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AD8354 датащи(PDF) 11 Page - Analog Devices |
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AD8354 датащи(HTML) 11 Page - Analog Devices |
11 / 16 page REV. A AD8354 –11– THEORY OF OPERATION The AD8354 is a two-stage feedback amplifier employing both shunt-series and shunt-shunt feedback. The first stage is degen- erated and resistively loaded, and provides approximately 10 dB of gain. The second stage is a PNP-NPN Darlington output stage, which provides another 10 dB of gain. Series-shunt feed- back from the emitter of the output transistor sets the input impedance to 50 Ω over a broad frequency range. Shunt-shunt feedback from the amplifier output to the input of the Darlington stage helps to set the output impedance to 50 Ω. The amplifier can be operated from a 3 V supply by adding a choke inductor from the amplifier output to VPOS. Without this choke inductor, operation from a 5 V supply is also possible. BASIC CONNECTIONS The AD8354 RF gain block is a fixed-gain amplifier with single- ended input and output ports whose impedances are nominally equal to 50 Ω over the frequency range 100 MHz to 2.7 GHz. Consequently, it can be directly inserted into a 50 Ω system with no impedance matching circuitry required. The input and output impedances are sufficiently stable versus variations in temperature and supply voltage that no impedance matching compensation is required. A complete set of scattering parameters is available at the Analog Devices website (www.analog.com). The input pin (INPT) is connected directly to the base of the first amplifier stage, which is internally biased to approximately 1 V, so a dc-blocking capacitor should be connected between the source that drives the AD8354 and the input pin, INPT. It is critical to supply very low inductance ground connections to the ground pins (Pins 1, 4, 5, and 8) as well as to the back- side exposed paddle. This will ensure stable operation. The AD8354 is designed to operate over a wide supply voltage range from 2.7 V to 5.5 V. The output of the part, VOUT, is taken directly from the collector of the output amplifier stage. This node is internally biased to approximately 3.2 V when the supply voltage is 5 V. Consequently, a dc-blocking capacitor should be connected between the output pin, VOUT, and the load that it drives. The value of this capacitor is not critical, but it should be 100 pF or larger. When the supply voltage is 3 V, it is recommended that an external RF choke be connected between the supply voltage and the output pin, VOUT. This will increase the dc voltage applied to the collector of the output amplifier stage, which will improve performance of the AD8354 to be very similar to the perfor- mance produced when 5 V is used for the supply voltage. The inductance of the RF choke should be approximately 100 nH. Care should be taken to ensure that the lowest series self-resonant frequency of this choke is well above the maximum frequency of operation for the AD8354. The supply voltage input, VPOS, should be bypassed using a large value capacitance (approximately 0.47 µF or larger) and a smaller, high frequency bypass capacitor (approximately 100 pF) physi- cally located close to the VPOS pin. The recommended connections and components are shown in the schematic of the AD8354 evaluation board (Figure 3). APPLICATIONS The AD8354 RF gain block may be used as a general-purpose fixed-gain amplifier in a wide variety of applications, such as a driver for a transmitter power amplifier (Figure 1). Its excellent reverse isolation also makes this amplifier suitable for use as a local oscillator buffer amplifier that would drive the local oscilla- tor port of an up or down converter mixer (Figure 2). AD8354 HIGH POWER AMPLIFIER Figure 1. AD8354 as a Driver Amplifier MIXER AD8354 LOCAL OSCILLATOR Figure 2. AD8354 as a LO Driver Amplifier NC = NO CONNECT COM1 NC INPT COM1 VOUT VPOS COM2 COM2 AD8354 C3 100pF C4 0.47 F VP OUTPUT 1 2 3 45 6 7 8 C2 1000pF C1 1000pF INPUT L1 Figure 3. Evaluation Board Schematic EVALUATION BOARD Figure 3 shows the schematic of the AD8354 evaluation board. Note that L1 is shown as an optional component, which is used to obtain maximum gain only when VP = 3 V. The board is powered by a single supply in the 2.7 V to 5.5 V range. The power supply is decoupled by 0.47 µF and 100 pF capacitors. Table I. Evaluation Board Configuration Options Component Function Default Value C1, C2 AC Coupling Capacitors. 1000 pF, 0603 C3 High Frequency Bypass Capacitor. 100 pF, 0603 C4 Low Frequency Bypass Capacitor. 0.47 µF, 0603 L1 Optional RF Choke. Used to increase current through output stage when VP = 3 V. Not recommended for use when VP = 5 V. 100 nH, 0603 |
Аналогичный номер детали - AD8354 |
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Аналогичное описание - AD8354 |
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