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Buffer amplifier
In electronics, a buffer amplifier is a unity gain amplifier that copies a signal from one circuit to another while transforming its electrical impedance to provide a more ideal source (with a lower output impedance for a voltage buffer or a higher output impedance for a current buffer). This "buffers" the signal source in the first circuit against being affected by currents from the electrical load of the second circuit and may simply be called a buffer or follower when context is clear.
A voltage buffer amplifier is used to transform a voltage signal with high output impedance from a first circuit into an identical voltage with low impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation, since without the voltage buffer, the voltage of the second circuit is influenced by output impedance of the first circuit (as it is larger than the input impedance of the second circuit). In the ideal voltage buffer (Figure 1 top), the input impedance is infinite and the output impedance is zero. Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
If the voltage is transferred unchanged (the voltage gain Av is 1), the amplifier is a unity gain buffer; also known as a voltage follower because the output voltage follows or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain.
As an example, consider a Thévenin source (voltage VA, series resistance RA) driving a resistor load RL. Because of voltage division (also referred to as "loading") the voltage across the load is only:
VA RL/RL + RA.
However, if the Thévenin source drives a unity gain buffer such as that in Figure 1 (top, with unity gain), the voltage input to the amplifier is VA, and with no voltage division because the amplifier input resistance is infinite. At the output the dependent voltage source delivers voltage Av VA = VA to the load, again without voltage division because the output resistance of the buffer is zero. A Thévenin equivalent circuit of the combined original Thévenin source and the buffer is an ideal voltage source VA with zero Thévenin resistance.
Typically a current buffer amplifier is used to transform a current signal with a low output impedance from a first circuit into an identical current with high impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit's current unacceptably and interfering with its desired operation. In the ideal current buffer (Figure 1 bottom), the output impedance is infinite (an ideal current source) and the input impedance is zero (a short circuit). Again, other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
For a current buffer, if the current is transferred unchanged (the current gain βi is 1), the amplifier is again a unity gain buffer; this time known as a current follower because the output current follows or tracks the input current.
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Buffer amplifier
In electronics, a buffer amplifier is a unity gain amplifier that copies a signal from one circuit to another while transforming its electrical impedance to provide a more ideal source (with a lower output impedance for a voltage buffer or a higher output impedance for a current buffer). This "buffers" the signal source in the first circuit against being affected by currents from the electrical load of the second circuit and may simply be called a buffer or follower when context is clear.
A voltage buffer amplifier is used to transform a voltage signal with high output impedance from a first circuit into an identical voltage with low impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation, since without the voltage buffer, the voltage of the second circuit is influenced by output impedance of the first circuit (as it is larger than the input impedance of the second circuit). In the ideal voltage buffer (Figure 1 top), the input impedance is infinite and the output impedance is zero. Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
If the voltage is transferred unchanged (the voltage gain Av is 1), the amplifier is a unity gain buffer; also known as a voltage follower because the output voltage follows or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain.
As an example, consider a Thévenin source (voltage VA, series resistance RA) driving a resistor load RL. Because of voltage division (also referred to as "loading") the voltage across the load is only:
VA RL/RL + RA.
However, if the Thévenin source drives a unity gain buffer such as that in Figure 1 (top, with unity gain), the voltage input to the amplifier is VA, and with no voltage division because the amplifier input resistance is infinite. At the output the dependent voltage source delivers voltage Av VA = VA to the load, again without voltage division because the output resistance of the buffer is zero. A Thévenin equivalent circuit of the combined original Thévenin source and the buffer is an ideal voltage source VA with zero Thévenin resistance.
Typically a current buffer amplifier is used to transform a current signal with a low output impedance from a first circuit into an identical current with high impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit's current unacceptably and interfering with its desired operation. In the ideal current buffer (Figure 1 bottom), the output impedance is infinite (an ideal current source) and the input impedance is zero (a short circuit). Again, other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
For a current buffer, if the current is transferred unchanged (the current gain βi is 1), the amplifier is again a unity gain buffer; this time known as a current follower because the output current follows or tracks the input current.