures that the parasitic base diode is always reverse biased and does not affect the flow of signals.
In a transmission gate, the base of the p-channel MOSFET is connected to the positive supply, while the base of the n-channel MOSFET is connected to the negative supply.
Function of the transmission gate
When the control input is logic zero, the gate of the n-channel MOSFET is held at a negative voltage, while the gate of the p-channel MOSFET is driven to a positive voltage by the inverter. This way, neither transistor conducts and the transmission gate remains closed. On the contrary, when the control input is logic one, the gate of the n-channel MOSFET becomes a positive voltage, causing the gate of the p-channel MOSFET to become a negative voltage, so that both begin to conduct and realize signal transmission.Through the transceiver design, the transmission gate can switch over the entire voltage range, enabling a wide range of applications.
Application of transmission gate
Electronic switch
Transmission gates are widely used in electronic switches and analog multiplexers. Through the coordination of multiple transmission gates, the signal transmission and blocking functions can be effectively realized. A typical example is the 4066 quad bidirectional switch, which can handle both analog and digital signals and is available on the market from several manufacturers.
Analog Multiplexer
In mixed-signal systems, analog multiplexers can route multiple analog input channels to a single analog-to-digital converter, thereby improving system flexibility and efficiency.
Logic Circuits
Transmission gates can also be used to build logic circuits that are often more compact and more amenable to silicon implementation than traditional CMOS pull-up and pull-down networks.
Negative voltage applications
When using a transmission gate to switch an AC voltage (such as an audio signal), it is important to ensure that the negative supply voltage is below the minimum signal voltage to ensure that the base diode remains non-conducting. While the transmission gates are still capable of switching to logic voltage levels, special versions are also available with an integrated level shifter. ConclusionThe unique design and efficient function of the transmission gate make it an indispensable component in modern electronic systems. As technology develops further, there will continue to be interest in the application and improvement of transmission gates. In the future, how will this technology further change the design and functionality of electronic products?
