CHAPTER 4 ULTRA WIDE BAND LOW NOISE AMPLIFIER DESIGN

Transcription

1 93 CHAPTER 4 ULTRA WIDE BAND LOW NOISE AMPLIFIER DESIGN 4.1 INTRODUCTION Ultra Wide Band (UWB) system is capable of transmitting data over a wide spectrum of frequency bands with low power and high data rates. CMOS technology is a promising candidate for UWB systems for providing scaling of the CMOS devices for high frequency operation and also facilitates the processing of large bandwidth analog signals with low power. First stage of a UWB RF receiver is a Low Noise Amplifier (LNA), whose main function is to provide enough gain to overcome the noise of subsequent stages. Main functions of LNA in radiofrequency chain are to increase the signal energy and to limit noise in the receiver as much as possible. Design of LNA demands low NF, sufficient gain with flatness over the wide frequency range, low power consumption and low cost. Block diagram of LNA is sketched in Figure 4.1 has input/output matching networks with core amplifier design [64]. Since it is difficult to implement open and short circuit conditions over wideband at high frequency, S- parameters are used to characterize the design at high frequencies. Hence, UWB LNA performance is characterized by S-parameters S 11, S 22, S 21 and S 12, where S 11 and S 22 are the input and

2 94 the output reflection coefficient to measure input/output matching, S 21 is forward transfer gain and S 12 is reverse isolation loss. Figure 4.1 Block Diagram of LNA In terms of performance specifications of a LNA, the following key parameters need to be measured. They are gain, gain flatness over wideband, operating frequency, bandwidth, input and output reflection coefficient, power supply requirements, noise figure, gain compression (P1dB) and intermodulation distortion (IIP3) Topologies of Wide Band LNA Numbers of topologies are available for wideband design. The most commonly used topologies are resistive terminated common source amplifier, common gate amplifier, resistive shunt feedback amplifier, inductively degenerated amplifier, cascode degenerated amplifier with LC ladder matching network. In case of resistive terminated common source design, a wide band impedance matching is attained by adding a 50Ω shunt resistor at the input of the common source LNA stage is illustrated in Figure 4.2 [65]. This topology has tradeoff between the noise figure and input impedance matching. Use of shunt resistor not only attenuates the input signal but also adds thermal noise, hence degrades the noise figure. Even though it consumes low power, it would not benefit good dynamic range.

3 95 Figure 4.2 Resistive Terminated Common Source Amplifier The common gate amplifier as given in Figure 4.3, provides perfect input impedance matching to 50 Ω through the resistance looking into its source used [34] has input impedance Z in =1/(g m + g mb ), where g m and g mb refer to the transconductance of the transistor. The major drawbacks are its higher NF larger than 3dB, low voltage gain and high power consumption [66]. Figure 4.3 Common Gate Amplifier Resistive shunt feedback amplifier topology of Figure 4.4 renders superior input matching for broad band and gain characteristics. For low NF, large feedback resistor is required, which increases the power consumption and also there is tradeoff between the gain and the linearity which hampers flat gain performance.

4 96 Figure 4.4 Resistive Shunt Feedback Amplifier Inductively degenerated amplifier shown in Figure 4.5 has good input matching for higher frequencies and allows good dynamic range with reasonable power consumption. Further, Input matching and NF trade-off is broken at the cost of linearity. Figure 4.5 Inductively Degenerated Amplifier Cascode degenerate amplifier with LC matching network of Figure 4.6 provides superior performance in terms of gain, noise figure and input matching [67]. If cascode transistor M2 is designed properly, a wideband input match with minimum noise figure can be achieved. Due to the benefits of this topology, the proposed design used cascode configuration with source degeneration.

5 97 Figure 4.6 Cascode Degenerate Amplifier with LC Matching Network Designing of UWB LNA is one of the challenging blocks in multiband receiver, with the designing issues of low noise characteristics over the entire band width of UWB, flat frequency response, input/output matching networks for maximum power transfer for a wideband, better linearity for large range of input power and stability. 4.2 CIRCUIT DESCRIPTION OF THE PROPOSED UWB LNA The design specifications for UWB LNA as reported by link budget of UWB receiver proposals [74, 75] is given in Table 4.1. Table 4.1 Design Specifications of UWB LNA Specifications Bandwidth Gain (S 21 ) Input reflection co-efficient (S 11 ) Power consumption Noise Figure Linearity (IIP3) Desired Range/Value of Parameters 4 13 GHz 15 db db 18mW 5dB Input power at -6dBm

6 98 Circuit schematic of Figure 4.7 is a two stage cascode UWB LNA with series connected L type LC sections for broad band input impedance matching. Cascode stages are preferred for high gain, high input/output impedance and large bandwidth with high voltage swing. Figure 4.7. Circuit Schematic of Proposed UWB LNA First cascode stage comprises M 1 common source stage with inductive source degeneration (L s ) to exhibit good linearity and stabilize the amplifier for entire frequency range. Even though, it simplifies the input impedance matching along with gate inductor L g, it can achieve only a narrow band matching. Thus, two series connected LC sections (C 1 L 1 and C 2 L 2 ) have been used to match the input impedance of L- degenerated amplifier across a wide bandwidth. First L section (C 1 & L 1 ) provides lower cutoff frequency and the second L section (C 2 & L 2 ) provides upper cutoff frequency for broad band match. M 3 act as a common gate stage for cascode

7 99 configuration to eliminate the Miller effect and provides better isolation for output return signal. Intermediate stage M 2 along with R 1, C 3 and L 5 forms a current reuse network where C 3 offers low impedance path and L 5 offers high impedance path for ac signal. Thus it reuses the charge stored across the capacitance C 3. The inductance L 5 increases the transconductance of common source stage (M 1 ) and also it provides good matching to feed the signal from M 1 to common gate stage (M 3 ). Second cascode stage (CS stage (M 5 ) and CG stage (M 4 )) is used to boost the gain and extend the band width with flatness. R 2 and R 5 are used for biasing the common source stages. For the purpose of shunt peaking at the high frequencies, the inductor loads L 3 and L 4 are used for the cascode stages. Inductive loads along with the drain parasitic capacitances of M 3 and M 4, provides gain roll at the high cut off frequencies of the band. Peaking inductor also features constant power gain over the entire bandwidth by compensating the decreasing impedance of capacitance with the increase of frequency [68]. Output buffer is a source follower which provides 50Ω wideband output matching for maximum power transfer. Common source stage of cascode networks contributes major noise in the system. Noise at the drain of the common source stage M 7 is 180 out of phase with the noise at the source of common drain M 6. Thus, the noise figure of the LNA has been reduced due to the noise cancellation by M 7. As this design uses LC section wideband impedance matching networks with current reuse and noise cancellation techniques, improve the performance of wideband LNA and make it more suitable for low power, low noise, and high gain wideband amplifier for RF front end.

8 Design of UWB LNA (a) Wide Band Input Matching Circuit For maximum power transfer, the input impedance of the amplifier has to be matched with the source impedance. Figure 4.8 shows the input matching network for the wideband LNA. Figure 4.8 Input Impedance Matching Network R s is the source resistance 50Ω has to be matched with R in of the amplifier for the entire band of frequencies. C 1 L 1 and C 2 L 2 are the two L sections used for wideband matching. Equivalent impedance Z eq is calculated from the following Equation (4.1), Zeq Rin jx L2 jxc2 jx L1 jxc1 L sections are resonant at f 1 and f 2, which are given as, (4.1) X X, XL 1 X C1 L2 C2 f f1 LC LC 1 1 At resonant frequencies f 1 and f 2 the equivalent impedance is simplified as given, (4.2) Z eq R j X R X X X 2 2 in L1 in L2 L2 L X X X R L1 L2 L1 in Equivalent resistance of Equation (4.4) is derived from the real part of Equation (4.3), R eq 1 R in 2 2 X X X R L1 L2 L1 in (4.3) (4.4)

9 101 Figure 4.9 Input Impedance of LNA Value of R in is the real part of the input impedance of the LNA with degenerative inductance L s and gate inductance L g. Input impedance is calculated from the first stage of the LNA as in Figure 4.9 and is expressed as Equation (4.5), Lg s m 1 Z j L L C C in g s gs gs (4.5) The real part of the input impedance is given as in Equation (4.6) R in Lg s C gs m (4.6) By adjusting the values of L 1 and L 2 of matching network and L s, C gs and g m of M 1, the input impedance of wideband LNA can be matched with source resistance of 50Ω. (b) Gain of UWB LNA The small equivalent circuit of the proposed UWB LNA (excluding input matching network) is shown in Figure 4.10, where the g m V refers the drain current of the corresponding transistors, r ds is the drain source resistance, C gd is the gate drain capacitance and C gs is the gate source capacitance.

10 102 Figure 4.10 Small Signal Equivalent Circuit of Proposed UWB LNA Apply KCL at the nodes 1 to 5, V in and V out to derive voltage gain A v of the design. At the node V out, Therefore, Voltage at the node 4 is given as, Voltage at the node 3 is given as, V 5 g V 1 out m6 0 (4.7) V5 V out (4.8) V 4 V g out g g ds4 ds4 m4 (4.9) V 3 V g j C g out ds4 gd 5 ds5 j C g gd 5 m5 (4.10) Voltage at the node 2 is, V 2 V ( g C C )( j C g g g )( g C )( j C g ) out ds3 gs5 gd 5 gd 5 ds4 ds4 ds5 ds4 gd 5 gd 5 m5 ( j C g )( g g )( g g ) gd 5 m5 ds4 m4 ds3 m3 (4.11) Apply KCL at the input node V in, ( V V ) C 0 (4.12) in 1 gd1 Therefore, V in = V 1.

11 103 Input voltage V in is given as, V in Vg 2 ds2 g g 1 g g g m1 ds1 1 m 2 ds2 j L5 (4.13) A v Sustituting V 2 in Equation (4.13) voltage gain A v is found as, 1 gm 1 gds 1 g1 gm2 gds2 gm3 gds3 gm4 gds4 scgd 5 gm5 V sl out 5 V g g C C sc g g g g g g C sc g in ds2 ds3 gs5 gd 5 gd 5 ds4 ds4 ds5 m4 ds4 ds4 gd 5 gd 5 m5 (4.14) where g m and g ds are the transconductances of the stages. It is seen that the gain of the UWB LNA is proportional to the product of (g m1 + g m2 ), g m3, g m4 and g m SIMULATION RESULTS Proposed UWB LNA has been designed and simulated in TSMC 0.18 μm CMOS technology using ADS tool. The performance of an amplifier is characterized by S parameters in terms of input/output reflection coefficient (S 11 and S 22 ), forward transfer gain (S 21 ) and reverse isolation (S 12 ), noise figure and linearity analysis. S-parameters describe the electrical behavior of linear electrical networks and have earned a prominent position in RF circuit design, analysis, and measurement. S-parameters change with the measurement frequency, so frequency must be specified for any S-parameter measurements stated, in addition to the characteristic impedance or impedance. To meet the design specifications, through the iteration process, the circuit parameters of the designed UWB LNA obtained are given in the Table 4.2.

12 104 Table 4.2 Circuit Parameters of UWB LNA Circuit Parameters Values M 1 (W/L)µm 8 / 0.18 M 2 (W/L) µm 4 /0.18 M 3 (W/L) µm 8 /0.18 M 4 (W/L) µm 5 / 0.18 M 5 (W/L) µm 8 /0.18 M 6 (W/L) µm 0.5 / 0.18 M 7 (W/L) µm 0.5 / 0.18 C 1 C 2 C 3 R s R 2 R 3 R 4 L 1 L 2 L 3 L 4 L 5 L g L s V bias 1pF 3.2 pf 4 pf 50Ω 1K 1K 2K 4 nh 9 nh 20 nh 4 nh 10 nh 5 nh 4 nh 0.8 V Input / Output Reflection Co-efficient (S 11 and S 22 ) and Reverse Isolation (S 12 ) For proper impedance matching, the input and output reflection coefficients should be less than 10dB for the required band of frequencies. For better reverse isolation S 12 should be less than 50dB.

13 105 Use of two LC sections has two resonant frequencies to fix the upper and lower cut off frequencies between 4 and 13 GHz as defined in Equation (4.2). Thus, these LC sections provide wide input matching for UWB range. The simulated results of Figures 4.11 and 4.12 have shown that S 11 and S 22 are less than -14dB and S 12 ranges from -100dB to -80dB over the entire band of frequencies 4-13 GHz respectively. Figure 4.11 Frequency Response of Input/output Reflection Co-efficient (S 11 and S 22 ) Figure 4.12 Frequency Response of Reverse Isolation Loss (S 12 )

14 106 Magnitude of input / output reflection coefficient S 11 and S 22 ranges from -14dB to -27dB and -25dB to -32dB for the UWB range has met the design specifications and also proved that the designed LNA has better input and output impedance matching. The reverse isolation loss S 12 ranges from -100dB to -80dB has shown that the LNA has better isolation between input and output Forward Transfer gain (S 21 ) The frequency response of forward transfer gain S 21 of designed UWB LNA shown in Figure It has attained a forward transfer gain more than 20dB for wide band of 4-13 GHz. This is because of using two stages of cascode amplifier along with two LC sections for wide band input impedance matching. The circuit consumes a low power of 18dB because of current reused network used in design. Figure 4.13 Frequency Response of Forward Transfer Gain (S 21 ) (Markers specify the frequency and the corresponding gain) Source inductive degeneration has made the circuit more stable with stability factor greater than or equal to one as shown in Figure 4.14.

15 107 Figure 4.14 Stability Measure of LNA Forward transfer gain is greater than 20dB for UWB range and stability is more than 1 has made the designed LNA more suitable for wide band RF front end Noise Figure One of the design considerations of the UWB LNA is to maintain a low noise figure of less than 5dB for the entire band of frequencies. The noise contributed by M 7 is out of phase with the noise contributed by M 1, thus it reduces the noise figure over the designed band of frequencies. Noise figure of the designed wide band LNA with and without noise cancellation technique are illustrated in Figure The LNA with noise cancellation technique has reduced noise figure ranges from 2dB to 4.6dB for the designed wide band of 4 10 GHz.

16 108 Figure 4.15 Noise Figure of LNA with and without Noise cancellation Designed LNA has attained a noise figure of less than 5dB which satisfies the design specifications Linearity Linearity is the criterion that defines the upper limit of detectable RF input power and sets the dynamic range of the receiver. The linearity of an amplifier is described in terms of 1dB compression point P1dB and IIP3. Since UWB signal seldom suffers from gain compression in LNA due to the low power of the received signal, P1dB and IIP3 are required to be calculated. P1dB compression point of Figure 4.16 is obtained at the input power of - 23dBm and output power of -8dBm.

17 109 Figure 4.16 P1dB Compression Point of LNA Input and output power characteristic for fundamental frequency 6GHz and third order frequency 6.01 GHz is shown in Figure The coordinates of IIP3 is -6dBm and 10dBm of input and output power respectively is attained as shown in Figure Figure 4.17 Third Order Intercept Point (IIP3) of LNA

18 110 Note: (dbm(mix(vout) is tool generated command for IIP3 where (2,-1) is the third order component and (1,0) is the fundamental frequency component. Dotted is the projected line for linearity measurement. Designed UWB LNA has better linear characteristics over the inter modulation distortion Layout The layout of the wideband LNA without noise cancellation technique illustrated in Figure 4.18 is obtained from ADS layout tool. Figure 4.18 Layout of UWB LNA

19 Figure of Merit To evaluate the performance of wideband LNA, a Figure Of Merit (FOM) [69] is defined as Equation (4.8). It is calculated to be GHz S. 21 BW FOM (4.8) mw NF P ( 1). D 4.4 PERFORMANCE COMPARISON Table 4.3 summarizes the performance comparison of the proposed wide band LNA with the references [32, 37, 39 and 40]. Table 4.3 Performance Comparison of Wideband LNAs Parameters Ref [32] Ref [37] Ref [39] Ref [40] Proposed Design Technology 180nm 180nm 180nm 180nm 180nm Band width(ghz) ~ Gain (S 21 ) db ± ~13 21 Input Return Loss (S 11 ) <-9 <-10.7 < 10 <-8 < -14 db Power dissipation (mw) Noise Figure(dB) ± IIP3 (dbm) @ 6GHz FOM(GHz/mW) Not 4.43 Not reported reported 3GHz Not reported 6GHz Comparison results prove that the designed UWB LNA has obtained a gain which is in the order of 38%, 25%, 37% and 53% greater than References [40], [39], [37] and [32] respectively. Further the use double LC input imepedance network provides good input impedance matching of -14 which is more negative than the other reported literatures. nearly 6dB greater than the reported literatures. It has achieved good input impedance matching, proved through the input return loss of -3dB lesser than the Ref [37 and 39} and -6dB lesser than Ref [40] for the designed band. Also the proposed design satisfies the design

20 112 specification of UWB LNA of Table 4.1 and is more suitable for wideband RF front end. 4.5 SUMMARY Various architectures of wideband LNA were discussed. It is seen that cascode is suited for ultra wideband LNA with high gain for broad band of 4 to 10GHz. Proposed UWB LNA with noise cancellation technique was designed and gain expression was obtained using small equivalent circuit. Two LC sections were used as input matching network to match to the source impedance for wide band of frequencies. The circuit was simulated and layout has been extracted using Agilent ADS tool. Finally, the performance parameters were compared with recently reported literatures and proved that the proposed LNA has better gain and noise figure.