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.

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. ==Voltage buffer== 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 A v 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 V A , series resistance R A ) driving a resistor load R L . Because of voltage division (also referred to as "loading") the voltage across the load is only: . 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 V A , and with no voltage division because the amplifier input resistance is infinite. At the output the dependent voltage source delivers voltage A v V A = V A 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 V A with zero Thévenin resistance. ==Current buffer== 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 β i 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 I A , parallel resistance R A ) driving a resistor load R L . Because of current division (also referred to as "loading") the current delivered to the load is only: . 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 I A , with no current division because the amplifier input resistance is zero. At the output the dependent current source delivers current β i I A = I A 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 I A with infinite Norton resistance. ==Voltage buffer examples== 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 to simply equal the input voltage (V out follows V in so the circuit is named op-amp voltage follower). The impedance 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. ====Impedance transformation using the bipolar voltage follower==== Using the small-signal circuit in Figure 4, the impedance seen looking into the circuit is R_{\rm in} = \frac {v_x} {i_x} = r_{\pi} + (\beta + 1) ({r_{\rm O}} || {R_{\rm L}}) (The analysis uses the relation g m r π = (I C /V T ) (V T /I B ) = β, which follows from the evaluation of these parameters in terms of the bias currents.) Assuming the usual case where r O >> R L , the impedance looking into the buffer is larger than the load R L without the buffer by a factor of (β + 1), which is substantial because β is large. The impedance is increased even more by the added r π , but often r π L , so the addition does not make much difference. ====Impedance transformation using the MOSFET voltage follower==== Using the small-signal circuit in Figure 5, the impedance seen looking into the circuit is no longer R L but instead is infinite (at low frequencies) because the MOSFET draws no current. As frequency is increased, the parasitic capacitances of the transistors come into play and the transformed input impedance drops with frequency. ====Chart of single-transistor amplifiers==== Some configurations of single-transistor amplifier can be used as a buffer to isolate the driver from the load. For most digital applications, an NMOS voltage follower (common drain) is the preferred configuration. These amplifiers have high input impedance, which means that the digital system will not need to supply a large current. ==Current buffer examples== Simple unity gain buffer amplifiers include the bipolar junction transistor in common-base configuration, or the MOSFET in common-gate configuration (called a current follower because the output current follows the input current). The current gain of a current buffer amplifier is (approximately) unity. ==References==

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