Transmission Gates

Transmission Gates

What's the big idea?

• A transmission gate is an idea where given an input:
• - If the gate is ON: input will be directly connected to output
• - If the gate is OFF: input is blocked from the output
• Therefore, acting as a switch!
• Why would we want transmission gates?
• - Multiplexing (Selecting)
• - Transistor logic

Components that act like a switch:

• NMOS
• - When gate voltage = 0V:
• - NMOS OFF, Output = 0V
• - GOOD, STRONG LOW 👍
• - When gate voltage = Vdd:
• - NMOS ON, Output = Vdd - Vtn
• - NOT GOOD, WEAK HIGH 👎
• PMOS
• - When gate voltage = Vdd : PMOS OFF -> Output = 0
• - When gate voltage = 0V, two scenarios:
• Input voltage = Vdd -> Output = Vdd 👍
• - STRONG HIGH
• Input voltage = 0V -> Output = Vt 👎
• - WEAK LOW

What's the Solution?

• We want the strong low from NMOS, while simultaneously having the strong low for PMOS, so what do we do?
• - Design a transmission gate where PMOS and NMOS are in parallel
• - IMPORTANT: the PMOS control signal must be inverted, so that both transistors will be on/off simultaneously
• 
• Caption: Control signal (A) is inverted for the PMOS

Dynamic Characteristics

• 
• - The total current charging the capacitor is the sum of the currents through the NMOS and PMOS
• - The PMOS enters triode region when the output voltage equals |Vtp|​, but it continues to conduct until the capacitor is fully charged, and output reaches the high output voltage
• - ensures a "good 1"
• - When input voltage goes low, PMOS and NMOS switch roles
• - The NMOS will continue to conduct until the capacitor is fully discharged and output voltage reaches the low output voltage
• - ensures a "good 0"
• Caption: The presence of the PMOS device provides additional current, leading to a lower tPLH compared to a single NMOS switch configuration.