CS 3651 – Prototyping Intelligent Appliances

Buffer Amplifiers

Georgia Institute of Technology

23 January 2009 1

Buffer Amplifiers

Terms and their specific meanings:Voltage, current, resistance, impedance, etc.

What is a Buffer Amplifier?Two types, current and voltage

What do they do?What are some examples?

Analog-to-Digital converters

Terms

Voltage – The difference in electrical potential between two points, i.e. the line integral of the relative field strength of the two points.

Current – The flow of electrical charge.

Resistance – opposition of current flow, DC.

Impedance – opposition to current flow, AC.

Power – Amount of energy delivered over time.

What is a buffer amplifier?

“A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another. Two main types of buffer exist: the voltage buffer and the current buffer.” (picture and quote from Wikipedia)

This means that by using a difference in impedance, the current or voltage can be transformed. The capital alpha and beta symbols represent the voltage and current gain, respectively.

Voltage Buffer

Typically used to transfer voltage from a circuit current with high impedance to a circuit current with low impedance.

This prevents the second circuit from being overloaded by the firstCould be disastrous to any circuit.

The ideal buffer has an infinite input impedance and zero output impedance.

Unity gain bufferGain coefficient of 1Even though voltage gain is 1, usually current is boosted, which means more power. Why? (I’m not sure )

Voltage Buffer (cont’d)

What does the circuit do?         This allows the output to draw more voltage without drawing it directly from the source, rather from a “reserve”Prevents the output from drawing too much and overloading the circuitSince the source isn’t being tapped, which means the current doesn’t rise, less power is used and less heat is created (if any at all)

This is just a unity gain voltagebuffer.

Current Buffer

Typically used to transfer current from a circuit with low impedance to a circuit with high impedance.

This prevents the second circuit from being overloaded by the firstCould be disastrous to any circuit. (Again!)

The ideal buffer has a zero input impedance and infinite output impedance.

Unity gain buffer

Current Buffer(cont’d)

Example:Imagine you want to have some device that has a high output current, but your computer would blow up if it received that much current.Too much current can cause wires to melt and fuse, or ignite its surroundings.By using a current buffer with a gain coefficient of < 1, the impedance can be used to reduce the current to levels which are safe for the computer

The number of situations in which these buffers are useful is without end.

Analog-to-digital converters (ADC converters)

“An electronic device that converts an input analog voltage (or current) to a digital number.” Wikipedia

The simplest form of output is binary, but other forms can be used, such as Gray code or two’s complement binary.