Fully differential amplifier
Encyclopedia
A fully differential amplifier, usually referred to as an 'FDA' for brevity, is a DC
-coupled
high-gain electronic voltage amplifier
with differential inputs and differential outputs. In its ordinary usage, the output of the FDA is controlled by two feedback paths which, because of the amplifier's high gain, almost completely determines the output voltage for any given input.
, infinite bandwidth, infinite input impedance
s resulting in zero input currents, infinite slew rate
, zero output impedance
and zero noise
.
In the ideal FDA, the difference of the output voltages is equal to the product of the difference of the input voltages, multiplied by the gain. The common mode voltage of the output voltages is not dependent of the input voltage. In many cases, the common mode voltage can be directly set by a third voltage input.
Input voltage: Vid = (Vin+) – (Vin–)
Output voltage: Vod = (Vout+) – (Vout–) = Vid * Gain
Output common-mode voltage: Voc = ((Vout+)-(Vout–))/2
A real FDA can only approximate this ideal, and the actual parameters are subject to drift over time and with changes in temperature, input conditions, etc. Modern integrated FET
or MOSFET
FDAs approximate more closely to these ideals than bipolar ICs where large signals must be handled at room temperature over a limited bandwidth; input impedance, in particular, is much higher, although the bipolar FDA usually exhibit superior (i.e., lower) input offset drift and noise characteristics.
Where the limitations of real devices can be ignored, an FDA can be viewed as a Black Box with gain; circuit function and parameters are determined by feedback
, usually negative. An FDA as implemented in practice is moderately complex integrated circuit
.
from input to output without any feedback
applied. For most practical calculations, the open-loop gain is assumed to be infinite; in reality it is obviously not. Typical devices exhibit open-loop DC gain ranging from 100,000 to over 1 million; this is sufficiently large for circuit gain to be determined almost entirely by the amount of negative feedback used. Op-amps have performance limits that the designer must keep in mind and sometimes work around. In particular, instability is possible in a DC amplifier if AC aspects are neglected.
. For example, an FDA with a gain bandwidth product of 1 MHz would have a gain of 5 at 200 kHz, and a gain of 1 at 1 MHz. This low-pass characteristic is introduced deliberately, because it tends to stabilize the circuit by introducing a dominant pole. This is known as frequency compensation
.
Typical low cost, a general purpose FDA exhibits a gain bandwidth product
of a few megahertz. Specialty and high speed FDAs can achieve gain bandwidth products
of hundreds of megahertz. Some FDAs are even capable of gain bandwidth products
greater than a gigahertz.
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
-coupled
Direct coupling
In electronics, direct coupling is a way of interconnecting two circuits such that, in addition to transferring the AC signal , the first stage also provides DC bias to the next...
high-gain electronic voltage amplifier
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
with differential inputs and differential outputs. In its ordinary usage, the output of the FDA is controlled by two feedback paths which, because of the amplifier's high gain, almost completely determines the output voltage for any given input.
The ideal FDA
For any input voltages the ideal FDA has infinite open-loop gainOpen-loop gain
The open-loop gain of an operational amplifier is the gain obtained when no feedback is used in the circuit.Open loop gain is usually exceedingly high; in fact, an ideal operational amplifier has infinite open-loop gain. Typically an op-amp may have an open-loop gain of around 10^5...
, infinite bandwidth, infinite input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
s resulting in zero input currents, infinite slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
, zero output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
and zero noise
Electronic noise
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
.
In the ideal FDA, the difference of the output voltages is equal to the product of the difference of the input voltages, multiplied by the gain. The common mode voltage of the output voltages is not dependent of the input voltage. In many cases, the common mode voltage can be directly set by a third voltage input.
Input voltage: Vid = (Vin+) – (Vin–)
Output voltage: Vod = (Vout+) – (Vout–) = Vid * Gain
Output common-mode voltage: Voc = ((Vout+)-(Vout–))/2
A real FDA can only approximate this ideal, and the actual parameters are subject to drift over time and with changes in temperature, input conditions, etc. Modern integrated FET
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
or MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
FDAs approximate more closely to these ideals than bipolar ICs where large signals must be handled at room temperature over a limited bandwidth; input impedance, in particular, is much higher, although the bipolar FDA usually exhibit superior (i.e., lower) input offset drift and noise characteristics.
Where the limitations of real devices can be ignored, an FDA can be viewed as a Black Box with gain; circuit function and parameters are determined by feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
, usually negative. An FDA as implemented in practice is moderately complex integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
.
DC imperfections
- Finite gainGainIn electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
— the effect is most pronounced when the overall design attempts to achieve gain close to the inherent gain of the FDA. - Finite input resistance — this puts an upper bound on the resistances in the feedback circuit.
- Nonzero output resistance — important for low resistance loads. Except for very small voltage output, power considerations usually come into play first. (Output impedance is inversely proportional to the idle current in the output stage — very low idle current results in very high output impedance.)
- Input bias current — a small amount of current (typically ~10 nA for bipolarBipolar junction transistor|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
FDAs, or picoamperes for CMOSCMOSComplementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
designs) flows into the inputs. This current is mismatched slightly between the inverting and non-inverting inputs (there is an input offset current). This effect is usually important only for very low power circuits. - Input offset voltageVoltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
— the FDA will produce an output even when the input pins are at exactly the same voltage. For circuits which require precise DC operation, this effect must be compensated for. - Common mode gain — A perfect operational amplifier amplifies only the voltage difference between its two inputs, completely rejecting all voltages that are common to both. However, the differential input stage of an FDA is never perfect, leading to the amplification of these identical voltages to some degree. The standard measure of this defect is called the common-mode rejection ratioCommon-mode rejection ratioThe common-mode rejection ratio of a differential amplifier is the tendency of the devices to reject the input signals common to both input leads...
(denoted, CMRR). Minimization of common mode gain is usually important in non-inverting amplifiers (described below) that operate at high amplification. - Temperature effects — all parameters change with temperature. Temperature drift of the input offset voltage is especially important.
AC imperfections
- Finite bandwidth — all amplifiers have a finite bandwidth. This is because FDAs use internal frequency compensationFrequency compensationIn electrical engineering, frequency compensation is a technique used in amplifiers, and especially in amplifiers employing negative feedback. It usually has two primary goals: To avoid the unintentional creation of positive feedback, which will cause the amplifier to oscillate, and to control...
to increase the phase marginPhase marginIn electronic amplifiers, phase margin is the difference between the phase, measured in degrees, of an amplifier's output signal and 180°, as a function of frequency. The PM is taken as positive at frequencies below where the open-loop phase first crosses 180°, i.e. the signal becomes inverted,...
. - Input capacitanceCapacitanceIn electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
— most important for high frequency operation because it further reduces the open loop bandwidth of the amplifier. - Common mode gain — See DC imperfections, above.
- Noise - all real electronic components (except superconductor) generate noise. You can find devices with 0.8 to several hundreds nV/rtHz noise performance.
Nonlinear imperfections
- SaturationSaturation (telecommunications)In telecommunications, the term saturation has the following meanings:*In a communications system, the condition at which a component of the system has reached its maximum traffic-handling capacity...
— output voltage is limited to a peak value, usually slightly less than the power supplyPower supplyA power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
voltage. Saturation occurs when the differential input voltage is too high for the op-amp's gain, driving the output level to that peak value. - Slewing — the amplifier's output voltage reaches its maximum rate of change. Measured as the slew rateSlew rateIn electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
, it is usually specified in volts per microsecond. When slewing occurs, further increases in the input signal have no effect on the rate of change of the output. Slewing is usually caused by internal capacitances in the amplifier, especially those used to implement its frequency compensationFrequency compensationIn electrical engineering, frequency compensation is a technique used in amplifiers, and especially in amplifiers employing negative feedback. It usually has two primary goals: To avoid the unintentional creation of positive feedback, which will cause the amplifier to oscillate, and to control...
, particularly using pole splittingPole splittingPole splitting is a phenomenon exploited in some forms of frequency compensation used in an electronic amplifier. When a capacitor is introduced between the input and output sides of the amplifier with the intention of moving the pole lowest in frequency to lower frequencies, pole splitting causes...
. - Non-linearLinearIn mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
transfer functionTransfer functionA transfer function is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a linear time-invariant system. With optical imaging devices, for example, it is the Fourier transform of the point spread function i.e...
— The output voltage may not be accurately proportional to the difference between the input voltages. It is commonly called distortion when the input signal is a waveform. This effect will be very small in a practical circuit if substantial negative feedback is used.
Power considerations
- Limited output powerElectric powerElectric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
— if high power output is desired, an op-amp specifically designed for that purpose must be used. Most op-amps are designed for low power operation and are typically only able to drive output resistances down to 2 kΩ. - Limited output currentCurrent limitingCurrent limiting is the practice in electrical or electronic circuits of imposing an upper limit on the current that may be delivered to a load with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load...
— the output current must obviously be finite. In practice, most op-amps are designed to limit the output current so as not to exceed a specified level thus protecting the FDA and associated circuitry from damage.
DC behavior
Open-loop gain is defined as the amplificationAmplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
from input to output without any feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
applied. For most practical calculations, the open-loop gain is assumed to be infinite; in reality it is obviously not. Typical devices exhibit open-loop DC gain ranging from 100,000 to over 1 million; this is sufficiently large for circuit gain to be determined almost entirely by the amount of negative feedback used. Op-amps have performance limits that the designer must keep in mind and sometimes work around. In particular, instability is possible in a DC amplifier if AC aspects are neglected.
AC behavior
The FDA gain calculated at DC does not apply at higher frequencies. To a first approximation, the gain of a typical FDA is inversely proportional to frequency. This means that an FDA is characterized by its gain-bandwidth productGain-bandwidth product
The gain–bandwidth product for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured....
. For example, an FDA with a gain bandwidth product of 1 MHz would have a gain of 5 at 200 kHz, and a gain of 1 at 1 MHz. This low-pass characteristic is introduced deliberately, because it tends to stabilize the circuit by introducing a dominant pole. This is known as frequency compensation
Frequency compensation
In electrical engineering, frequency compensation is a technique used in amplifiers, and especially in amplifiers employing negative feedback. It usually has two primary goals: To avoid the unintentional creation of positive feedback, which will cause the amplifier to oscillate, and to control...
.
Typical low cost, a general purpose FDA exhibits a gain bandwidth product
Gain-bandwidth product
The gain–bandwidth product for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured....
of a few megahertz. Specialty and high speed FDAs can achieve gain bandwidth products
Gain-bandwidth product
The gain–bandwidth product for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured....
of hundreds of megahertz. Some FDAs are even capable of gain bandwidth products
Gain-bandwidth product
The gain–bandwidth product for an amplifier is the product of the amplifier's bandwidth, and the gain at which the bandwidth is measured....
greater than a gigahertz.
See also
- Operational amplifier applicationsOperational amplifier applicationsThis article illustrates some typical applications of operational amplifiers. A simplified schematic notation is used, and the reader is reminded that many details such as device selection and power supply connections are not shown....
- Instrumentation amplifierInstrumentation amplifierAn instrumentation amplifier is a type of differential amplifier that has been outfitted with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment...
- Active filterActive filterAn active filter is a type of analog electronic filter that uses an amplifier stage. Amplifiers included in a filter design can be used to improve the performance, stability and predictability of a filter. An amplifier prevents the impedance of source or load stages from affecting the...
- Analog computerAnalog computerAn analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...
- Current-feedback operational amplifierCurrent-feedback operational amplifierThe current feedback operational amplifier otherwise known as CfoA or CfA is a type of electronic amplifier whose inverting input is sensitive to current, rather than to voltage as in a conventional voltage-feedback operational amplifier . The CFA was invented by David Nelson at Comlinear...
- Operational transconductance amplifierOperational transconductance amplifierThe operational transconductance amplifier is an amplifier whose differential input voltage produces an output current. Thus, it is a voltage controlled current source . There is usually an additional input for a current to control the amplifier's transconductance...
External links
- Fully-Differential Amplifiers (TI Application Report)
- Fully Differential Amplifiers (TI Analog Applications Journal)