CMOS Circuits Design FAQ

1Q: Give the expression for CMOS switching power dissipation

Given a frequency f for low-to-high output transitions, the power drawn from the supply is C*Vdd*Vdd*f

2Q: For CMOS logic, give the various techniques you know to minimize power consumption

Reduce transistor width, increase threshold voltage, reduce supply voltage, substrate biasing

3Q: What are the approximate values for MOSFET drain to source resistance in different modes ?

Ron = 100 Ohm, Roff > 5 MOhm

4Q: What should be done to the threshold voltage of CMOS to reduce the subthreshold leakage current ?

Threshold voltage should be increased to reduce subthreshold current.

5Q: What is a MOSFET channel length modulation ?

With drain-to-source voltage drop increase, the depletion region at the drain junction grows reducing the efffective channel length. As a result drain-to-source current increases with the associated voltage drop (non-ideal current source in a saturation mode). Devices with bigger channel length are used whenever high-impedance current source is required.

6Q: What is a MOSFET body effect ?

Body effect caused by the increase in source to substrate voltage drop and therefore increase in threshold voltage of a MOS devices usually due to the their serial connection which results in lower currents and slower circuits performance.

7Q: Why don’t we use just one NMOS or PMOS transistor as a transmission gate ?

Transmission gate has better noise margin vs. single pass transistor. Also transmission gate is more efficient to implement some complex logic (XOR, MX, DMX).

8Q: Explain sizing of the inverter

In invertor transistor gates usually sized as 2:1 (p:n) due to the mobility factor (pmos mobility is half of nmos).

9Q: Draw a transistor level two input NAND gate. Explain its sizing for equal rise and fall times

For the equal rise and fall time, transistors sized as 4:1 (p:n)

10Q: How do you size NMOS and PMOS transistors to increase the threshold voltage ?

Threshold voltage is proportional to the channel length i.e. larger channel length means higher threshold voltage and lower leakage power.

11Q: Let A & B be two inputs of the NAND gate. Say signal A arrives at the NAND gate later than signal B. To optimize delay, of the two series NMOS inputs A & B, which one would you place near the output ?

The transistor with the latest arrival time at the gate should be kept near the output to minimize delay.

12Q: In the design of a large inverter, why do we prefer to connect small transistors in parallel (thus increasing effective width) rather than lay out one transistor with large width ?

To reduce parasitic effects.