Application Note 5460

Transcription

1 MGA-89 High Linearity Amplifier with Low Operating Current for 9 MHz to. GHz Applications Application Note 6 Introduction The Avago MGA-89 is a high dynamic range amplifier designed for applications in the MHz to 6 GHz frequency range. By using an Avago proprietary.-micron GaAs E-pHEMT process (Enhancement-mode pseudomorphic High Electron Mobility Transistor), the MGA-89 achieves high linearity with low current consumption. The MGA-89 is a two-stage cascode amplifier, operates with a single V supply and is self-biased at ma. Simple external input and output matching circuitry allows the MGA-89 to achieve the desired performance without complex circuitry or extra PCB area. With the frequency range extending up to 6 GHz, the MGA-89 is ideal for Cellular/PCS/W CDMA, WLAN and WLL applications. The combination of high linearity, low noise figure and high gain makes the MGA-89 ideal as a second-stage LNA in a receive chain or as a driver amplifier in the transmit chain. Housed in a surface mount, plastic SOT-89 package with a low thermal resistance of 6. C/W, the MGA-89 is suitable for base station applications. Application Circuit Schematic The MGA-89 is biased by a single V supply connected to the output pin. The device is internally biased for optimal performance at a quiescent current of ma. A good rule of thumb for biasing is to use an RF choke,, as shown in Figure to isolate the in-band signal from the DC supply. Choke is chosen to have a high reactance at the operating frequency, but it must be chosen carefully since it will also contribute to the output match. serves as a RF bypass for in-band signals, and serves as a DC bypass to eliminate out-of-band low frequency signals. An optional resistor,, may be added to de-q any resonance created between and. A resonance effect between and may cause a gain peak at lower frequencies that can cause stability concerns. To avoid dropping the device power supply voltage excessively, is usually between. to Ω. Figure. Typical MGA-89 application circuit The input matching circuit consists of and, which forms a high-pass configuration. Besides setting the required impedance match the high-pass configuration improves stability by diminishing low frequency gain. At the output, acts as a DC blocking capacitor to isolate the supply voltage from following stages. and form the output matching circuit which is also a high-pass configuration.

2 Figure. MGA-89 demonstration board PCB layout Demonstration Board Layout The MGA-89 demonstration board is a two-layer printed circuit board using FR material (dielectric constant of.6) with a thickness of.8 mm. The top layer consists of microstrip lines and the bottom layer is a solid ground plane. The put and output traces have Ω characteristic impedances. In order to maintain stability and optimize RF performance, the MGA-89 GND pin should be connected to the solid ground plane with plated through via holes. The ground vias should be placed as close to the SOT-89 package terminal as possible to reduce inductance in the ground path. The best practice is to use multiple vias to further minimize the ground path inductance. Demonstration Board Performance The MGA-89 can be designed for various applications using the schematic shown in Figure. The following examples demonstrate how the basic MGA-89 can be configured for either maximum OIP or for optimum OIP over a wide operating temperature range. 9 MHz Amplifier Designed for Maximum OIP A common application for the MGA-89 is in a PCS 9MHz radio receiver where the amplifier is typically optimized for maximum linearity. The MGA-89 can achieve a maximum OIP of 9 dbm at C. For maximum linearity, the input and output are matched to the Γ s and Γ L values listed in the datasheet. For maximum OIP at 9 MHz, Γ s is and Γ L is..68. The table below lists the components used for this matching and for biasing.. pf Murata.8 pf Kyocera pf Murata. mf Murata nh Toko.7 nh Toko nh Toko Ω KOA Figure. Γ s, Γ L,, for 9 MHz maximum OIP match Figure. Bill of materials for a 9 MHz amplifier with maximum OIP

3 Using the components in Figure, the OIP is 9.7 dbm, and the PdB is 8. dbm. Noise figure (NF) is. db. Small signal gain (SSGain) at 9 MHz is 6. db. Input return loss (IRL) is 6.8 db, and output return loss (ORL) is 9. db. As shown in Figure 7, this circuit is unconditionally stable up to GHz with the k-factor >. For the circuit in Figure, a maximum OIP of 9.7 dbm was measured at C. However, OIP performance will degrade at - C and 8 C. The measured OIP variation over temperature is approximately 9. db Figure. Input and output return loss Figure 6. Small signal gain and reverse isolation Gain Output Input For the OIP measurement, the two fundamental tones were set at F = 9 MHz and F = 9 MHz. The output power level of F and F was set at.9 dbm. Because OIP degrades at the extremes of the operating temperature range the matching circuits can be designed to give good OIP performance over temperature. The match conditions and circuit shown in Figures 9 and optimize OIP over temperature without sacrificing high gain or a low noise figure. OIP - dbm TEMPERATURE - C Figure 8. OIP over temperature for a 9 MHz amplifier, designed for maximum OIP 9 MHz Application Designed for Optimum OIP over Temperature To tune for optimum linearity over a wide a wide operating temperature range, the input and output is matched to the Γ s and Γ L values listed in the datasheet. For optimum 9 MHz OIP over temperature Γ s is and Γ L is..8. Figure shows the completed design and the components needed for matching and biasing. K-Factor 6 8 Figure 7. K-Factor for a 9 MHz amplifier 6 8 Figure 9. Γ s, Γ L and, for a 9 MHz amplifier optimized for OIP over temperature

4 As shown in Figure, the temperature optimized OIP design showed a reasonable db OIP variation over temperature. The maximum OIP performance changed to 6.6 dbm. The two-tone measurement was done using a MHz frequency spacing (Fspacing) with a.9 dbm output power per tone. There are insignificant changes in other parameters with the OIP optimized design. SSGain was 6. db, IRL was. db and ORL was 7.7 db. OPdB remained at 8.8 dbm while the NF was about. db. As shown in Figures, and, the optimized circuit is unconditionally stable up to GHz Output Input. pf Murata. pf Murata pf Murata. mf Murata. nh Coilcraft.9 nh Toko Not Used Not Used Not Used R KOA Figure. Bill of materials for a 9 MHz amplifier optimized for OIP over temperature OIP - dbm TEMPERATURE - C Figure. Optimized OIP performance over temperature Figure. Input and output return loss Figure. Small signal gain and reverse isolation K-Factor Gain Figure. K-Factor for the 9 MHz amplifier optimized for OIP over temperature - - -

5 9 MHz Cellular Band Application The MGA-89 can also be used in the 9 MHz cellular band. Optimum linearity is achieved with the input and output match set to a Γ s of.9-7. and a Γ L of.66-9., as listed in the datasheet. Figure and 6 detail the components used for 9 MHz matching and for biasing. The 9 MHz design had an SSGain of 7.9 db with an IRL of. db and an ORL of.6 db. Noise figure is. db. This design had 6.6 dbm OIP and 8.7 dbm OPdB measured performance. This circuit is unconditionally stable up to GHz with a k-factor greater than. Overall circuit performance is shown in Figures 7, 8 and 9. IRL - ORL Figure 7. Input and output return loss Figure. Γ s, Γ L and, for 9 MHz match - 8. pf Murata 8. pf Murata pf Murata. mf Murata nh Toko 7 nh Toko 9 nh Toko R KOA Figure 6. Bill of materials for a 9 MHz amplifier - SSGain Figure 8. Small signal gain and reverse isolation K-Factor Figure 9. K-factor for the 9 MHz amplifier

6 MHz WLAN Application This example is for a WLAN amplifier operating at MHz. For the design goal of optimum linearity, the input and output are matched to a Γ s of and a Γ L of. 7.8, as listed in the datasheet. Figure shows the components used for the MHz match and for circuit biasing. The MHz design had an SSGain of db with an IRL of.8 db and an ORL of. db. The noise figure was.7 db. This design also had a measured OIP of 7.6 dbm and OPdB of 7.7 dbm. The circuit is unconditionally stable up to GHz, where the k-factor is more than. Measured circuit performance is shown in Figures, and Input Output Figure. Input and output return loss Figure. Γ s, Γ L and, for MHz match Gain pf Murata 6.8 pf Murata pf Murata. mf Murata nh Toko 68 nh Toko.7 nh Colicraft R KOA Figure. Small signal gain and reverse isolation k-factor Figure. Bill of materials for a MHz amplifier Figure. K-factor for the MHz amplifier 6

7 Summary The high linearity and low noise figure capability of the MGA-89 has been demonstrated through several design examples across various frequency bands. IP temperature variation has also been taken into consideration. Performance measurements, using the demonstration board, are summarized in Table. Resources. Avago MGA-89 data sheet: com/docs/av-76en. MAG-89 ADS Model: docs/mga-89_69 Table. MGA-89 performance summary Parameter 9MHz 9Mz MHz Noise Figure, NF. db. db.7 db Gain 7.9 db 6. db db Output Third-order Intercept Point, OIP 6.6 dbm 6.6 dbm 7.6 dbm Output Power at db Gain Compression, PdB 8.6 dbm 8.8 dbm 7.7 dbm Input Return Loss. db. db.8 db Output Return Loss.6 db 7.7 db. db.9 db db. db For product information and a complete list of distributors, please go to our web site: Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies in the United States and other countries. Data subject to change. Copyright - Avago Technologies. All rights reserved. AV-EN - August,