Ratioed Circuits

• Write short notes on ratioed circuits.

• The ratioed gate consists of an nMOS pulldown network and pullup device called the static load.
• When the pulldown network is OFF, the static load pulls the output to 1.
• When the pulldown network turns ON, it fights the static load.
• The static load must be weak enough that, the output pulls down to an acceptable 0. Hence, there is a ratio constraint between the static load and pulldown network.

Advantage: 

• Stronger static loads produce faster rising outputs.

Disadvantages:

• Degrade the noise margin and burn more static power when the output is 0.
• A resistor is a simple static load, but large resistors consume a large layout area in typical MOS processes.

• Another technique is to use an nMOS transistor with the gate tied to VGG (Shown in fig.(b)). If VGG =VDD, the nMOS transistor will only pull up to VDD – Vt.
• Figure (c) shows depletion load ratioed circuit.

nMOS ratioed gates