CHAPTER 3 CMOS LOW NOISE AMPLIFIERS

Transcription

1 46 CHAPTER 3 CMOS LOW NOISE AMPLIFIERS 3.1 INTRODUCTION The Low Noise Amplifier (LNA) plays an important role in the receiver design. LNA serves as the first block in the RF receiver. It is a critical building block since its performance greatly affects both sensitivity and selectivity of the receiver. The main function of LNA is to amplify extremely low signals without adding noise, preserving the required Signal-to Noise Ratio (SNR) of the system at extremely low power levels. Additionally, for large signal levels, the LNA should eliminate channel interference. The purpose of LNA is to boost the signal power level for the other circuits following the LNA in the receiver. Impedance matching is important in LNA design as the system performance could be strongly affected by the quality of the termination. For instance, the frequency response of the antenna filter that precedes the LNA will deviate from its normal operation if there are reflections from the LNA back to the antenna filter. LNA can be designed using lumped elements or distributed elements. LNA design using lumped elements and distributed elements for wideband applications are proposed by Challal et al (2008). Distributed LNA operating in wide band from 0.5GHz to 5GHz proposed by Errikos Lourandakis et al (2008) uses distributed elements. Distributed elements are preferred wideband systems. For narrow band systems it is difficult to tune

2 47 the components. Hence LNA with active device and lumped elements are considered for narrow band applications. In the wireless systems LNAs are implemented using CMOS technology. CMOS LNAs with lumped elements can be either single-ended or differential. Differential architecture requires the use of a balun to transform the single-ended signal from the antenna into a differential signal. Practical baluns introduce extra loss which adds directly to the noise figure of the system, so single-ended architectures are preferred (Bosco Leung 2002). Single-ended LNAs are preferred since most of the antenna output is single-ended. Another reason for preferring single-ended LNA is power consumption is less. In this chapter performance of single ended LNA with lumped elements is studied. In this chapter, narrow band single-ended Low Noise Amplifiers (LNAs) using CMOS technology, operating in the 900MHz band and 2.4GHz band are proposed and studied for beamforming in the receiver of wireless systems. Two different LNAs, namely LNA1 working at center frequency of 902.5MHz with bandwidth of 25MHz from MHz and LNA2 working at center frequency of 2.4 GHz with bandwidth of 20 MHz from MHz are implemented using 0.35µm CMOS technology in ADS. Their performance is compared with that of existing LNAs. It is found that the proposed LNAs perform better in terms of various parameters. 3.2 LITERATURE SURVEY While the literature is full of examples of LNA work in GaAs and bipolar technology, there are few examples of CMOS studies. Table 3.1 shows literature study on LNAs operating in the range of 0.75GHz to 2.5GHz. Their performance is compared based on the parameters like Noise figure (NF), Gain, IIP3 (3 rd order Input Intercept Point) Power and Center frequency of the amplifier.

3 48 Design of various LNAs is proposed by Shaeffer and Lee (1999). Resistive termination LNA proposed by Chang et al (1993) using 2µ m CMOS technology provides a NF of 6dB. Two effects are responsible for the degradation in noise figure. First, the added resistor contributes its own noise to the output which equals the contribution of the source resistance. Second, the input is attenuated by the resistor. Table 3.1 Previous works on LNA Author Noise Figure (db) Gain (db) IIP3 (dbm) Power (mw) Frequency (GHz) Chang et al (1993) Sheng et al (1996) Andrew (1996) Karanicolas Shaeffer and Lee (1997) Shahhani et al (1997) Johnson et al (1997) Shunt-series feedback LNA proposed by Sheng et al (1996) using 0.8µm CMOS technology provides a NF of 7.5dB. Evgeniy Balashov and Alexander Korotklov (2008) proposed LNA with source degeneration and shunt series feedback for wideband systems. LNA using shunt-series feedback often has high power dissipation compared to others with similar noise performance. The higher power is partially due to the fact that such amplifiers are naturally broadband and hence techniques which reduce the power consumption through LC tuning are not applicable. LNA with shunt-series feedback is not suitable for narrowband applications.

4 49 LNA with inductive source degeneration proposed by Shahhani et al (1997) using 0.35µm CMOS technology provides good gain and low NF but the stability of the amplifier is not discussed. Pietro Andreani and Henrik Sjoland (2001) has proposed LNA with cascode transistor and noise optimization. Only noise is minimized and gain, bandwidth and other parameters are not discussed. There is still a requirement for stable LNAs with high gain, low power consumption, low noise figure and good linearity. In this thesis, an attempt is made to propose LNAs with high gain, low NF, stability and good linearity. 3.3 CMOS NARROW BAND LNA CMOS Narrowband LNAs has the advantages like low power consumption and low-noise performance (Challal et al 2008). The primary design challenge in LNA design is to maximize gain without adding excessive noise into the signal. As LNA is next immediate to antenna, proper impedance matching is required between these two devices to avoid the reflection of power back to antenna. The input and output impedance of the LNA is 50 as specified in specifications of wireless systems. Since the input impedance of the MOS transistor is almost purely capacitive, providing a good match to the source without degrading noise performance is a challenge. There is a subtle difference between impedance matching and power matching. The condition for impedance matching occurs when the load impedance is equal to the characteristic impedance. However, the condition for power matching occurs when the load impedance is the complex conjugate of the characteristic impedance. When the impedances are real, the conditions for power matching and impedance matching are the same. An impedance is matched when Z S = Z L as shown in Figure 3.1. The LNAs proposed are compared with existing LNAs based on number of parameters like voltage gain, Noise Figure (NF), SNR, S-

5 50 parameters, stability, Spurious Free Dynamic Range (SFDR), 1-dB compression point, 3-dB compression point and Third order Intercept Point (IP3). Voltage gain gives the amount by which the signal at input of the LNA is amplified. Noise Figure (NF) specifies the additive noise inherent in the LNA circuit. NF is used to describe the degradation of Signal to Noise Ratio (SNR) by the active device in the LNA. NF is defined as the ratio of SNR at input of the LNA to SNR at the output of the LNA. NF must be very low at the center frequency for the designed LNA. SNR can be found at the input and output of any amplifier circuit. The SNR at the output of LNA must be very high. SFDR is defined by Joel Lawrence Dawson and Thomas Lee (2004) as the SNR when the power in each 3rd order intermodulation product equals noise power at the output. High value of SFDR is preferred for the LNA circuit to be free from spurious signals. Z S =R S +jx S V S Z L =R L +jx L =Z S Figure 3.1 Conditions for Impedance Match The stability of the LNA is found using stability factors like Rollet stability factor and Edwards-Sinsky stability parameter (Pozar 2005). The Rollet stability factor (K) in terms of S-parameters is given by (3.1) as

6 S11 S22 K 2S S (3.1) where is given as (S 11 S 22) (S12 S 21) (3.2) If K > 1 and < 1, then the circuit will be unconditionally stable for any combination of source and load impedances. For K < 1 the device is potentially unstable and will most likely oscillate with certain combinations of source and load impedance. The Edwards-Sinsky stability parameter (µ 1 ) is given by (3.3) as 1 1 S11 1 S S S S (3.3) If µ 1 >1 and <1, the circuit is unconditionally stable. Based on the S-parameters various gains like Power Gain (PG), Transducer Power Gain (TPG) and Available Power Gain (APG) are found for the LNA. Power Gain is defined as the ratio of the power delivered to the load and the average power delivered to the circuit from the input and is given by (3.4) as 2 S21 (3.4) 2 PG 1 S 11 The Power Gain is same as the voltage gain in LNA when the circuit is perfectly impedance matched. Transducer Power Gain (TPG) is defined as the ratio of the output power delivered to a load by a source and the maximum power available from the source and is given in (3.5). It includes the effects of input and output impedance matching. Component resistive losses are neglected.

7 S 21 (1 S ) (1 L ) TPG 2 [(1 S 11 S )(1 S 22 L )] S 12 S 21 L S (3.5) The Available Power Gain (APG) is the ratio of the power available from the output port of the network to the power available from the source and is given as 2 S21 (3.6) 2 APG 1 S 22 1 db compression point of an LNA is the input voltage level at which the gain of the LNA drops by 1dB. 1dB compression point is a measure of gain compression which indicates the nonlinearity of LNA. When the input signal given to the amplifier has large amplitude, the amplifier saturates and there is clipping of the signal at output. When the strength of the input is further increased, the output signal is no longer amplified and at this point the output is compressed. 3dB compression point is also of importance in studying the performance of LNA. An LNA, like any active component in the receiver, has a dynamic range. If a signal of sufficient amplitude is presented to its input, the LNA output will compress and the gain of the LNA will reduce. The 3dB compression point is the input signal amplitude at which the LNA gain is reduced by 3dB. When the LNA goes into compression, there is less gain available to amplify the wanted signals, thereby reducing the receiver s sensitivity. In the wireless receivers, this phenomenon occurs when the receiver is close to a cellular phone tower. There are two ways to improve the performance of the receiver. One is to select a receiver with LNA having the highest 3dB compression point and second method is to place a filter in front of the LNA that attenuates out-of-band signals.

8 53 IP3, the third order inter-modulation product rejection, is a measure of how well a receiver handles inter-modulation. Basically, inter-modulation occurs when two or more strong signals (also called tones) are being received from transmitters transmitting on frequencies other than the receiver s current frequency. When the two tones hit the down-conversion mixer, they produce additional frequencies that are the sum and difference. The sum product is usually of no concern because it is well outside the receiver s pass band. The difference product, however, is important because it could easily fall inside the receiver s pass band. So, rejecting these product frequencies (also called inter-modulation) is important. Therefore, the higher the IP3 number, the better. The specifications for the design of LNA operating at 900MHz band and 2.4GHz band are given in the Table 3.2. Table 3.2 Specifications for design of LNAs Parameter Specification Frequency band 900MHz 2.4GHz Center frequency 902.5MHz 2.4GHz Bandwidth 25MHz 20MHz Stability factor K and K>1 and <1 Noise figure Gain Power consumption SFDR SNR <6dB >10dB <100mW >30dB >70dB 3.4 CMOS SINGLE ENDED LNAs Commonly existing narrowband single ended LNA architectures with lumped elements are Resistive termination, shunt-series feedback and

9 54 inductive source degeneration. An NMOS transistor added in cascode to the existing amplifier improves the performance of the LNAs as proposed by Hajiz Fouad et al (2002). The performance of LNA with resistive termination, shunt-series feedback and inductive source degeneration are compared with existing LNAs LNA with Resistive Termination LNA uses resistive termination of the input port to provide 50 impedance to suppress reflection at the input. Also the input impedance of the LNA must match the output impedance of the preceding stage. Resistive termination is the most straightforward approach to achieve the broadband 50 matching at the input of LNA as shown in Figure 3.2. V DD =3.3V + - L Vout MOSFET 2 Vin R1 MOSFET 1 R2 Figure 3.2 Circuit of Resistive termination matched LNA The 50 -resistor (R1) is placed across the input terminal of the LNA and hence providing a broadband input matching. The bandwidth of this amplifier is determined by the input capacitance of the transistor MOSFET 1

10 55 and is normally very high. MOSFET 2 is the transistor added in cascode to improve the performance. However, the resistor R1 adds its own thermal noise to the circuit as well as attenuates the incoming signal before it hits the gate of the transistor. These two effects result in an unacceptably high NF of the circuit and less tuning capability of the amplifier bandwidth and hence not practically used in most applications LNA with Shunt-Series Feedback The second method used for getting a good input matching is the shunt-series feedback amplifier which uses resistive shunt-series feedback to set the input and output impedances of the LNA as shown in Figure 3.3. V DD =3.3V + - L2 R4 Vout C3 MOSFET 2 Vin R1 L1 MOSFET 1 C4 Figure 3.3 Circuit of Shunt-series feedback matched LNA Unlike in the resistive termination case, this circuit does not attenuate the signal by a noisy attenuator before reaching the gate of the amplifying device and hence the Noise Figure is expected to be much better.

11 56 The advantage of having inductance L1 at the gate of the NMOS is that this inductance cancels out the gate to source capacitance of the NMOS at the resonant frequency and hence making the impedance at the input of the NMOS to be real i.e., only input resistance (R in ). However, the feedback resistor continues to generate thermal noise of its own. This results in the relatively high Noise Figure, usually a few decibels above the given specification. The shunt-series architecture requires on-chip resistors of reasonable quality, which are generally not available in CMOS technology LNA with Inductive Source Degeneration The inductive source degeneration provides a perfect match without adding any noise to the system or giving any restrictions on the device transconductance (g m ). It uses an inductor as a source degeneration device and has another inductor connecting to the gate. This architecture employs inductive source degeneration to generate a real term in the input impedance. Tuning of the amplifier input becomes necessary, making this a narrow-band amplifier. Inductive source degeneration is the most prevalent method used for many amplifiers. It offers the possibility of achieving the best noise performance of any architecture. The LNA with inductive source degeneration performs better than resistive termination LNA and shunt-series feedback LNA. The performance of LNA with inductive source degeneration is improved by proposing LNA with diode connected NMOS transistor at the input acting as resistance and an additional NMOS transistor in cascode configuration at the load of the LNA. Diode connected transistor at input provides better gain and tuning of bandwidth. LNA1 is LNA operating at center frequency of 902.5MHz and LNA2 is LNA operating center frequency of 2.4GHz. The performance of LNA1 and LNA2 is compared with that of existing narrow band LNA architectures.

12 Proposed Single ended LNA without Cascode transistor LNA with inductive source degeneration and diode connected transistor is proposed. LNA without diode connected transistor is proposed by Shahhani et al (1997) and Johnson et al (1997). These LNAs provides high gain but large bandwidth and are wide band amplifiers. To make the amplifier as narrow band diode connected transistor is used at the input for biasing and tuning of bandwidth. The circuit of proposed LNA without cascade transistor is shown in Figure 3.4. V DD =3.3V + - R3 MOSFET 2 L3 R2 Vout Vin R1 C1 L2 MOSFET 1 L1 Figure 3.4 Proposed Single ended LNA circuit without Cascode transistor Diode connected transistor, MOSFET2 used at the input acts as resistance to improve the impedance matching at the input of the LNA. The advantage of using inductor L1 is that biasing is done without significant dc voltage drop. L1 acts as source degeneration for MOSFET1 which increases the linearity without noise penalty. Maximization of the quality factor (Q) of the inductor allows low losses in the circuit and superior performance when used in matching networks, oscillators and biasing networks. High Q results in low phase noise, low noise figure of the LNA, low loss matching networks,

13 58 lower power consumption and improved receiver sensitivity. Proposed LNAs are designed with inductor having high Q Proposed Single ended LNA with Cascode transistor In the proposed LNA, transistor is used in cascode configuration at the load. The advantage of cascode configuration is given by Hajiz Fouad et al (2002). Cascode is a combination of a common-source device with a common-gate load. This has the effect of increasing the output impedance. If a resistive load is used and if LNA is to be connected to another stage i.e., another LNA or mixer, then the load will be capacitive. This capacitance will limit the frequency response of the first amplifier stage and results in lower gain due to the Miller effect. Proposed Single ended LNA circuit with Cascode transistor is shown in Figure 3.5. V DD =3.3V - + L3 Vout R3 MOSFET 3 MOSFET 2 R2 Vin R1 C1 L2 MOSFET 1 L1 Figure 3.5 Proposed Single ended LNA circuit with Cascode transistor The inductor, L3 between the cascode source and supply in the Figure 3.5 blocks any RF leaking to the supply rail and may be varied in value to optimize the gain response of the LNA. The additional cascode device has been configured as a diode (i.e., at DC the gate is connected to the source) as shown in Figure 3.5.

14 PARAMETERS OF MOSFET FOR PROPOSED SINGLE ENDED LNAs The design of LNA given in Shaeffer et al (1999) is used in this thesis. The NMOS Transistor with 0.35µm CMOS technology chosen for design of LNAs has the following parameters: oxide thickness (T ox ), 7.7 nm; Threshold Voltage (V t ), V; minimum length (L min ), 0.4µm; minimum width (W min ), 0.90µm; mobility of electrons (µ n ), cm 2 /V-s; permittivity of silicon ( Si ), 11.7 o and permittivity of the substrate ( ox ), 3.97 o, where o =8.85e -12 F/m. The equations for finding various parameters of the LNA without cascode transistor are given as follows: The cut-off frequency of MOSFET is given by (3.7) as g m O (3.7) Cgs O is the angular cut-off frequency, gm is the transconductance of the MOSFET and C gs is the gate to source capacitance of the MOSFET. Optimal Q of the inductor is given by 1 Q 1 (3.8) p where p 2 (3.9) 5 The parameters of p are dependent on the CMOS technology used and are specified in the model file of the transistors ( The

15 60 parameters are typically set as =2; =4 and =0.9 from the model file. The value of p is calculated from (3.9) as The inductance (L2) at the gate of the NMOS is given by the equation L2 Q R L in L1 (3.10) o where Q L is the Q of the inductor, R in is the input resistance at the gate of NMOS, o is the angular center frequency given by o =2 f o. f o is the center frequency in Hz. The gate to source capacitance (C gs ) of the NMOS is given by the equation C gs 2 o 1 (L 2 L1) (3.11) The width of NMOS is given as 3 C W 2 C L gs (3.12) ox m in where C ox is the oxide capacitance given by C ox = ox /T ox. The transconductance (g m ) of the NMOS is given as g C (3.13) m O gs and the total voltage to be applied at the gate of the NMOS is V GS which is sum of effective voltage (V eff ) and threshold voltage (V t ) of NMOS. The effective voltage at gate of NMOS is calculated as V eff g C m (3.14) n ox W The bias current needed at the drain of NMOS is given by (3.15) as

16 61 1 I D g m Veff (3.15) 2 The design procedure of LNA with cascode transistor is same as for LNA without cascode transistor. The W/L ratio of MOSFET3 is same as that of MOSFET1 (Hajiz Fouad 2002). The circuit parameters of the MOSFET used for the simulation of LNAs with cascode and without cascode transistor are given in Table 3.3. Table 3.3 Circuit Parameters of MOSFET for LNA LNA1 LNA2 LNA1 LNA1 with LNA2 without LNA2 with Parameters without Cascode Cascode Cascode Cascode transistor transistor transistor transistor M1(W/L) µm 500/ / / /4.643 M2(W/L) µm 50/ / / /0.653 Cascode transistor - 500/ /4.643 M3(W/L) µm L1 (nh) L2(nH) L3(nH) R1( ) R2( ) R3( ) C1(pF) C2(pF)

17 SIMULATION RESULTS Various simulations like AC simulation, S-parameter simulation and Harmonic Balance simulation are performed to study the performance of LNAs. The performance of the LNAs is verified using parameters like Gain, Bandwidth, NF, SNR, SFDR, VSWR, 1-dB compression, stability, power gain and IP3. The maximum frequency that the 0.35µm CMOS technology can support is given in terms of unity-gain frequency (f T ) (Hassan Hassan et al 2006). Unity-gain frequency of N-channel MOSFET (NMOS) used for the design of LNA using 0.35µm CMOS technology is the frequency at which the current gain of the MOSFET is unity. The unity-gain frequency for this NMOS is found using AC simulation. In the AC simulation the operating point of NMOS device is selected as 2.5V for both VGS and VDS, saturated with a significant overdrive voltage in order to minimize the non-quasi static (NQS) effects. Input current of 1mA is given at the gate of the NMOS transistor which is biased with dc voltage of 2.5V. The voltage at the drain is 3.3V. AC simulation is performed to find the variation of current gain with respect to frequency. The f T obtained from AC simulation is shown in Figure 3.6 for LNA1 and in Figure 3.7 for LNA2. For the LNA1 the unity gain frequency is 19GHz. If this NMOS is used for implementing LNA at center frequency of 902.5MHz there will not be any NQS effects as there will not be any phase deviation till 20% of 19GHz i.e., 3.8GHz and no amplitude variations till f T. Hence this NMOS can be used to implement LNAs at center frequency of 902.5MHz.

18 63 Figure 3.6 f T of MOSFET for LNA1 Figure 3.7 f T of MOSFET for LNA2 The unity gain frequency of LNA2 is 16GHz. If this NMOS is used for implementing LNA at center frequency of 2.4GHz there will not be any NQS effects as there will not be any phase deviation till 20% of 16GHz i.e., 4GHz and no amplitude variations till f T. The drain current measured for the NMOS is 22mA at 902.5MHz and is 7mA at 2.4GHz. The operating point

19 64 (V DS, I D ) for NMOS device at 902.5MHz and 2.4GHz are (2.5V, 22mA) and (2.5V, 7mA) respectively. The AC simulation is performed to find the voltage gain, center frequency of operation, bandwidth, NF and SNR of the LNA. The S- parameter simulation is performed to find the S-parameters, stability, Power gain, Available Power Gain (APG), Transducer Power Gain (TPG) and Voltage Standing Wave Ratio (VSWR). Harmonic balance simulation is performed to find the parameters like IP3, total power consumption, SFDR, 1- db compression point and 3-dB compression point of the LNA LNA with Resistive Termination and LNA with Shunt Series Feedback Performance of LNA with resistive termination and shunt series feed back are found using simulation. Parameters Table3.4 Simulation results of LNA LNA with Resistive Termination LNA with Shunt-Series Feedback Gain (db) S 11 (db) S 12 (db) S 21 (db) S 22 (db) NF(dB) dB input power (dbm) db output power (dbm) db input power (dbm) db output power (dbm)

20 65 From the various parameters of resistive LNA and shunt series feedback LNA given in Table 3.4, it is inferred that the gain is very low and the NF is high. Another drawback in the above two circuits is, tuning of the circuits for the required bandwidth is not easy as the components in the circuit are mostly resistors. Hence LNA with inductive source degeneration is implemented to provide better results Single Ended LNA1 The LNA1 without cascode transistor provides a maximum gain of dB at the designed center frequency of 902.5MHz and 3-dB bandwidth of 36.5MHz against the requirement of 25MHz as shown in Figure 3.8 (a). (a) Gain without Cascode transistor (b) Gain with Cascode Transistor Figure 3.8 Gain of LNA1

21 66 The LNA1 with cascode transistor at the output provides a maximum gain of dB at the designed center frequency of 902.5MHz and bandwidth of 61MHz as shown in Figure 3.8 (b). The gain and bandwidth of the LNA1 with cascode transistor has increased when compared with LNA1 without cascode transistor. Noise Figure of LNA with cascode and without cascode transistor is shown in Figure 3.9. a) NF of LNA1 without cascode transistor b) NF of LNA1 with cascode transistor Figure 3.9 Noise Figure of LNA1

22 67 The LNA1 without cascode and with cascode transistor provide a very low NF of 1.683dB and 3.273dB respectively. The NF of LNA1 with cascode transistor has increased which shows that noise has been added in the circuit due to the addition of transistor in cascode, but still the NF satisfies the design specification. The SNR obtained for the LNAs is shown in Figure a) SNR of LNA1 without cascode transistor b) SNR of LNA1 with cascode transistor Figure 3.10 SNR of LNA1

23 68 The LNA1 without cascode transistor gives a SNR of dB and with cascode gives a SNR of dB. There is no significant difference in SNR of both the LNAs. From the S-parameter simulation performed for the LNA1 the various S-parameters obtained are given in Table 3.5. Harmonic balance simulation is performed to find the parameters related to linearity of the amplifier and the dynamic range of the LNA. The results of Harmonic balance simulation are given in Table 3.6. Table 3.5 S-parameter simulation results of LNA1 S-parameters LNA1 without cascode transistor LNA1 with cascode transistor S 21 (db) S 11 (db) S 12 (db) S 22 (db) Rollett stability factor K Stability measure Edwards-Sinsky stability parameter µ 1 VSWR Power gain (db) APG (db) TPG (db)

24 69 Table 3.6 Harmonic balance simulation results of LNA1 Parameters (unit) LNA1 without cascode transistor LNA1 with cascode transistor IIP3 (dbm) OIP3 lower (dbm) OIP3 upper (dbm) Power (mw) consumed SFDR (db) dB Gain Compression Point 3-dB Gain Compression Point power of 4.23dBm input power of 8.409dBm power of 5.978dBm power of 15.6dBm Single Ended LNA2 The LNA2 without cascode transistor provides a maximum gain of 35.84dB at the designed center frequency of 2.4GHz and 3-dB bandwidth of 90MHz against the requirement of 20MHz as shown in Figure 3.11 (a). The LNA2 with cascode transistor at the output provides a maximum gain of dB at the designed center frequency of 2.4GHz and bandwidth of 50MHz as shown in Figure 3.11 (b).

25 70 (a) Gain without Cacode transistor (b) Gain with Cascode transistor Figure 3.11 Gain of LNA2 The gain and bandwidth of the LNA2 without cascode transistor is high when compared with LNA2 with cascode transistor. The LNA2 without cascode and with cascode transistors provide a very low NF of 1.15dB and 1.591dB respectively as shown in Figure 3.12.

26 71 a) NF of LNA2 without cascode transistor b) NF of LNA2 with cascode transistor Figure 3.12 Noise Figure of LNA2 The Noise Figure of LNA2 with cascode transistor has increased which shows that noise has been added in the circuit due to the addition of transistor in cascode, but still the Noise Figure satisfies the design specification.

27 72 The LNA2 without cascode transistor gives a SNR of 88.50dB and with cascode gives a SNR of 88.01dB as shown in Figure The SNR is high for both the LNA without cascode and with cascode transistors. From the S-parameter simulation performed for the LNA2 the various S-parameters obtained are given in Table 3.7. a) SNR of LNA2 without cascode transistor b) SNR of LNA2 with cascode transistor Figure 3.13 SNR of LNA2

28 73 Table 3.7 S-parameter simulation results of LNA2 S-parameters LNA2 without cascode transistor LNA2 with cascode transistor S 21 (db) S 11 (db) S 12 (db) S 22 (db) Rollett stability factor, K Stability measure, Edwards-Sinsky stability parameter, µ 1 VSWR Power gain (db) APG (db) TPG (db) Harmonic balance simulation is performed to find the parameters related to linearity of the amplifier and the dynamic range of the LNA. The results of Harmonic balance simulation are given in Table 3.8. Table 3.8 Harmonic balance simulation results of LNA2 Parameters (unit) LNA2 without cascode transistor LNA2 with cascode transistor IIP3 (dbm) OIP3 lower (dbm) OIP3 upper (dbm) Power consumed (mw) 2 2 SFDR (db) dB Gain Compression Point 3-dB Gain Compression Point 24.04dBm@input power of 16.93dBm 11.90@ input power of 2.781dBm 12.86dBm@input power of 3.122dBm 11.98dBm@input power of 0.24dBm

29 74 The performance of proposed LNAs is compared with existing architectures namely resistive termination and Shunt-Series feedback. The proposed LNA with inductive source degeneration performs better. In the proposed LNAs, LNA with cascode transistor at performs better than the LNA without cascode transistor at 900MHz band and LNA without cascode transistor at performs better than the LNA with cascode transistor in 2.4GHz band. 3.7 CONCLUSION LNA circuits are designed and analyzed for 900MHz and 2.4GHz wireless systems. The results obtained from S-Parameter simulation, AC simulation, DC simulation, and Harmonic balance simulation for IIP3, XDB simulation for 3-dB gain compression and 1-dB gain compression show that, the circuits are suitable for GSM900 systems and wireless systems operating at 2.4GHz. In the 900MHz frequency band the LNA with resistive termination provides a voltage gain of dB and minimum noise figure of LNA with Shunt-series feedback provides a gain of dB and a noise figure of LNA1 with Inductive source degeneration provides a gain of dB without cascode transistor. LNA1 with Inductive source degeneration provides a gain of dB with cascode transistor. The NF of LNA1 is lower than the noise figure of LNA with resistive termination and shunt-series feedback. The LNA1 with inductive source degeneration provides better results compared to resistive termination and shunt series feedback LNAs. LNA1 provides better results than the similar LNAs available in the literature. LNA2 operating in the 2.4GHz frequency band provides high gain and low noise figure when compared to the similar LNAs in the literature. LNA2 also provides high SNR, SFDR and good linearity. The LNA1 and LNA2 proposed are highly stable.