Principle and application of CMOS transmission gate

One 、 Basic principle of transmission gate （ Understand one ）

Two 、 Basic principle of transmission gate （ Understanding two ）

   The so-called transmission gate （TG） It is an analog switch that transmits analog signals .CMOS The transmission gate consists of a P Ditch and a N Channel enhanced MOSFET In parallel .TG The left side of the is the input , On the right is the output . Above is the control signal C’ Input end , Below is the control signal C Input end .

   When C’=0、C=1 when ,TG Conduction , The output signal is equal to the input signal .

   Set their starting voltage |VT|=2V And the variation range of the input analog signal is -5V To +5V. The operation of the transmission gate is as follows ： When C Terminate low voltage -5V when TN The grid voltage of is -5V,vI take -5V To +5V Any value in the range ,TN No conduction .

   meanwhile 、TP The grid voltage of is +5V,TP Nor is it conductive . so , When C When terminating low voltage , The switch is off . To turn the switch on , Can be C Terminate high voltage +5V. here TN The grid voltage of is +5V,vI stay -5V To +3V Within the scope of ,TN Conduction . meanwhile TP The shed pressure is -5V,vI stay -3V To +5V Within the scope of TP Turn on the . It can be seen from the above analysis , When vI《-3V when , have only TN Conduction , And when vI》+3V when , have only TP Conduction when vI stay -3V To +3V Within the scope of ,TN and TP Both tubes are conducting . Further analysis also shows , The deeper a tube is connected , The conductivity of the other tube is correspondingly reduced .

   let me put it another way , When the on resistance of a tube decreases , Then the on resistance of the other tube increases . Because the two piping systems operate in parallel , It can be approximately considered that the on resistance of the switch is approximately a constant . This is a CMOS Advantages of transmitting out . In normal operation , The on resistance of the analog switch is about hundreds of ohms , When it is connected in series with an operational amplifier with an input impedance of megaohm , Negligible .

3、 ... and 、 Basic principle of transmission gate （ Understanding three ）