PROBLEM 096 · Medium · The transmission gate
Both Ways
A transmission gate for a high input
The problem
Build a sampling switch for an input held at 1.5 V. An NMOS alone (gate on “clk”) loses its overdrive as the capacitor charges and stops near VDD − Vth − the voltage needed: it cannot pass 1.5 V in the time allowed. Add a PMOS in parallel with it whose gate is on “clkb” (the inverted clock, supplied by the test bench) so that the pair conducts over the whole input range.
A transmission gate has the NMOS pass low voltages and the PMOS high ones: the on-resistance of the pair stays small whatever the input. The capacitor starts at 0 V and the clock is high from 1 ns to 4 ns. Settle to within 5 mV of 1.5 V in under 1.5 ns.
The test bench
- Supply: VDD = 1.8 V.
- The test bench holds “in” at 1.5 V, drives “clk” high from 1 ns to 4 ns and “clkb” as its inverse (low during that time), and hangs a 1 pF capacitor from “out” to ground, starting at 0 V. It measures how long “out” takes to settle within 5 mV of 1.5 V.
Numbers to hit
- Settling time to 5 mV of 1.5 V at most 1.5 ns
- Voltage before the clock falls between 1.49 V and 1.51 V
Design it yourself
Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.
Sign in or create a free account →More on switched-capacitor and converters
- Switch OnA bigger switch settles faster but kicks more charge into the capacitor when it opens. Find the size that satisfies both.
- Tick TockReplace the input resistor with a switched capacitor: an integrator whose time constant is a ratio of capacitors and a clock.
- Binary LadderTwo resistor values and a binary-weighted output: the cleanest way to turn bits into a voltage.
- Eight StepsEight equal resistors between the reference and ground give seven evenly spaced taps: the heart of a flash ADC and of a string DAC.
- Charge ThiefHalf the channel charge goes to the capacitor when the switch opens. A half-size dummy switch on the opposite clock takes it back.