Datasheet LTC6431-20 (Analog Devices) - 10

LTC6431-20
OPERATION
The LTC6431-20 is a highly linear, fixed-gain amplifier that linearity. Shunt and series feedback are added to lower the is configured to operate single ended. Its core signal path input/output impedance and match them simultaneously consists of a single amplifier stage, minimizing stability to the 50Ω source and load. Meanwhile, an internal bias issues. The input is a Darlington pair for high input imped- controller optimizes the internal operating point for peak ance and high current gain. Additional circuit enhancements linearity over environmental changes. This circuit archi- increase the output impedance and minimize the effects tecture provides low noise, excellent RF power handling of internal Miller capacitance. capability and wide bandwidth — characteristics that are The LTC6431-20 starts with a classic RF gain block to- desirable for IF signal chain applications. pology but adds enhancements to dramatically improve
APPLICATIONS INFORMATION
The LTC6431-20 is a highly linear fixed gain amplifier which wound inductors are preferred, and multilayer ceramic is designed for ease of use. Implementing an RF gain stage chip inductors should be avoided for an RF choke. Since is often a multi-step project. Typically an RF designer must the LTC6431-20 is capable of such wideband operation, choose a bias point and design a bias network. Next we a single choke value will probably not result in optimized need to address impedance matching with input and output performance across its full frequency band. Table 1 lists matching networks and finally add stability networks to target frequency bands and suggested corresponding ensure stable operation in and out of band. These tasks inductor values: are handled internally within the LTC6431-20.
Table 1. Target Frequency Bands and Suggested Inductor Values
The LTC6431-20 has an internal self-biasing network
FREQUENCY BAND INDUCTOR MODEL
which compensates for temperature variation and keeps
(MHz) VALUE (nH) NUMBER MANUFACTURER
the device biased for optimal linearity. Therefore input and 20 to100 1500nH 0603LS Coilcraft output DC blocking capacitors are required. www.coilcraft.com 100 to 500 560nH 0603LS Both the input and output are internally impedance matched 500 to1000 100nH 0603LS to 50Ω from 20MHz to 1400MHz. Similarly, an RF choke 1000 to 2000 51nH 0603LS is required at the output to deliver DC current to the de-
DC Blocking Capacitor
vice. The RF choke acts as a high impedance (isolation) to the DC supply which is at RF ground. Thus, the internal The role of a DC blocking capacitor is straightforward; block LTC6431-20 impedance matching is unaffected by the the path of DC current and allow a low series impedance biasing network. The open-collector output topology can path for the AC signal. Lower frequencies require a higher deliver much more power than an amplifier whose collector value of DC blocking capacitance. Generally, 1000pF to is biased through a resistor or active load. 10000pF will suffice for operation down to 20MHz. The LTC6431-20 is relatively insensitive to the choice of block-
Choosing the Right RF Choke
ing capacitor. Not all choke inductors are created equal. It is always
RF Bypass Capacitor
important to select an inductor with low RLOSS, as this will drop the available voltage to the device. Also look for an RF bypass capacitors act to shunt AC signals to ground inductor with high self-resonant frequency (SRF) as this with a low impedance path. It is best to place them as will limit the upper frequency where the choke is useful. close as possible to the DC power supply pins of the de- Above the SRF, the parasitic capacitance dominates and vice. Any extra distance translates into additional series the choke impedance will drop. For these reasons, wire inductance which lowers the self-resonant frequency and 643120f 10 For more information www.linear.com/LTC6431-20 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Order Information DC Electrical Characteristics AC Electrical Characteristics Typical Performance Characteristics Block Diagram Test Circuit A Operation Applications Information Demo Board S Parameters Package Description Typical Application Related Parts