Freescale Semiconductor Application Note AN3100 Rev. 0, 3/2005 General Purpose Amplifier Biasing by: Jeff Gengler Freescale Semiconductor INTRODUCTION Freescale Semiconductor’s General Purpose Amplifier (GPA) devices are all designed to operate from a single positive voltage supply. The GPAs have output powers ranging from 12 to 34 dBm. They are currently designed with three different circuit techniques: • Darlington Pair • Discrete with integrated current mirror • Field Effect Transistor (FET) operating at zero gate voltage drain leakage current (IDSS) and use two different device technologies: • Indium Gallium Phosphide Heterostructure Bipolar Transistors (InGaP HBT) • GaAs Heterostructure Field Effect Transistor (HFET) The required biasing methods for the different circuit schemes are described in this application note. GPA CIRCUIT DESIGN METHODS Freescale’s InGaP HBTs are designed using one of two different circuit methods. The low power GPAs (P1dB from 12 to 24 dBm) are designed using a Darlington Pair (Fig. 1). The Darlington Pair is biased when voltage is applied to the collector of discrete devices Q1 and Q2. Resistor R1 is used for negative feedback of the amplifier but is also part of the voltage divider with R2 to establish the base bias on Q1. VSUPPLY R6 HBT devices are current - driven; therefore, Freescale recommends that designers use a constant current source to minimize the impact of shifts in supply voltage and shifts in the temperature of the operating environment. Deviations from the optimal current can impact both power and linearity performance. A series resistor between the voltage supply and collectors of the Darlington is the easiest way to emulate a constant current source (R6 in Fig. 1). This is necessary for the MMG3001NT1, MMG3002NT1 and MMG3003NT1 devices. Because the RF output of the Darlington Pair is also used for the DC bias, an RF choke is required (L1) to connect the voltage supply to the output. RF coupling capacitors may also be required on the RF input and RF output because the input and output of the devices are DC coupled. Since the release of these first three devices, Freescale has developed a method to eliminate the need for an external resistor and to enable the devices to operate directly from a positive 5 Volt supply. This approach has exceptional current stability over temperature and has a pending patent. All Darlington HBT products with the exception of the MMG3001NT1, MMG3002NT1 and MMG3003NT1 use this approach (Fig. 2). VSUPPLY VCC RF INPUT Q1 VCC C3 L1 Q1 R2 R3 R4 C2 Q2 R2 R3 R4 RF OUTPUT R1 C1 RF OUTPUT R1 C1 RF INPUT C3 L1 R5 C2 Q2 R5 PACKAGED DEVICE Figure 2. Improved Darlington Pair InGaP HBT Bias Scheme PACKAGED DEVICE Figure 1. Darlington Pair InGaP HBT Bias Scheme Freescale Semiconductor, Inc., 2005. All rights reserved. RF Application Information Freescale Semiconductor AN3100 1 The second circuit method is used on the intermediate power amplifiers (P1dB ranging from 27 to 34 dBm). These are designed with a MMIC that contains a discrete device, Q1, with an integrated current mirror to drive the base (Fig. 3). This active bias approach means that the bias current has minimal shift with normal supply voltage deviations over the specified operating temperature range. R6 in Fig. 3 is an external dropping resistor that is required to establish the reference voltage on the current mirror that drives the bias of Q1. The HFETs are discrete devices that operate directly from a 5 Volt supply voltage (Fig. 4). The DC blocking capacitor that is integrated in the feedback loop prevents the gate voltage from being established with R1 and R2; therefore, the HFET devices operate at IDSS when 5 Volts are applied to the drain. L1 is again required as an RF choke as well as the RF coupling capacitors, C2 and C3. VSUPPLY C4 VCC R6 L1 C6 L1 Vref RF INPUT R1 C5 C2 Q2 R4 C2 Q3 R2 R1 C3 R2 PACKAGED DEVICE RF OUTPUT R3 RF INPUT Figure 4. HFET Bias Scheme C4 Q1 C3 C1 Q1 R5 RF OUTPUT C1 PACKAGED DEVICE Figure 3. Intermediate Power InGaP HBT Bias Scheme The reference voltage (Vref) is different for each device based on its size. The data sheets for each device list the specific reference voltage required for optimal bias current. L1 is required to prevent the DC supply line from improperly loading the RF output. RF coupling capacitors (C3 and C4 in Fig. 3) are also required. The third circuit approach in GPAs is used for the HFET devices. Bias of this type of device is very similar to the Darlington circuit technology. SUMMARY The GPA lineup from Freescale is designed to operate from a single positive voltage supply, which makes them easy to use. Designers using these devices should be careful to bias the devices correctly using the appropriate method for the type of device used. If the current is set too low, linearity and power will degrade. If the current is set too high, there is some risk of compromising reliability. The techniques outlined here are a guide to the bias approaches for the different technologies and products available from Freescale. The data sheets for each device should be followed to achieve optimal performance from all GPAs. AN3100 2 RF Application Information Freescale Semiconductor NOTES AN3100 RF Application Information Freescale Semiconductor 3 How to Reach Us: Home Page: www.freescale.com E - mail: [email protected] USA/Europe or Locations Not Listed: Freescale Semiconductor Technical Information Center, CH370 1300 N. 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Freescalet and the Freescale logo are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. Freescale Semiconductor, Inc. 2005. All rights reserved. AN3100 Document Number: AN3100 Rev. 0, 3/2005 4 RF Application Information Freescale Semiconductor