A sense amplifier is a circuit that is used to amplify and detect small signals in electronic systems. It is commonly used in

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...

circuits, such as dynamic random access memory (DRAM), to read and amplify the weak signals stored in memory cells. In modern computer memory, a sense amplifier is one of the elements which make up the circuitry on a

semiconductor memory Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a si ...

chip (

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

); the term itself dates back to the era of magnetic core memory. 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 (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 of ...

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 A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

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 () 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 relationship between a signal ...

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 Dram, DRAM, or drams may refer to: Technology and engineering * Dram (unit), a unit of mass and volume, and an informal name for a small amount of liquor, especially whisky or whiskey * Dynamic random-access memory, a type of electronic semicondu ...

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 (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

in tiny

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

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.

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