AN0014

AN0014 15 AN0014 Bias Scheme for NBB-Series Amplifiers Bias Scheme for NBB-Series Amplifiers Introduction RFMD’s NBB-series amplifiers are monolithic integrated circuits (IC's) using InGaP/GaAs HBT technology. The NBB-series uses a Darlington-pair transistor configuration with bias and feedback resistors properly selected to determine the gain, input and output match and bias (both voltage and current) parameters. A schematic representation of the NBB-series amplifier is shown in Figure 1. +VCC NBB-Series Amplifier RCC C1 L1 RF C2 Q1 RBB RBIAS Q2 C3 Figure 1. Schematic representation of Darlington-pair feedback amplifier used in the NBB-series amplifiers. (On-chip components are shown inside the dotted outline.) The amplifier may be analyzed as a two-port network with the input (left) and output (right) as shown in Figure 1. The output node is also used to provide bias to the amplifier through the bias network (top), which is shown in the figure. The packaged part has a total of four ports: input, output and bias, and two ground connections for minimization of ground inductance. Bias Circuit Topology The input voltage of the amplifier is fixed by the base-emitter voltage of Q1 and Q2, and the output voltage is determined by the voltage divider established by RBB and RF. Hence, the output voltage is a designed parameter and the amplifier is controlled by the current supplied to the output node. Thus, the amplifier is biased using a current source rather than a voltage source. The simplest current source is a resistor (RCC) connected to a voltage source (VCC) as shown in Figure2. The current source, comprising voltage source VCC and resistor RBIAS, should be selected such that the designed amplifier voltage (VD) appears at the output with the desired current (ICC) flowing into the amplifier. The governing relationship for the bias circuit shown in Figure 2 is: 15 TECHNICAL NOTES AND ARTICLES ( V CC – V D ) I CC = --------------------------R BIAS Eq. 1 The recommended amplifier current (ICC) and design value of the amplifier voltage (VD) for each NBB-series amplifier is specified on the data sheet. Hence, if a VCC is selected, then the desired RCC may be calculated from Equation 1. A bias resistor table is provided with each datasheet which is calculated using this method. Copyright 1997-2002 RF Micro Devices, Inc. 15-23 AN0014 Typical Bias Configuration RBIAS 4 CBLOCK IN 1 2 3 VD LCHOKE VCC (optional) OUT CBLOCK Figure 2. Typical bias configuration for the NBB-series amplifier. The device is current-controlled due to the nature of the circuit. The simplest current source which can be connected is a resistor (RBIAS) and voltage supply (VCC) as shown. Two blocking capacitors (CBLOCK) are shown to prevent DC-loading of the circuit by an adjacent component. Alternatively, a current steering circuit (see Sedra and Smith, Microelectronic Circuits, 2nd Edition, page 508) may be used to bias the amplifier as shown in far right of Figure 3. Such circuits are commonly used to bias various stages of an IC. The circuit uses one positive power supply (VCC). The DC reference current IREF is generated in the branch that consists of the diode-connected transistor, Q1, resistor R1, and the diode-connected transistor Q2. Assuming all transistors have high current gain and hence the base currents are negligibly small, then the reference current is given by: V CC – V BE 1 – V BE 2 I REF = ----------------------------------------------R1 Eq. 2 Diode-connected transistor Q1 forms a current mirror with Q3. Thus Q3 will supply a constant current ICC equal to IREF. Transistor Q3 can supply this current to any load as long as the voltage that develops at the collector does not exceed that at the base (VCC -VBE3). +VCC Q1 +VCC Q3 +VCC ICC ICC RBIAS IREF R1 ICC Q2 15 TECHNICAL NOTES AND ARTICLES Figure 3. The family of NBB-series amplifiers is biased using a current source (left). The simplest current source is a resistor connected to a voltage source (middle). Alternatively, a current steering circuit (right) may be used as well to produce a constant current (see Sedra and Smith, Microelectronic Circuits, 2nd Ed., page 508). RF Choke Selection The RF choke in series with the bias resistor is recommended as the bias line will effectively load the output of the amplifier. When considering power delivered from the amplifier to a load, it is useful to model the output of the amplifier as a Thevenin equivalent: a voltage Vth with internal impedance ZO (50 Ω for a matched amplifier). When an amplifier is driven in a 50 Ω load, only half the Thevenin voltage (Vth/2) appears across the load resistor. If a RF choke is not used then the voltage across the load reduces to Vth xRBIAS/(2RBIAS +50). The power deliver to the load is reduced by the reduction in the voltage which appears across the load resistor. Taking this ratio, the reduction in power (in dB) by not using an RF choke can be expressed by: P REDUCTION 15-24 = ( V th ⁄ 2 )   20 × LOG  -------------------------------------------------------------  V th R BIAS ⁄ ( 2 R BIAS + 50 ) Copyright 1997-2002 RF Micro Devices, Inc. Eq. 3 AN0014 ( 2 R BIAS + 50 ) P REDUCTION = 20 × LOG  ----------------------------------  2 R BIAS  Eq. 4 In the case of the NBB-300 with a bias resistor of 120 Ω (VCC =10V), the reduction in power delivered to a 50 Ω load is 1.64dB. In order to prevent loading of the output of the amplifier, a reactance of five to 10 times the characteristic impedance is desired. At upper microwave frequencies where lumped element chokes are not available, a microstrip bypass circuit is desirable. Such as choke circuit would consist of a 90° high-impedance line with a short-circuit radial stub. If the tolerances are an issue with the short-circuit stub, a short circuit may be provided from a suitable capacitor instead. In such instances the self-resonance frequency of the capacitor must be considered. Bias Resistor Selection The output voltage of the amplifier (VD) varies as a function of both the bias current (ICC) and the temperature. The variation of device voltage versus current is supplied on each data sheet. From this data, a coefficient may be calculated for the change in VD versus current (see Figure 4). Notice that all coefficients are positive, indicating that the device voltage increases with increasing current. A large bias resistor is desirable because it reduces the variation in bias current, reducing the change in important amplifier parameters such as P1dB and IP3. Selecting a large bias resistor, RBIAS, requires selecting a higher voltage supply (VCC) to maintain the desired bias current (ICC). The current steering circuit (see Figure 3) provides a steady current and minimizes variations in amplifier parameters as well. Table 1. Summary of the coefficient of the change in device voltage (VD) versus amplifier current (ICC). The data is calculated from the plot provided with each datasheet. The positive coefficient indicates that the device voltage increases with increasing current.) Typical Device Voltage Variation with Current, NBB Amplifier Model Number δVD/δICC (in V/A) NBB-300 NBB-301 NBB-400 NBB-401 NBB-410 NBB-500 +4 +4 +7 +3 Device voltage (VD) decreases with increasing temperature as shown in Figure 5. An average rate of change of the device voltage versus temperature is calculated and provided in the table. The device voltage can be expressed as a function both current and temperature as follows. δ VD δ VD V D ( I CC, T ) = V O +  ----------- × I CC +  ---------  × ( T – T O )  δT   δ I CC Table 2. Summary of the dependence of the amplifier output voltage (VD) versus temperature. The coefficient of the change in device voltage (VD) versus ambient temperature is calculated from the tabular data. The negative coefficient indicates that the device voltage decreases with increasing current. Device Voltage (VD) versus Temperature Model Amplifier Temp. Coef. Number Current (mA) (mV/°C) -45°C +25°C +85°C NBB-300 NBB-301 NBB-400 NBB-401 NBB-410 NBB-500 50 50 65 35 4.03 4.09 4.19 4.12 3.86 3.90 4.00 3.94 3.70 3.74 3.88 3.78 -2.75 -2.80 -2.85 -2.70 Eq. 5 15 TECHNICAL NOTES AND ARTICLES Copyright 1997-2002 RF Micro Devices, Inc. 15-25 AN0014 Equation 5 may be substituted into Equation 1 and solved for ICC to get: I CC = δ VD V CC –  ---------  × ( T – T O )  δT  ----------------------------------------------------------δ VD  R BIAS +  ---------- δ I CC Eq. 6 Equation 6 may be differentiated with respect to temperature to get an expression which is useful to observe the effect of the bias resistor (and supply voltage) selected by the user: δ VD –  ---------  δ I CC  δT  ----------- = -------------------------------------δT δ VD R BIAS +  -----------  δ I CC Eq. 7 For example, an NBB-400 biased from a 5V supply (VCC) using a 22 Ω resistor (RCC) will exhibit the following current change with respect to temperature, δ I CC – ( – 2.8 ) ----------- = ------------------- = 0.108 mA ⁄ ° C δT ( 22 + 4 ) Eq. 8 which translates into a variation of 14mA over the temperature range from -40°C to +85°C. By comparison, if a 12V supply rail is with a 162 Ω bias resistor, then a change in current with respect to temperature reduces to 0.016mA/°C. This translates into a current variation of only 2.2mA over the same operating temperature range. 15 TECHNICAL NOTES AND ARTICLES 15-26 Copyright 1997-2002 RF Micro Devices, Inc.