Buffer amplifier

A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents (or voltages, for a current buffer) that the load may be produced with. The signal is 'buffered from' load currents. Two main types of buffer exist: the voltage buffer and the current buffer. A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents (or voltages, for a current buffer) that the load may be produced with. The signal is 'buffered from' load currents. Two main types of buffer exist: the voltage buffer and the current buffer. A voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal voltage buffer in the diagram, the input resistance is infinite and the output resistance zero (output impedance of an ideal voltage source 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 transfer a current from a first circuit, having a low output impedance level, to a second circuit with a high input impedance level. 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 in the diagram, 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. As an example, consider a Norton source (current IA, parallel resistance RA) driving a resistor load RL. Because of current division (also referred to as 'loading') the current delivered to the load is only IA RA / ( RL + RA ). However, if the Norton source drives a unity gain buffer such as that in Figure 1 (bottom, with unity gain), the current input to the amplifier is IA, with no current division because the amplifier input resistance is zero. At the output the dependent current source delivers current βi IA = IA to the load, again without current division because the output resistance of the buffer is infinite. A Norton equivalent circuit of the combined original Norton source and the buffer is an ideal current source IA with infinite Norton resistance. A unity gain buffer amplifier may be constructed by applying a full series negative feedback (Fig. 2) to an op-amp simply by connecting its output to its inverting input, and connecting the signal source to the non-inverting input (Fig. 3). Unity gain here implies a voltage gain of one (i.e. 0 dB), but significant current gain is expected. In this configuration, the entire output voltage (β = 1 in Fig. 2) is fed back into the inverting input. The difference between the non-inverting input voltage and the inverting input voltage is amplified by the op-amp. This connection forces the op-amp to adjust its output voltage simply equal to the input voltage (Vout follows Vin so the circuit is named op-amp voltage follower). The importance of this circuit does not come from any change in voltage, but from the input and output impedances of the op-amp. The input impedance of the op-amp is very high (1 MΩ to 10 TΩ), meaning that the input of the op-amp does not load down the source and draws only minimal current from it. Because the output impedance of the op-amp is very low, it drives the load as if it were a perfect voltage source. Both the connections to and from the buffer are therefore bridging connections, which reduce power consumption in the source, distortion from overloading, crosstalk and other electromagnetic interference.