Subject: VLSI Design

Topic: Data Path Design

Difficulty: Low

• Various kinds of sense amplifiers are used to detect a signal difference on data lines such as Current-mirror differential, semi-latch, and full latch types.
• The cross-coupled differential amplifier is used to achieve high speed, low power consumption, and small area penalty.
• In the full CMOS type, OUT and OUT bar are precharged to Vss by PSAE.
• When PSAE goes to high, the amplifier starts to amplify a signal difference. As a node (A or B) is discharged, the other output (B) becomes isolated from the other input signal (IN bar), the sensing speed can be accelerated, and power consumption can be small. Since there is also no static power consumption after the sensing is completed, unlike the current-mirror type, this structure can be used in low power applications.
• However, a precharge signal (PSAE) is required and output nodes are precharged properly before starting a new sensing operation.
• A valid signal difference must be guaranteed because its operation cannot be reversed; that is, the states of output nodes cannot be reset once they are latched until the next precharge signal arrives.
• Performance of the semilatch type sense amplifier is between that of the current- mirror and that of the full CMOS latch type.

• The data due to invalid signal differences can be corrected with speed penalty, unlike the CMOS latch type. Figure shows various voltage sense amplifiers. (a) Differential-output current-mirror type, (b) pMOS latch type, c) full CMOS latch type.