Low Noise Figure , High Gain Single LNA Cascaded with Cascoded LNA Amplifiers using Optimized Inductive Drain Feedback for Direct Conversion RF Front-end Receiver at Wireless Application

This study presents a design of low noise figure, high gain LNA at 5.8 GHz with cascaded and cascoded techniques using inductive drain feedback that is applicable for the WiMAX 802.16 standard. The amplifier uses Pseudomorphic High Electron Mobility Transistor FHX76LP super HEMT low noise FET. The Ansoft Designer SV as an Electromagnetic (EM) simulator was used during the design process. The LNA was designed using the inductive drain feedback, inductive generation to the source and the T-network as a matching technique was used at the input and output terminal. The cascaded and cascoded Low Noise Amplifier (LNA) produced a gain (S21) of 43.94 dB and the Noise Figure (NF) of 0.61 dB. The input reflection (S11), output reflection (S22) and return loss (S12) are -10.65, -20.02 and -52.23 dB, respectively. The measured 3 dB bandwidth of 1.24 GHz has been achieved. The input sensitivity is -84 dBm exceeded the standards required by IEEE 802.16 has been observed.


INTRODUCTION
Over the past few years, wireless communication is an area that is growing rapidly and the latest technology applications created for the growth of the relationship between the progress of people or companies around the world.Where they can measure and control the entire work process, analyze data in real time, lower cost, improve reliability and requires a continuous and nonstop searching for new technologies with greater capacity (Fourty et al., 2005).
Expansion in coverage, increase data speed and capacity consumers is a challenge that must be addressed and it is always a requirement for on a different generation wireless communication systems.WiMAX is one of the new standards developed recently.It covers a large area and allows a huge data rate.Theoretically, WiMAX can achieve transmission rates up to 70 Mb/sec and a service area of around 50 km for fixed stations and 5-15 km for mobile stations (Roger, 2004).The construction of the RF front-end receiver for WiMAX application becomes an arduous task considering the complicated challenges of achieving the standard set.
Figure 1 shows the available RF bands that can be used for WiMAX.The system is based on IEEE 802.16 standards and uses several licensed and unlicensed bands respectively) to transmit data (Roger, 2004).In this research, focus will be on the unlicensed spectrums at 5.8 GHz frequency.
To get the best performance in the RF front-end receiver, design optimization of Low Noise Amplifier (LNA) is needed to produce a good input impedance match, sufficient power gain and low noise figure at a predetermined band (Ruey et al., 2008).There are two types of amplifier topology used for LNA, i.e., the cascading and cascoding technique.The cascoded topology is the most famous since this technique introduces a higher gain, due to the increase in the output impedance and isolation that-occurs at the input and output ports which could be further reduced (Othman et al., 2012).While cascaded topology is used to increase the gain of the amplifier.
However, most of the researches that are focused on the single stage LNA or cascoded LNA only exhibit gain of around 20 dB (Othman et al., 2012).By taking consideration the extension of communication distance of up to 50 km, the increased in gain with low noise Fig. 1: 2 to 6 GHz centimeter bands available for BWA (Broadband Wireless Access), (Farahani and Ismail, 2005) Fig. 2: Direct conversion RF front-end receiver using a single LNA cascaded with cascoded LNA configuration figure is crucial to ensure the best signal generated.In this project, three transistors using PHEMT (LNA) were used under two different operating conditions.The first stage LNA is designed with the same parameters published in (Xuezhen and Robert, 2003).The second stage for two LNA is optimized to achieve a minimum noise figure with maximum gain available.This study describes the addition of inductive feedback in LNA to provide the higher gain and bandwidth of the RF frontend receiver.
In this study, a low noise amplifier with gain of more than 35 dB is proposed.A single LNA cascaded with cascoded LNA front-end amplifier is used.To achieve a gain more than 35 dB, a single LNA with cascoded amplifier for direct conversion RF front-end receiver architecture WiMAX at 5.8 GHz is introduced as shown in Fig. 2. The development of LNA at the front-end of the receiver will be focused.

METHODOLOGY
LNA theory: In designing the low noise amplifiers, analysis of the transistor stability, gain, noise figure, bandwidth, optimization and proper matching network selection will have to be carried out.The selection of the transistor for the LNA is based on input sensitivity and noise figure.Bipolar transistors are capable of high gain and power capacity at lower frequencies.GaAs is less expensive than bipolar devices and can operate at higher frequencies with better noise figures (Pozar, 2001).Table 1 shows the comparison between the FET and BJT transistors by Pozar (2001).
With reference to Table 1, the GaAs HEMT transistor is selected due to its low Noise Figure (NF) and reasonable gain at the required frequencies.For frequencies between 4 to 8 GHz, the GaAs HEMT could provide gains up to 20 dB while maintaining a minimum noise figure below 0.5 dB.However since the    Initially, the gain, bandwidth and noise figures were compared for different transistors.All the transistors satisfy the requirements of the design.The input sensitivity and bandwidth of the transistors are compliant with WiMAX standard.The stability of each transistor was then checked with reference to the data sheet.FHX76LP from Eudyna Devices Inc was found to have the best stability for the required frequency range of 5.2-6.5 GHz.This transistor is a super low noise HEMT and provides a low noise figure of 0.4 dB at 12 GHz and a high gain of 18 dB at 6 GHz.The transistor consumes very low current of 10 mA with drain-source voltage of 2 volts.Hence FHX76LP was selected for the LNA designed (Othman et al., 2010).
Figure 3 shows the basic concept of high frequency am plifierde sign.It shows there quired impedance to match the input and output impedance of the transistor at high frequencies.Using S-parameters, the frequency characteristics at a DC-bias point are set based on the source impedance and load impedance (Othman et al., 2010).The main purpose of obtaining input and output matching conditions is to avoid reflection and improve the efficiency of signal transmission from the source to the load.
The targeted S-parameter specification for the single LNA cascaded with cascoded LNA amplifier is shown in Table 2.

Minimum noise figure:
One of the most critical steps in the LNA design procedure is the noise optimization (IEEE Computer Society and IEEE Microwave Theory Technique and Society, 2004).It can only be done using constant gain circles and circles of constant noise figure to select usable trade-off between noise figure and gain.Typically, noise figure of 2-port transistor has a minimum value at the specified admittance given by the Eq. ( 1), (Ibrahim et al., 2011): For low noise transistors, manufacturers usually provide F min , R N , Y opt by frequencies.N defined by formula for desired noise figure, shows in Eq. ( 2): After stability of the active device is determined, input and output matching circuits should be designed so  that reflection coefficient of each port can be correlated with conjugate complex number as given in Eq. ( 3) and (4): To get a minimum noise figure using 2-port transistor, the source reflection coefficient should match with Γ opt and load reflection coefficient should match with Γ* out with a complex conjugate number as formulated in Eq. ( 5) and (6): Power gain: Amplifier operation can be explained in more detail through the input/output circuit for two port networks.As shown in the Fig. 3, power gain of 2 port networks with circuit impedance or load impedance of the power amplifier are represented with scattering coefficient classified into Operating Power Gain, Power Transducer and Available Power Gain (Lorenzo and De Leon, 2010).
Operating power gain is the ratio of the power dissipated in the load Z L (P L ) to the power delivered to the input (P in ) of the two-port network (Lorenzo and De Leon, 2010).The Operating Power Gain can be expressed as an Eq. ( 7), (Ibrahim et al., 2011): )1 Transducer Power Gain is the ratio of P avs , maximum power available from source to P L , power delivered to the load.The maximum power cans be obtained, when the input impedance Г in of the network is terminated conjugately matched to the source impedance Г s ., if Г in = Г s , Transducer Power Gain can be expressed in Eq. ( 8), (Ibrahim et al., 2011): Available Power Gain, G A is the ratio of P avs , power available from the source, to P avn , power available from 2-port network, that is: The power gain is P avn when Г in = Г s * .Therefore Available Power Gain is given by Eq. ( 9), (Ibrahim et al., 2011):  3 PHEMT Transistor FHX76LP was used to design a single LNA cascaded with cascoded LNA.S-parameters for PHEMT is shown in Table 3, where the parameters were obtained at V DD = 2V and I DS = 10 mA of bias set at PHEMT.
From the observation of the single LNA cascaded with cascoded LNA amplifiers, there are some passive component that should be optimized to achieve a targeted specification as required for WiMAX.This passive component values determine the high gain, low noise, good stability and maintain the value of the required bandwidth despite the increasing stage of LNA amplifiers.For example, CA 11 capacitance passive component was influencing the overall system noise figure.This can be shown in Fig. 8.
From Fig. 8, the changes in value of CA 11 from 0.1 to 1 pF caused noise figure of the amplifier to change from 0.61 to 1.6 dB.This change also resulted in decreased bandwidth amplifier from 1.24 to 1.05 GHz.Meanwhile, changes in the value of a single LNA inductive L 15 resulted in changes in the overall system gain when the length of the micro-strip L 15 was changed from 1 to 14 mm as shown in Fig. 9.
In addition, changing the value of inductive L15 also affects the noise figure from 5.9 to 0.69 dB but bandwidth increase from 0.59 to 1.24 GHz.L10 is used as an inductive generation for the single LNA amplifier to allow more flexibility in matching to terminal 50 ohm at the input stage.
In the second stage, a cascoded LNA amplifier, the inductive L25 is placed between M1 and M2.The variable value of the inductance L25 will give the cascoded LNA real input impedance and helps in getting the input and output of the optimal matching.When this condition occurs, it enhances the grain and stability as shown in Fig. 10.
The cascoded transistor M 2 suppresses the Miller capacitance of M 1 thereby increases the reverse isolation.The suppression of the parasitic capacitances of the input transistor also improves the high frequency operation of the amplifier (Lorenzo and De Leon, 2010).When inductive components for the L 26 values raised from the set value, will cause an increase in the gain change dramatically.Additionally inductive L 26 allows output matching at 50 Ohm impedance terminals, it will show at Fig. 11.
At the input, the matching network consists of passive elements such as L11, L12, L13 and CA11.While the passive elements in the output matching network are L27, L28, L29 and CA22.Good selection of passive component in the input and output matching cause LNA amplifiers to be on high gain at the desired frequency.Capacitor CB1 and CB2 are acting as DC block to the single LNA cascaded with cascoded LNA circuit, in which they proposed is worth 10 times the original value of the CB1 or CB2 because it acts as a bypass capacitor (Roger, 2004).

RESULTS
Table 5 shows the s-parameters output for comparison of topology LNA.It is simulated using Ansoft Designer SV.The recorded result for the single LNA cascaded with cascoded LNA gain S 21 is 43.94 dB.The input reflection S 11 is -10.65 dB while the output reflection loss S 22 is -20.02 dB, overall Noise Figure (NF) is 0.61 dB and the return loss S 12 is -52.23 dB.These values are within the design specification as stated in Table 2.The stability factor and noise figure obtained after matching load is 1.35 at 5.8 GHz frequency.The value of stability obtained is greater than 1, the LNA amplifiers are currently in a state ofunconditionally stable and, provides no isolation.From Figure 11 c, it is observed that, the 3dB bandwidth of around 1.24 GHz was obtained and this complies with the targeted result of more than 1 GHz.The output S-parameter for different LNA topology are shown in Figure 11 6 shows the comparison of recently reported LNA.

CONCLUSION
The single LNA cascaded with cascoded LNA amplifier using inductive drain feedback was successfully designed, and developed compliant with  (Kim et al., 2007) and (Othman et al., 2012).

Fig. 11 :
Fig. 11: Affect changes value the L 26 to the overall gain and output matching

Figure 5
Figure 5 shows the complete schematic single stage LNA amplifier.The design of single LNA topology as given in the schematic used T-matching network placed at the input and output impedance.Passive elements contained in the input matching network at the LNA circuit is built as follows L 11 , L 12 , L 23 and CA 11 .While there are passive elements in the output matching network, they are L 17 , L 18 , L 19 and CA 12 , which characteristics of the T-network element used lump reactive element and micro-strip line impedance.There are additional features in the single LNA designed placing inductive source generation L 10 at source M 1 and L 5 inductor peaking structure at the drain of M 1The design of the cascoded LNA has a new technique and topology, where we introduced an inductive feedback L6 is connected to the drain of the M1, inductive source generation L10, which is connected to the source M3 while inductive L5 is placed between the source and drain of M2 and M3 (Fig.6).The design of single cascoded LNA topology as given in the schematic also used T-matching network placed at the input and output impedance.Passive elements

Fig. 16 :
Fig. 16: Noise figure and stability for cascoded LNA a to c.While noise figure and stability are shown in Fig. 11 d to f, respectively.Table

Table 1 :
The comparison between FET and BJT transistors GaAs FET

Table 2 :
Targeted S-parameters for a single LNA cascaded with

Table 5 :
The S-parameters output for comparison of topology LNA IEEE standard 802.16WiMAX.Observations made from the results of S-parameters in the single LNA cascaded with cascoded LNA amplifier is better than the predetermined specifications listed in Table2.The double-stage cascoded LNA amplifier designed achieved the lowest noise figure and high gain due to the noise optimization in the implementation of input matching using passive element CA 11, inductive L 15 peaking structure and inductive drain feedback L26.Recorded result for single LNA cascaded with cascoded LNA amplifier observed provide the gain S21 43.94 dB at frequency of 5.8 GHz.While the input reflection loss S11 -10.65 dB and, the output reflection loss S22 was -20.02 dB.The S12 return loss was -52.23 dB.The stability (K) and Noise Figure (NF) were 1.35 and 0.61 dB, respectively.In conclusion, it has been shown that by using a single LNA cascaded with cascoded LNA amplifier, a minimum noise figure, higher gain, and wider the bandwidth can be obtained and which are better than the recent research the