Application Note 5038

Transcription

1 MGA-6P8 Buffer Amplifier for 10 MHz Application Application Note 038 Introduction The MGA-6P8 is a high isolation buffer amplifier based on Avago Technologies EPHEMT process. This application note discusses the circuit configuration and performance obtainable from the MGA-6P8 operating at 10 MHz on an application demoboard. Application Circuit Figure 1 shows the MGA-6P8 on the application demoboard configured to operate at 10 MHz. The supply required at + V current drain is typically 40 ma. Figure 2 shows the schematic diagram of the 10 MHz amplifier. All the information on the components required to build the demoboard amplifier is given in Table 1. Due to the high reverse isolation featured in the MGA- 6P8, it is a very easy device to match for low VSWR at both input and output ports. When one of the ports is matched closely to s, it remains well matched when proceeding to match the other port. This is seldom the case for amplifiers designed with discrete transistors or other MMICs. At 10 MHz, a 68 nh (L1) shunt inductor is required to match the MGA-6P8 input good return loss. As the MGA 6P8 requires that the input and output be DC isolated from external circuits, coupling capacitors of 180 pf provide sufficient coupling of the RF signal at input and output. Higher capacitance can be used for coupling, but is not necessary. On the output of the MGA-6P8 a shunt L C circuit is used to deliver the bias to the device. The choice of the shunt L (L2) is not as straightforward as it may seem. An inductor with a high inductance (1.2 µh to 1.8 µh) may be commonly used for such VHF amplifiers as RF choke to deliver bias. However, such inductors are usually lossy, introducing loss to the output circuit. Using an 82 nh inductor to deliver bias to the output pin of the MGA-6P8 at 10 MHz results in about a 1.0 db higher gain than when a high inductance RF choke is used. In both cases, the MGA 6P8 is matched close to s at input and output. When an 82 nh inductor is used to deliver the bias, a series 1 pf capacitor (C3) following the shunt inductor can be used to complete the match. Figure 1. A populated application demoboard for MGA 6P8 at 10 MHz. Table 1. Component Designators Manufacturer and Part Number R1 KOA RM73B1E1R0J 0402 resistor R2 KOA RM73B1E1R0J 0402 resistor R3 KOA RM73B1E1R0J 0402 resistor R4 080 OΩ jumper R KOA RM73B1E1R0J 0402 resistor L1 Coilcraft 0603CS_68NX_BC 0603 inductor L2 Coilcraft 0603CS_82NX_BC 0603 inductor C1 ROHM MCH1A181JK 0402 capacitor C2 Murata GRM40X7R102K0 080 capacitor C3 ROHM MCH1A10JK 0402 capacitor C4 Murata GRM40X7R102K0 080 capacitor U1 Avago Technologies MGA-6P8 SMA connectors (2 pcs) EF Johnson Demoboard for MGA-6P8 Avago Technologies DEMO-MGA-6P8

2 Measured RF Performance on Application Demoboard The RF performance of the MGA 6P8 shown in this application results from measurement of the device on the application demoboard as a whole. P 1dB and Psat measured on the application demoboard at 10 MHz is approximately dbm and +21 dbm, respectively. Current consumption of the device is approximately 38 ma at small input signal level and increases to about 42 ma when the device is driven into 1dB gain compression. R3 can be increased to reduce the Psat and current consumption whenever applications of this device allow. When the board is matched to good return loss as shown in Figures, 6, and 7, the insertion gain obtainable from the amplifier on board is around 33.6 db (Figure 8). The MGA-6P8 is an excellent candidate for VCO buffer and for other applications that require an amplifier with high reverse isolation. High isolation on VCO buffer is desired in RF systems to minimize VCO frequency pull as a result of load impedance fluctuation during system operations. Figure 9 shows that the demoboard amplifier achieves the isolation at 10 MHz in excess of 0 db and is sufficiently high for most applications pf +V Id approx.38 ma at V supply P1dB (+21 dbm) approx. 42 ma 6 82 nh 1000 pf 68 nh 180pF 2 MGA-6P pf Figure 2. Schematic diagram of a MGA-6P8 amplifier on application demoboard. Input SMA 0 (R1) MGA-6P8 (U1) 180pF (C1) 1pF (C3) 82nH (L2) 1nF (C2) 68nH (L1) (R2) (R3) (R) Output SMA 1nF 080 (C4) 080 (R4) Avago Technologies GND +V +V GND Figure 3. Component placement on application demoboard. 2

3 Figure 4. A blank application demoboard for MGA-6P8. Figure. Board level S11 (Input) on Smith Chart. 3

4 Figure 6. Board level S22 (Output) on Smith Chart. MGA-6P8 Board Level Return Loss 40 MGA-6P8 Board Level Insertion Gain 0 3 RETURN LOSS (db) Output Return Loss Input Return Loss GAIN (db) Figure 7. Board level measured input and output return loss. Figure 8. Board level insertion gain. 4

5 -38 MGA-6P8 Board Level Isolation REVERSE GAIN (db) Figure 9. Board level isolation. 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, Limited in the United States and other countries. Data subject to change. Copyright Avago Technologies, Limited. All rights reserved EN June 2, 2010