WideBand CMOS LowNoise Amplifier CMOS宽频低噪声放大器的设计.docx

1、WideBand CMOS LowNoise Amplifier CMOS宽频低噪声放大器的设计Wide-Band CMOS Low-Noise AmplifierAbstractKnown elementary wide-band amplifiers suffer from a fundamental tradeoff between noise figure (NF) and source impedance matching, which limits the NF to values typically above 3 dB. Global negative feedback can

2、 be used to break this tradeoff, however, at the price of potential instability. In contrast, this paper presents a feedforward noise-canceling technique, which allows for simultaneous noise and impedance matching, while canceling the noise and distortion contributions of the matching device. This a

3、llows for designing wide-band impedance-matching amplifiers with NF well below 3 dB, without suffering from instability issues. An amplifier realized in 0.25- m standard CMOS shows NF values below 2.4 dB over more than one decade of bandwidth (i.e., 1502000 MHz) and below 2 dB over more than two oct

4、aves (i.e., 2501100 MHz). Furthermore, the total voltage gain is 13.7 dB, the 3-dB bandwidth is from 2 MHz to 1.6 GHz, the IIP2 is +12 dBm, and the IIP3 is 0 dBm. The LNA drains 14 mA from a 2.5-V supply and the die area is 0.3 0.25 mm2.Index TermsBroadband, distortion canceling, low-noise amplifier

5、 (LNA), noise canceling.I. INTRODUCTION WIDE-BAND low-noise amplifiers (LNAs) are used in receiving systems where the ratio between bandwidth (BW) and its center frequency can be as large as two. Application examples are analog cable (50850 MHz), satellite (9502150 MHz), and terrestrial digital (450

6、850 MHz) video broadcasting. Moreover, a wide-band LNA can replace several LC-tuned LNAs typically used in multiband and multimode narrow-band receivers. A wide-band solution saves chip area and fits better with the trend towards flexible radios with as much signal processing (e.g., channel selectio

7、n, image rejection, etc.) as possible in the digital domain (toward “software radio”). High-sensitivity integrated receivers require LNAs with sufficiently large gain, noise figure (NF) well below 3 dB, adequate linearity, and source impedance matching . The latter is to avoid signal reflections on

8、a cable or alterations of the characteristics of the RF filter preceding the LNA, such as pass-band ripple and stop-band attenuation 1. These requirements must be achieved over a wide range of frequencies while also allowing some variable gain to handle interference generated by strong adjacent chan

9、nels. Traditional wide-band LNAs built of MOSFETs and resistors have difficulties in meeting the above-mentioned requirements. Known elementary amplifiers 2, 3 fail to achieve low NF upon . On the other hand, amplifiers exploiting global negative feedback might achieve low NF with , but they are pro

10、ne to instability 4. In this paper, a thermal-noise canceling technique is presented that allows for designing LNAs with low NF and source impedance matching over a wide range of frequencies without instability problems. In earlier work 2, 5, a limited form of noise cancellation was already presente

11、d. However, it does not allow for low NF dB upon . In contrast, the technique presented in this paper can reach much lower NF, as was validated through the design of a sub-2-dB noise figure wide-band LNA in a 0.25- m CMOS 6. This paper analyzes the noise-canceling technique and its properties in dep

12、th. The paper is organized as follows. Section II reviews existing wide-band CMOS low-noise techniques. Section III introduces the noise-canceling technique. Section IV analyzes properties and limitations of noise canceling. Section V describes the IC design of a wide-band CMOS LNA. Section VI deals

13、 with the measurements. Finally, Section VII presents the conclusions.II. REVIEW OF EXISTING TECHNIQUES In this section, common wide-band CMOS low-noise techniques are reviewed in order to highlight their NF limitations. A MOSFET in saturation is modeled as a voltage-controlled current source with t

14、ransconductance. Fig. 1(a)(e) shows known elementary wide-band amplifiers capable of matching a real source impedance (biasing not shown). These amplifiers suffer from a fundamental tradeoff between their noise factor NF and impedance matching . The tradeoff between and source impedance matching can

15、 be broken, exploiting negative feedback properly. Fig. 1(g) shows a commonly used wide-band feedback amplifier capable of a low upon . In this case, the feedback resistor determines the minimum noise factor2 ). The latter can be well below 2 (i.e., 3 dB), provided adequate gain is available. Despit

16、e its noise performance, this amplifier suffers from important drawbacks, as follows, motivating the search for alternatives. Sufficient gain and gigahertz bandwidth often mandate the use of multiple cascaded stages within the feedback loop 2 in Fig. 1(g), making its operation prone to instability.

17、For , the open-loop gain is lower than 1. Thus, the closed-loop linearity is not much better than that of the loop amplifier A. If A consists of cascaded stages and most of the gain is in the first one (i.e., to optimize noise), linearity can be poor 4. depends on and , so it is sensitive to process

18、 variations. Next, and are directly coupled and variable gain at is not straightforward.III. NOISE-CANCELING TECHNIQUE In this section, a wide-band low-noise technique is presented, which is able to decouple from without needing global negative feedback or compromising the source match. This is achi

19、eved by canceling the output noise of the matching device without degrading the signal transfer.A. Noise Canceling Principle This is done by creating a new output, where the voltage at node Y is added to a scaled negative replica of the voltage at node X. A proper value for this scaling factor rende

20、rs noise canceling at the output node, for the thermal noise originating from the matching device. Fig. 3(a) shows a straightforward implementation using an ideal feedforward voltage amplifier A with a gain (with ). By circuit inspection, the matching device noise voltages at node X and Y are:(1)The

21、 output noise voltage due to the noise of the matching device:(2)Output noise cancellation, , is achieved for a gain:(3)where the index denotes the cancellation. On the other hand, signal components along the two paths add constructively, leading to an overall gain:(4) From (3), two characteristics

22、of noise canceling are evident. Noise canceling depends on the absolute value of the real impedance of the source, (e.g., the impedance seen “looking into” a properly terminated coax cable). The cancellation is independent on and on the quality of the source impedance match. This is because any chan

23、ge of equally affects the noise voltages Fig. 3(b) shows an elementary implementation of the noisecanceling LNA in Fig. 3(a). Amplifier A and the adder are replacedwith the common-source stage M2M3, rendering an output voltage equal to the voltage at node X times the gain . Transistor M3 also acts a

24、s a source follower, copying the voltage at node Y to the output. The superposition principle renders the final addition of voltages with an overall gain. Note that any small signal that can be modeled by a current source between the drain and source of the matching device is cancelled as well (e.g.

25、, noise, thermal noise of the distributed gate resistance, and the bias noise current injected into node Y). However, the noise of R is not cancelled. B. Noise Factor The noise factor F of the LNA in Fig. 3(a) can be written as：(5)where the excess noise factor EF is used to quantify the contribution

26、 of different devices to , where index refers to the matching device, to the resistor , and to amplifier A. For the implementation in Fig. 3(b), expressions for EF for are (assuming equal NEF)(6)Upon cancellation, (6) becomes(7)The noise factor at cancellation, , is thus only determined by EF and EF

27、 , neither of which are constrained by the matching requirement. EF can be made arbitrarily smaller than 1 by increasing of its input stage, at the price of power dissipation. The LNA concept in Fig. 3(a) was simulated using MOS model 9 in a 0.25- m CMOS process using an ideal noiseless amplifier A

28、(i.e., a voltage-controlled voltage source). The matching stage provides with and a voltage gain of dB. Fig. 3(c) shows the transfer function from to the LNA output (right axis) versus . It is evaluated at 1 GHz, which is morethan a factor of ten below the 3-dB bandwidth of the matching stage. This

29、noise transfer is zero for , meaning that the noise from the matching device cancels at the output. On the other hand, the noise transfer rises for due to imperfect cancellation. Fig. 3(c) also shows the simulated NF versus at 1 GHz (left axis). The NF drops from a maximum of 6 dB for , (i.e., NF of

30、 the matching stage standalone) to NF dB for (i.e., the contribution of ), which is very close to the value predicted from (3) and (7).C. Generalization The concept of noise canceling can be generalized to other circuit topologies according to the model shown in Fig. 4(a). It consists of the followi

31、ng functional blocks: 1) an amplifier stage providing the source impedance matching, ; 2) an auxiliary amplifier sensing the voltage (signal and noise) across the real input source; and 3) a network combining the output of the two amplifiers, such that noise from the matching device cancels while si

32、gnal contributions add. Fig. 4(b) shows another implementation example (biasing not shown) among several alternatives 9. Noise cancellation occurs for , while low requires high . The 2-MOSFETs in Fig. 4(b) is a well-known transconductor 10, also used for a double-balanced active mixer 11. However, in both cases, noise canceling was apparently not recognized. As shown in the previous section, the noise-canceling technique is capable of NF well below 3 dB upon . Moreover, it offers advantages compared to feedback techniques. It is