In modern

computer memory In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...

, a sense amplifier is one of the elements which make up the circuitry on a semiconductor memory chip ( integrated circuit); the term itself dates back to the era of magnetic

core memory Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...

. A sense amplifier is part of the read circuitry that is used when data is read from the memory; its role is to sense the low power signals from a ''bitline'' that represents a data

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

(1 or 0) stored in a memory cell, and amplify the small voltage swing to recognizable

logic level In digital circuits, a logic level is one of a finite number of states that a digital signal can inhabit. Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. The range ...

s so the data can be interpreted properly by logic outside the memory. Modern sense-amplifier circuits consist of two to six (usually four)

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s, while early sense amplifiers for core memory sometimes contained as many as 13 transistors. There is one sense amplifier for each column of memory cells, so there are usually hundreds or thousands of identical sense amplifiers on a modern memory chip. As such, sense amplifiers are one of the few remaining

analog circuit Analogue electronics ( en-US, analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional relat ...

s in a computer's memory subsystem.

Basic structure

Sense amplifier is required during the data read and refresh operation from the memory concerned.

Memory chip operation

The data in a semiconductor memory chip is stored in tiny circuits called memory cells. Sense Amplifiers are primarily applied in

Volatile memory Volatile memory, in contrast to non-volatile memory, is computer memory that requires power to maintain the stored information; it retains its contents while powered on but when the power is interrupted, the stored data is quickly lost. Volatile ...

cells. The memory cells are either SRAM or DRAM cells which are laid out in rows and columns on the chip. Each line is attached to each cell in the row. The lines which run along the rows are called ''wordlines'' which are activated by putting a voltage on it. The lines which run along the columns are called ''bit-line'' and two such complementary bitlines are attached to a sense amplifier at the edge of the array. Number of sense amplifiers are of that of the "bitline' on the chip. Each cell lies at the intersection of a particular wordline and bitline, which can be used to "address" it. The data in the cells is read or written by the same ''bit-line''s which run along the top of the rows and columns.

SRAM operation

To read a bit from a particular memory cell, the wordline along the cell's row is turned on, activating all the cells in the row. The stored value (Logic 0 or 1) from the cell then comes to the Bit-lines associated with it. The sense amplifier at the end of the two complementary bit-lines amplify the small voltages to a normal logic level. The bit from the desired cell is then latched from the cell's sense amplifier into a buffer, and put on the output bus.Sense Amplifier for SRA

Prof: Der-Chen Huang, National Chung Hsing University

DRAM operation

The sense amplifier operation in dynamic random access memory, DRAM is quite similar to the SRAM, but it performs an additional function. The data in DRAM chips is stored as

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

in tiny

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s in the memory cells. The read operation depletes the charge in a cell, destroying the data, so after the data is read out the sense amplifier must immediately write it back in the cell by applying a voltage to it, recharging the capacitor. This is called

memory refresh Memory refresh is the process of periodically reading information from an area of computer memory and immediately rewriting the read information to the same area without modification, for the purpose of preserving the information."refresh cycle" i ...

Design objectives

As part of their designs, sense amplifiers aim at a minimum sense delay, required level of amplification, minimum power consumption, fit into restricted layout areas, and high reliability and tolerance.

References

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External links

Current Shunt Monitoring Products Current Sensing Overview

High-speed sense amplifier for SRAM applications

Data caching in DRAM row buffers
Memory Semiconductor technology