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LTC2153-14 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2153-14 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC6431-15 11 643115f The LTC6431-15 is a highly linear fixed-gain amplifier which is designed for ease of use. Implementing an RF gain stage is often a multistep project. Typically an RF designer must choose a bias point and design a bias network. Next the designer needs to address impedance matching with input and output matching networks and, finally, add stability networks to ensure stable operation in and out of band. These tasks are handled internally within the LTC6431-15. The LTC6431-15 has an internal self-biasing network which compensates for temperature variation and keeps the device biased for optimal linearity. Therefore, input and output DC blocking capacitors are required. Boththeinputandoutputareinternallyimpedancematched to 50Ω from 20MHz to 1700MHz. Similarly, an RF choke is required at the output to deliver DC current to the device. The RF choke acts as a high impedance (isolation) to the DC supply which is at RF ground. Thus, the internal LTC6431-15 impedance matching is unaffected by the biasing network. The open collector output topology can delivermuchmorepowerthananamplifierwhosecollector is biased through a resistor or active load. Choosing the Right RF Choke Not all choke inductors are created equal. It is always important to select an inductor with low RLOSS, as this will drop the available voltage to the device. Also look for an inductor with high self-resonant frequency (SRF) as this will limit the upper frequency where the choke is useful. Above the SRF, the parasitic capacitance dominates and the choke impedance will drop. For these reasons, wire wound inductors are preferred, and multilayer ceramic chip inductors should be avoided for an RF choke. Since the LTC6431-15 is capable of such wideband operation, a single choke value will probably not result in optimized performance across its full frequency band. Table 1 lists target frequency bands and suggested corresponding inductor values. Table 1. Target Frequency Bands and Suggested Inductor Values FREQUENCY BAND (MHz) INDUCTOR VALUE (nH) MODEL NUMBER MANUFACTURER 20 to 100 1500nH 0805LS Coilcraft www.coilcraft.com 100 to 500 560nH 0603LS 500 to 1000 100nH 0603LS 1000 to 2000 51nH 0603LS DC Blocking Capacitor The role of a DC blocking capacitor is straightforward: block the path of DC current and allow a low series imped- ance path for the AC signal. Lower frequencies require a highervalueofDCblockingcapacitance.Generally,1000pF to 10000pF will suffice for operation down to 20MHz. The LTC6431-15 is relatively insensitive to the choice of blocking capacitor. RF Bypass Capacitor RF bypass capacitors act to shunt AC signals to ground with a low impedance path. It is best to place them as close as possible to the DC power supply pins of the device. Any extra distance translates into additional series in- ductance which lowers the self-resonant frequency and useful bandwidth of the bypass capacitor. The suggested bypass capacitor network consists of two capacitors: a low value 1000pF capacitor to handle high frequencies in parallel with a larger 0.1µF capacitor to handle lower frequencies. Use ceramic capacitors of an appropriate physical size for each capacitance value (e.g., 0402 for the 1000pF, 0805 for the 0.1µF) to minimize the equiva- lent series resistance (ESR) of the capacitor. applicaTions inForMaTion |
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