Voltage Follower
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In
electronics Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...
, a buffer amplifier is a unity gain
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
that copies a
signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...
from one circuit to another while transforming its
electrical impedance In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of Electrical_resistance, resistance and Electrical_reactance, reactance in a electrical circuit, circuit. Quantitatively, the impedan ...
to provide a more ideal source (with a ''lower''
output impedance In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
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 An electrical load is an electrical component or portion of a Electric Circuit, circuit that consumes (active) electric power, such as electrical appliances and Electric light, lights inside the home. The term may also refer to the power Power con ...
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 In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
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 '. 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.


Current buffer

Typically a current buffer amplifier is used to transform a current signal with a low
output impedance In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...
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. As an example, consider a Norton source (current ''IA'', parallel resistance ''RA'') driving a resistor load ''RL''. Because of
current division In electronics, a current divider is a simple linear circuit that produces an output current (''IX'') that is a fraction of its input current (''IT''). Current division refers to the splitting of current between the branches of the divider. The c ...
(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 ''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.


Voltage buffer examples


Op-amp implementation

A
unity Unity is the state of being as one (either literally or figuratively). It may also refer to: Buildings * Unity Building, Oregon, Illinois, US; a historic building * Unity Building (Chicago), Illinois, US; a skyscraper * Unity Buildings, Liverpoo ...
gain buffer amplifier may be constructed by applying a full series
negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused ...
(Fig. 2) to an
op-amp An operational amplifier (often op amp or opamp) is a DC-coupled electronic voltage amplifier with a differential input, a (usually) single-ended output, and an extremely high gain. Its name comes from its original use of performing mathem ...
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 (Vout follows Vin 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 to 10 ), 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 A voltage source is a two-terminal (electronics), terminal device which can maintain a fixed voltage. An ideal voltage source can maintain the fixed voltage independent of the load resistance or the output Electric current, current. However, a r ...
. Both the connections to and from the buffer are therefore bridging connections, which reduce power consumption in the source,
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
from overloading,
crosstalk In electronics, crosstalk (XT) is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, ...
and other electromagnetic interference.


Simple transistor circuits

Other unity gain buffer amplifiers include the
bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor (FET), uses only one kind of charge carrier. A ...
in
common-collector In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit, the base termin ...
configuration (called an ''emitter follower'' because the emitter voltage follows the base voltage, or a ''voltage follower'' because the output voltage follows the input voltage); the
field effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...
in
common-drain In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the t ...
configuration (called a source follower because the source voltage follows the gate voltage or, again, a ''voltage follower'' because the output voltage follows the input voltage); or similar configurations using
vacuum tube A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
s ( cathode follower), or other active devices. All such amplifiers actually have a gain of slightly less than unity (though the loss may be small and unimportant) and add a
DC offset In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform. A waveform with zero mean or no DC bias is known as a ''DC balanced'' or ''DC ...
. Only one transistor is shown as the active device in these schematics (however, the current source in these circuits may require transistors too).


Impedance transformation using the bipolar voltage follower

Using the small-signal circuit in Figure 4, the impedance seen looking into the circuit is :: R_ = \frac = r_ + (\beta + 1) ( , , ) (The analysis uses the relation ''gmrπ = (IC /VT) (VT /IB)'' = β, which follows from the evaluation of these parameters in terms of the bias currents.) Assuming the usual case where ''rO'' >> ''RL'', the impedance looking into the buffer is larger than the load ''RL'' 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 ''rπ'', but often ''rπ'' << (β + 1) RL, 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 ''RL'' 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.


Logic buffer amplifiers

A non-linear buffer amplifier is sometimes used in digital circuits where a high current is required, perhaps for driving more gates than the normal
fan-out In digital electronics, the fan-out is the number of gate inputs driven by the output of another single logic gate. In most designs, logic gates are connected to form more complex circuits. While no logic gate input can be fed by more than one ...
of the logic family used, or for driving displays, or long wires, or other difficult loads. It is common for a single package to contain several discrete buffer amplifiers. For example, a hex buffer is a single package containing 6 buffer amplifiers, and an octal buffer is a single package containing 8 buffer amplifiers. The terms inverting buffer and non-inverting buffer effectively correspond with high-current capability single-input NOR or OR gates respectively.


Speaker array amplifiers

The majority of amplifiers used to drive large speaker arrays, such as those used for rock concerts, are amplifiers with 26-36dB voltage gain capable of high amounts of current into low impedance speaker arrays where the speakers are wired in parallel.


Driven guards

A driven guard utilizes a voltage buffer to protect a very high impedance signal line by surrounding the line with a shield driven by a buffer to the same voltage as the line, the close voltage matching of the buffer prevents the shield from leaking significant current into the high impedance line while the low impedance of the shield can absorb any stray currents that could affect the signal line.


Current buffer examples

Simple unity gain buffer amplifiers include the
bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor (FET), uses only one kind of charge carrier. A ...
in common-base configuration, or the
MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...
in
common-gate In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit, the source terminal of the transistor serves ...
configuration (called a ''current follower'' because the output current follows the input current). The current gain of a current buffer amplifier is (approximately) unity.


Simple transistor circuits

Figure 6 shows a bipolar current buffer biased with a current source (designated ''IE'' for DC emitter current) and driving another DC current source as active load (designated ''IC'' for DC collector current). The AC input signal current ''iin'' is applied to the emitter node of the transistor by an AC Norton current source with Norton resistance ''RS''. The AC output current ''iout'' is delivered by the buffer via a large coupling capacitor to load ''RL''. This coupling capacitor is large enough to be a short circuit at frequencies of interest. Because the transistor output resistance connects input and output sides of the circuit, there is a (very small) backward voltage feedback from the output to the input so this circuit is not unilateral. In addition, for the same reason, the input resistance depends (slightly) upon the output load resistance, and the output resistance depends significantly on the input driver resistance. For more detail see the article on
common base amplifier Common may refer to: As an Irish surname, it is anglicised from Irish Gaelic surname Ó Comáin. Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common, a central public park in Boston, Massachusetts * Cambridge Com ...
.


See also

*
Preamplifier A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier a ...
*
Common base In electronics, a common-base (also known as grounded-base) electronic amplifier, amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier. In t ...
*
Common gate In electronics, a common-gate electronic amplifier, amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a Electric current, current buffer amplifier, buffer or voltage amplifier. In ...
*
Common collector In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit, the base term ...
*
Common drain In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the t ...
* Current differencing buffered amplifier *
Negative feedback amplifier A negative-feedback amplifier (or feedback amplifier) is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal. The applied negative feedback can improve its perfo ...
* Driven shield * Voltage controlled voltage source filter


References

{{Reflist Electronic amplifiers