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1、New CCII Current ConveyorBy:John RobinsonMar 27, 2008Abstract: Offering higher bandwidth than its voltage-feedback counterpart, the second-generation current conveyor operational amplifier (CCII) can be used in RF mixers, high-frequency precision rectifiers, and medical applications such as electric
2、al impedance tomography. The conventional operational amplifiers cannot be used in the high-frequency applications due to their limited gain-bandwidth product.IntroductionThe current conveyor has been around since the original design, or CCI (which can be regarded as an ideal transistor), was initia
3、lly proposed by Smith and Sedra in 19681,2. CCI was then replaced by a more versatile second-generation device in 1970, the CCII3. Current conveyor designs have mainly been BJT due to their high transconductance values compared to their CMOS counterparts. They are used as current-feedback operationa
4、l amplifiers such as the MAX4223 low-power amplifier, which features current feedback rather than the conventional voltage feedback used by standard operational amplifiers. This means that the current feedback operational amplifier is not restricted by the conventional gain bandwidths of a standard
5、operational amplifier, and can offer a much higher bandwidth solution than its voltage-feedback counterpart.Current conveyors are used in high-frequency applications where the conventional operational amplifiers cannot be used, because the conventional designs are limited by their gain-bandwidth pro
6、duct. In theory, the current conveyor is only limited by the ft of the transistors used in its design. Some applications where current conveyors are used today include RF mixers, high-frequency precision rectifiers, and medical applications such as Electrical Impedance Tomography (EIT).A Bipolar Con
7、veyorThe diagram in Figure 1 below shows a current conveyor implemented using bipolar devices.Figure 1. A bipolar CCII.From Figure 1 it can be seen that the CCII conveyor can be modelled as an ideal transistor:Y being the base/gateX being the emitter/sourceZ being the collector/drainThis type of cir
8、cuit works well as a circuit with BJTs, as the transconductance and Early voltages of BJTs are much greater than that of CMOS devices. Therefore, current conveyors work well as source followers. Gain X/Y is close to 1; Z has a natural high-output impedance which cannot be mimicked by their CMOS coun
9、terparts.The CMOS Source FollowerAs previously explained, the major problem with a CMOS follower is the low gm and poor Early voltage (1/lambda). This equates to poor gain, because the gain for a voltage follower heavily relies on these two parameters to be large. This can be observed in the equatio
10、n below:1Ga r =1 + 0 + 触) 3mWhere gL is the load conductance, gds is the drain source conductance and gm is the transconductance of the CMOS device.A typical simulated gain with a TSMC 0.18m with a load of 1kQ gave a gain of 0.7. Compared to the ideal gain of 1, this represents a 30% loss in output
11、gain.Current Conveyor Source FollowerOne can use an unbuffered amplifier (Figure 2 a) to mimic a source follower with a gain of one. Then this modification can be added to the basic design in Figure 1 to make a CCII currentconveyor.Figure 2a. A simple source follower.Figure 2a can be implemented as
12、shown below in Figure 2b.Figure 2b. The CCII unbuffered source follower and implementation.From Figure 2b it can be seen that output X is fed back to one of the long tail pairs of inputs (X). The other input to the long tail pair is Y, as input Y changes the current through M1. M2 differs as M3, and
13、 M4 is a current mirror.There is a current difference between M2 and M4. This imbalance is addressed by pulling current from, or to, the gate/source capacitance Cgsof device M5. Until the output X matches that of Y, the bandwidth limit is defined as the rate at which this transistor can be discharge
14、d and charged. Thus, the bandwidth limit can be defined as:Banduu(dtr(w)=Current Conveyor (CCII+) Using an Unbuffered AmplifierFrom Figure 2, the first part of the current conveyor (CCII+) can be realized. To build the rest of the current conveyor (CCII+), the current from output X simply has to be
15、mirrored. To give Zs output, see the example in Figure 3.Figure 3. The current conveyor (CCII+) using an unbuffered amplifier.The current from M5/M6 is simply mirrored by M7/M8, giving the output Z(-) of the CCII+.The output impedance of Z can be improved by adding in a cascode to M7/M8 if necessary
16、. One must be aware that to mimic the current successfully, the output impedance of X must match that of Z, i.e., the same transistor types and confirmation must be used on M5/M6 as on M7/M8.The gain of the CCII is simply:Converting from a CCII+ to a CCII-Taking the bias point Yb (Figure 3), simply
17、add the extra connections as shown in Figure 4.Figure 4. The current conveyor configured as a CCII-.From Figure 4, if all transistor dimensions are the same and if Yb (the bias point from Figure 3) is taken, then a current 2i is generated from M10 and M11. This is mirrored by M9 to give a current of
18、 2i through M13. M12 provides a current of i and gives a current of -i through Z(+),thus giving a true CCII- output. There is a problem with this approach: the Z(+) now has an -i DC term instead of a +i term. Consequently, a 2i DC term needs to be added to the output of Z(+) to compensate for -i. Fi
19、gure 5 below shows the addition.M15r7敦-别DC】4 叩Figure 5. The CCII- output with added DC bias.From Figure 5, transistors M14 and M15 provide the appropriate current to compensate for the DC current taken by M13. (Note that M14 and M15 must match M12). Make the current through R3 equal to i(DC) - i. Re
20、member that R3 and R2 must match; any mismatch in their values will mean that their output DC values will differ.The VBiAs CircuitTo create the necessary and appropriate voltage, VDCB|as，that will maintain the DC currents in M14 and M15, then VDCB|as (Figure 5) must have the same DC value as Node Yb
21、 (Figures 4 and 5). To do this, simply mimic the front-end stage and take the DC value of the input signal as the input bias voltage into this stage (V|nDC), as shown in Figure 6 below.TOMu AND(FIGURF 5)Figure 6. VBIAScircuit for the DC compensation circuit (Figure 5).The only problem with this desi
22、gn is that another resistor (R4) is needed, and R4 must again match R2 and R3.Simulation ResultsUsing the CCII+ as in Figure 3 and using a TSMC 0.18pm process with R1 = 1kQ and R2 = 1kQ, results in a gain of 1. The 3dB bandwidth of the device was 2.5GHz and had a gain of 0.972 with a power-supply re
23、ject ratio (PSRR) of 41dB.These results were improved by using a cascode device to replace M5/M6 and M7/M8, which gave a bandwidth of 900MHz and an improved gain of 0.993. The PSRR was also improved at 51dB.References1K.C. Smith and A. Sedra, The Current-Conveyor A New Circuit Building Block, IEEE P
24、roc, Vol. 56, 1968, pp. 1368-1369.2C. Toumazou, John Lidgey & Alison Payne, Practical Integrated Current-Conveyors, Current Mode Circuits Techniques in Analog High Frequency Design, July 1996, Chapter 5.2, pp. 69-80.3K.C. Smith and A. Sedra, A Second Generation Current-Conveyor and its Applications,
25、 IEEE Trans, CT-17, 1970, pp. 132-134.This application note is based on an article published in Chip Design Magazine, August/September 2007.MAX4112Single/Dual/Quad, 400MHz, Low-Power, CurrentFeedback AmplifiersMAX477300MHz High-Speed Op Amp新型CCII电流传输器John RobinsonJul 23, 2008摘要：第二代电流传输器运算放大(CCII)与采用
26、电压反馈的类似器件相比可以提供更宽的频 带，适用于月F混频器、高频精密整流器以及医疗产品，例如：电阻抗断层成像系统。传统的运 算放大器受其增益带宽积限制，不能胜任高频应用。概述电流传输器或CCI (可以看作一个理想的晶体管)的概念最初是由Smith和Sedra于19681,2提出 的。之后，在1970年，CCI被更加通用的第二代器件CCI所取代。现在的传输器设计主要采 用BJT，它们与CMOS相比具有更高的跨导，非常适合电流反馈运算放大器的设计，例如 MAX4112低功耗放大器，其特点是电流反馈，而不是标准运算放大器中使用的电压反馈方式。 因此，电流反馈运算放大器不像标准运算放大器那样受到增益
27、带宽积的限制，它可以提供比电压 反馈器件更宽的频带。电流传输器通常用于传统运算放大器无法支持的高频产品，因为传统设计的增益带宽积有限。理 论上讲，电流传输器只受设计中晶体管ft的限制。目前采用电流传输器的应用主要包括：RF混 频器、高频精密整流器以及医疗产品，比如电阻抗断层成像系统(EIT)。双极型传输器图1所示框图是使用双极型器件构成的电流传输器。图1.双极型CCII从图1可以看出CCII传输器可以当作一个理想的晶体管模型：Y是基极/栅极X是发射极/源极Z是集电极/漏极这种利用BJT构成的电路能够很好地工作，因为BJT的跨导和Early电压比CMOS器件高。因 此，电流传输器可以很好地用作源
28、极跟随器。增益X/Y接近于1, Z具有高输出阻抗，这是CMOS 电路望尘莫及的。CMOS源极跟随器如同上述说明，CMOS跟随器的主要问题是gm和Early电压(1/lambda)较低，等同于低增益， 因为电压跟随器的增益很大程度上依赖于这两个参数的提高。通过下式可以看到这个关系：Ga r =1(GL + Qds)9 m式中，gL是负载电导，gds是漏源间电导，gm是CMOS器件的跨导。利用TSMC 0.18口m工艺，在负载为1k。时，典型的仿真增益可以达到0.7。同理想增益1相比， 存在30%的输出增益损失。电流传输器源极跟随器利用一个不带缓冲的放大器(图2a)可以模拟增益为1的源极跟随器，然
29、后在图1设计的基础上 增加这一电路，构成CCII电流传输器。图2a.简单的源极跟随器图2a可以按照下面的图2b实现。图2b. CCII无缓冲源极跟随器及其实现从图2b可以看出输出X被反馈到长尾晶体管对的一个输入(X)。长尾晶体管对的另一个输入是 丫，输入Y通过M1改变电流。M2与M3不同，M4是电流镜。M2和M4之间存在电流差。从器件M5的栅极/源极电容Cgs拉电流或馈入电流，可以解决不平 衡问题。在输出X与Y达到匹配之前，带宽限制定义为晶体管充电和放电的速率。因此，带宽 限制可以定义为：GprifllongTailPfi rj BandwidthCw) =采用非缓冲放大器的电流传输器(CCI
30、I+)按照图2,可以实现电流传输器(CCII+)的第一部分。为了完成电流传输器(CCII+)的剩余部分， 输出X电流只需进行镜像，参考图3,它给出了 Z的输出电路。图3.采用无缓冲放大器的电流传输骸C/时M7/M8对来自M5/M6的电流进行简单镜像，得到CCII+的输出Z(-)。必要时，可以给M7/M8增加一个共源共栅结构，以提高Z的输出阻抗。需要注意的是：为了正 确模拟电流，X的输出阻抗必须与Z匹配，比如，M5/M6必须使用与M7/M8相同的晶体管类型。CCII的增益可简单表示为:Gain =依从CCII+转变为CCII-选取偏置点Yb(图3)，增加图4所示附加连接。图4.电流传输器配置为C
31、C-图4中，如果所有晶体管规格一致，并且选取Yb(图3中的偏置点)，M10和M11所产生的电 流将为2i。通过M9镜像，在M13得到2i电流。M12提供电流为I,并通过Z(+)提供电流-i, 由此得到一个真正的CCII-输出。这种方案存在一个问题：Z(+)有一个直流项-i，而不是+i。因此， Z(+)输出需要增加2i的直流电流补偿-I，图5提供了这个附加项。2i(DC)v1 DCbias叩图5.增加直流偏置后的CC/-输出 图5中，晶体管M14和M15提供适当的电流补偿M13吸取的直流电流(注意：M14和M15必 须与M12匹配)。令R3电流等于i(DC) - i，R3和R2必须匹配。它们阻值
32、的任何不匹配都会导 致输出直流值的差异。VBIAS电路为了得到所要求的电压，VDCbias将保持M14和M15的直流电流，VDCbias (图5)也必须与节点 Yb(图4和图5)具有相同的直流值。实现这一步只需要模拟前端电路，并将输入信号的直流值 (V|nDC)作为这一级的输入偏压，如图6所示。图6.直流补偿电蹈图仞的VBIA路本设计的唯一问题是需要另一个电阻(R4)，而且R4必须与R2、R3匹配。仿真结果使用图3所示CCII+，并采用TSMC 0.18顷工艺，R1 = 1kQ，R2 = 1kQ，可以得到增益为1。器件的3dB带宽为2.5GHz，电源抑制比(PSRR)为41dB，增益为0.97
33、2。使用共源共栅器件代替M5/M6和M7/M8可以改善性能，使带宽达到900MHz，增益提高到0.993, PSRR 也提高到 51dB。参考文献1K.C. Smith and A. Sedra, The Current-Conveyor A New Circuit Building Block, IEEE Proc, Vol. 56, 1968, pp. 1368-1369.2C. Toumazou, John Lidgey & Alison Payne, Practical Integrated Current-Conveyors, Current Mode Circuits Techniques in Analog High Frequency Design, July 1996, Chapter 5.2, pp.69-80.3K.C. Smith and A. Sedra, A Second Generation Current-Conveyor and its Applications, IEEE Trans, CT-17, 1970, pp. 132-134.本应用笔记基于一篇发表在Chip Design Magazine2007年8/9月刊的文章。相关型号MAX4112单/双/四路、400MHz、低功耗、电流反馈放大器MAX477300MHz、高速运算放大器
