AN3100

www.DataSheet4U.com 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 tempera...