电流反馈放大器

Current Feedback Amplifiers National Semiconductor Corporation电流反馈放大器美国国家半导体公司Current Feedback Amplifiers声明：本文为中国电子网社区网友NE5532翻译，在翻译中得到了中国电子网和广大社区网友的帮助，在这里一并致谢。

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西南科技大学（SWUST）江海波（NE5532）2004-7电流反馈放大器1. The Conventional OP Amp (6)2. Gain Bandwidth Tradeoff (10)3. Slew-Rate Limiting (13)4. The Current-Feedback Amplifier (16)5. No Gain-Bandwidth Tradeoff (22)6. Absence of Slew-Rate Limiting (25)7. Second-Order Effects (27)8. CF Applications Considerations (32)9. CF Integrators (34)10. Stray Input-Capacitance Compensation (36)11. 译注： (37)Current Feedback AmplifiersIn their effort to approximate the ideal OP amp, manufacturers 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 particularly 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 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 because of the Miller effect which multiplies the stray capacitance 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 configurations 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. 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 possible to achieve monolithics speeds that were hitherto available only in hybrid form. t fThe advantages of the current-feedback topology are best appreciated by comparing it against that of the conventional OP amp (Reference 3, Reference 4).电流反馈放大器在我们为实现理想OP放大器的努力中，生产商所要做的不仅仅是增大开环增益，减小输入相关的误差因素，例如失调电压、偏置电流，噪声情况，也应该确保足够带宽和建立时间特性。

在高速DAC和ADC系统、S/H（采样保持电路）、ATE（Automatic Test Equipment 自动测试系统）驱动器、视频系统和信号采集驱动器的应用领域，放大器的动态指标就显得尤为重要了。

（见参考文献1）作为一个电压处理型的器件，OP的速度被其内部特性所限制，主要是节点杂散电容和晶体管截止频率的瓶颈影响。

其中最严重的是杂散电容的影响，在作为高增益反相放大级使用的时候，杂散电容将造成输出和输入点之间的偶合，并且由于弥勒效应的影响，杂散电容的（效应）将被本级所放大。

另一方面，我们早就认识到，对电流信号处理的速度在本质上高于对电压信号的处理速度。

在电路中，杂散电感的影响比杂散电容要小得多；另外，三极管切换电流信号的速度也比切换电压信号要快得多。

以这些理论为支持，射极耦合器件、双极DAC、电流传送器，以及被称为电流反馈运放的高速运算放大器技术。

（见参考文献2）要想实现真正的电流处理，电路中的所有节点都应该保持在一个固定的电压上，以避免由于杂散电容带来的缓变现象。

但是，由于OP输出的必须是一个电压量，在一些节点上我们也要采用高速电压处理的措施。

采用一些在结构上能够避免弥勒效应的电路，如采用共集电极（射极跟随器）、共射——共基结构，采用推挽电路来驱动节点，以使得其能够对杂散电容迅速充电、放电。

为了确保上升时间和下降时间的对称性，推挽结构使用的NPN管和PNP管必须要有相匹配的性能参数，特别是截至频率f t。

在传统的单片IC中，PNP管的性能较其配对的NPN管低很多的情况一直困扰着我们。

但是，近期真互补高速工艺的发展，使得对管能够达到单片IC 的速度，但迄今为止，这仅限于混合模式中。

于传统模式的OP相比较，可以让我们更深入地了解电流反馈OP的优势所在（见参考文献3、参考文献4）。

Current Feedback Amplifiers1. The Conventional OP Amp传统运算放大器：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 isd o V jf a V )(=------- (1)Whereis the output voltage; o V n p d V V V −= is the differential inputvoltage; and , a complex function of frequency, is the open-loopgain. )(jf a t fConnecting 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.o i d V R R R V V 211+−=------- (2)Substituting into Eq. (1), collecting, and solving for the ratio V yields thefamiliar non-inverting amplifier transfer characteristic i o V /)(/111)1()(12jf T R R V V jf A i o ++==∆------- (3)Where represents the closed-loop gain, and)(jf A电流反馈放大器12/1)()(R R jf a jf T +=-------(4)Represents the loop gain.The designation loop gain stems from the fact that if we break the loop as in Figure 2a and inject a test signal withsuppressed, the circuit will firstattenuate to produce x V i V x V 12/R R V xn =1+V and then amplify to producen VCurrent Feedback Amplifiersn o aV V −=/V o T . Hence, the gain experienced by a signal in going-around theloop is . The negative of this ratio represents theloop gain, )/1(12R R a V x +−=)/(x o V V −≡ Hence, Eq. (4).12/1R R +d V ∞→d p V o a V (a n V =V V −=The loop gain gives a measure of how closeA is to the ideal value, also called the noise gain of the circuit. By Eq. (3), the larger T ,the better. To ensure substantial loop gain over a wide range of closed-loop gains, OP amp manufacturers strive to makeA as large as possible.Consequently, will assume extremely small values since . Inthe limitwe obtain V , that is, . This forms thebasis of the familiar OP amp rule: when operated with negative feedback, an OP amp will provide whatever output voltage and current are needed to ideally force to follow . a V V o d /=0→p n V V →传统的运算放大器由一个高输入阻抗的差分放大器和一个高增益的放大级组成，其中放大级的末级为一个低输出阻抗的放大器。