Current Feedback Amplifiers
In their effort to approximate the ideal op amp,manufactur-ers must not only maximize the open-loop gain and minimize input-referred errors such as offset voltage,bias current,and noise,but must also ensure adequate band-width and settling-time characteristics.Amplifier dynamics are particu-larly important in applications like high-speed DAC buffers,subranging ADCs,S/H circuits,ATE pin drivers,and video and IF drivers (Reference 1)
Being basically voltage-processing devices,op amps are subject to the speed limitations inherent to voltage-mode operation,stemming primarily from the stray capacitances of nodes and the cutoff frequencies of transistors.Particularly severe is the effect of the stray capacitances between the input and output nodes of high-gain inverting stages be-cause of the Miller effect which multiplies the stray capaci-tance by the voltage gain of the stage.
On the other hand,it has long been recognized that current manipulation is inherently faster than voltage manipulation.The effect of stray inductances in a circuit is usually less severe than that of its stray capacitances,and BJTs can switch currents much more rapidly than voltages.These technological reasons form the basis of emitter-coupled logic,bipolar DACs,current conveyors,and the high-speed amplifier topology know as current-feedback (Reference 2)For true current-mode operation,all nodes in the circuit should ideally be kept at fixed voltages to avoid the slow-down effect by their stray capacitances.However,since the output of the amplifier must be a voltage,some form of high-speed voltage-mode operation must also be provided at some point.This is achieved by employing gain configura-tions that are inherently immune from the Miller effect,such as the common-collector and the cascode configurations,and by driving the nodes with push-pull stages to rapidly charge/discharge their stray capacitances.
To ensure symmetric rise and fall times,the npn and pnp transistors must have comparable characteristics in terns of cutoff frequency f t .Traditionally,monolithics pnp’s have been plagued by much poorer performance characteristics than their npn counterparts.However,the recent development of truly complementary high-speed processes makes it pos-sible to achieve monolithics speeds that were hitherto avail-able only in hybrid form.
The advantages of the current-feedback topology are best appreciated by comparing it against that of the conventional op amp (Reference 3,Reference 4).
The Conventional Op Amp
The conventional op amp consists of a high input-impedance differential stage followed by additional gain stages,the last of which is a low output-impedance stage.As shown in the circuit model of Figure 1A,the op amp transfer characteristic is
V O =a(jf)V d
(1)where V O is the output voltage;V d =V p −V n is the differen-tial input voltage;and a (jf),a complex function of frequency f t ,is the open-loop gain.
Connecting an external network as in Figure 1B creates a feedback path along which a signal in the form of a voltage is derived from the output and applied to the non-inverting input.By inspection.
(2)
Substituting into Eq.(1),collecting,and solving for the ratio V O /V I yields the familiar non-inverting amplifier transfer char-acteristic
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FIGURE 1.Circuit model of the conventional op amp,
and connection as a non-inverting amplifier.National Semiconductor OA-31
November 1992
Current Feedback Amplifiers
OA-31
©2002National Semiconductor Corporation AN012775www.national
The Conventional Op Amp(Continued)
where A(jf)represents the closed-loop gain,and
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Slew-Rate Limiting
To fully characterize the dynamic behavior of an op amp,we also need to know its transient response.If an op amp with the response of Eq.(5)is operated as a unity-gain voltage follower and is subjected to a suitably small voltage step,its dynamic behavior will be similiar to that of an RC network. Applying an input step∆V i will cause the output to undergo an exponential transition with magnitude∆V o=∆V i,and with time-constantτ=1/(2πf t).For the741op amp we haveτ= 1/(2πx106)≅170ns.
The rate at which the output changes with time is highest at the beginning of the exponential transition,when its value is ∆V o/τ.Increasing the step magnitude increases this initial rate of change,until the latter will saturate at a value called the slew-rate(SR).This effect stems from the limited ability of the internal circuitry to charge/discharge capacitive loads, especially the compensation capacitor responsible for open-loop bandwidth f a.
To illustrate,refer to the circuit model of Figure3,which is typical of many op amps.4The input stage is a transconduc-tance block consisting of differential pair Q1−Q2and current mirror Q3−Q4.The remai
ning stages are lumped together as an integrator block consisting of an inverting amplifier and the compensation capacitor C.Slew-rate limiting occurs when the transconductance stage is driven into saturation, so that all the current available to charge/discharge C is the bias current/of this stage.For example,the741op amp has I=20µA and C=30pF,so that SR=I/C=0.67V/µs. The step magnitude corresponding to the onset of slew-rate limiting is such that∆V i/τ=SR,that is,∆V i=SR xτ=(0.67 V/µs)x(170ns)=116mV.As long as the step is less than 116mV,a741voltage follower will respond with an expo-nential transition governed byτ≅170ns,whereas for a greater input step the output will slew at a constant rate of 0.67V/µs.
In many applications the dynamic parameter of greatest concern is the settling time,that is,the time it takes for the output to settle and remain within a specified band around its final value,usually for a full-scale output transition.Clearly, slew-rate limiting plays an important role in the settling-time characteristic of the device.
The Current-Feedback Amplifier
As shown in the circuit model of Figure4,the architecture of
the current-feedback amplifier(CF amp)differs from the
conventional op amp in two respects:2
1.The input stage is a unity-gain voltage buffer connected
across the inputs of the op amp.its function is to force V n
to follow V p,very much like a conventional op amp does
via negative feedback.However,because of the low out-
put impedance of this buffer,current can easily flow in or
out of the inverting input,though we shall see that in
normal operation this current is extremely small.
2.Amplification is provided by a transimpedance amplifier
which senses the current delivered by the buffer to the
external feedback network,and produces an output volt-
age V o such that
Vo=z(jf)I n(8) where A z(jf)represents the transimpedance gain of the
rain图片amplifier,in V/A orΩ,ind I n is the current out of the inverting微甜的回忆
input.
instructive to examine the simplified circuit diagram of Figure
5a.The input buffer consists of transistors Q1through Q4.
While Q1and Q2form a low output-impedance push-pull
stage,Q3and Q4provide V BE compensation for the
push-pull pair,as well as a Darlington function to raise the
input impedance.
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The Current-Feedback Amplifier
(Continued)
Summing currents at the inverting node yields I1−I2=I n,
where I1and I2are the push-pull transistor currents.Two
Wilson current mirrors,consisting of transistors Q9−Q10−Q11
and Q13−Q14−Q15,reflect these currents and recombine
them at a common node,whose equivalent capacitance to
ground has been designated as C.By mirror action,the
current through this capacitance is I c=I1−I2,that is
I c=I n(9)
The voltage developed by C in response to this current is
城里的月光 伴奏
then conveyed to the output via a second buffer,consisting
of Q5through Q8.The salient features of the CF amp are
summarized in block diagram form in Figure5b.
When the amplifier loop is closed as in Figure4b,and
whenever an external signal tries to imbalance the two in-
puts,the input buffer will begin sourcing(or sinking)an
imbalance current I n to the external resistances.This imbal-
ance is the conveyed by the Wilson mirrors to capacitor C,
causing V o to swing in the positive(or negative)direction
until the original imbalance I n is neutralized via the negative
feedback loop.thus,I n plays the role of error signal in the
唐璜的回忆system.
To obtain the closed-loop transfer characteristic,refer again
to Figure4b.Summing currents at the inverting node yields
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The Current-Feedback Amplifier(Continued)
No Gain-Bandwidth Tradeoff
The transimpedance gain of a practical CF amp rolls off with
frequency according to
5
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