how to generate qr code in vb.net NOTE : When the transmitter s PA stage is connected to a filter network and/or narrowband in Software

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NOTE : When the transmitter s PA stage is connected to a filter network and/or narrowband
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antenna, then the stability, IP3, PAE, as well as the level of the RF power output itself, may change drastically This is caused by the nonlinear transistor s generated harmonics being reflected off the filter s and/or antenna s stopbands, and being returned, at various amplitudes and phases, back into the amplifier s output port The more nonlinear the device, the stronger the harmonics, and the more the PA will be affected by this Harmonics at the PA s input port will also have an influence Thus, after an initial tune of the PA alone as a single stage, it is always wise to perform a complete PA tune with any filter inserted at the amplifier s output, in the exact PCB layout of the final and completed circuit Further, due to a PA s output rarely being at a perfect 50 , any filter at its output will be strongly influenced by this poor termination, shifting the filter s amplitude and frequency response away from the as-expected design values
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CRF 84 pF RVcc 464 RFC 42 nH
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CRF 84 pF
C5 364 pF Port_2 50 L4 419 nH
Port_1 50 +8 dBm INPUT L1 19 nH
Q1 C2 415 pF
25 20 15 10 dB (LS21) P2 (dBm) 5 0 5 10 15 20 25 0 19 38 57 Frequency (GHz) P2 76 95
20 9 2 13 24 35 46 57 68 79 dB(LS21) dB(LS11)
1909e+9 , 13111
5 25 0 25 5 75 10 dB (LS11)
1909e+9 , 14015
125 15 175 20 6e+9
90 100e+6 128e+9 246e+9 364e+9 482e+9 Frequency
FIGURE 373 An example design of a complete Class AB power ampli er with active bias and matching, showing the Agilent Genesys nonlinear simulation of its RF output power and large signal S-parameters
A Quick Example Design an RF Power Amplifier (Fig 373) Goal: Create a stable Class-AB RF power amplifier with diode bias and 50- matching The specifications and parameters for the circuit are: VCC = 36 V fr = 19 GHz Ga = 13 dB PIN = 8 dBm POUT = > 21 dBm Transistor = NXP BFG480W (IC(MAX) = 250 mA, hFE(MIN) = 40)
Three
One possible solution: 1 RV(cc) = 464 2 D1 = any small-signal diode capable of dissipating the maximum power required 3 L1 = 19 nH, C2 = 415 pF, C5 = 364 pF, L4 = 419 nH (BFG-480W s data sheet states that it has an input impedance of 85 + j14, and wants to see a load of 25 + j2 when at 36 VCE, 19 GHz, 100 mW The L1/C2 and C5/L4 input and output L networks are designed to satisfy these requirements) 4 RFC = 42 nH, CRF = 84 pF, RS = 5
NOTE : When PA is modeled and/or built with real, nonideal passive parts, all of the above
84 pF RF decoupling capacitors would be replaced by approximately 18 pF (for 0402 case size) values to take advantage of their series resonance frequency (SRF) This applies equally to the 42 nH RFC inductors for their parallel self resonant frequency (SRF) RS is used to minimize undesirable parallel resonant interactions of the large value CAC s with the RF decoupling capacitors CRF s Use a ferrite bead for improved performance
Power Amplifier Stability, Tests, and Cures
Instability in RF amplifiers can take the form of oscillations at almost any frequency, and may even damage or destroy the transistor These spurious oscillations will arise at specific, or very wide ranging, frequency or frequencies, and over a particular bias, drive level, temperature, or output load impedance Eliminating this instability will normally require a reduction of stage gain or output power through the complete retuning of the PA, or by the application of negative feedback all of which will reduce the PAE Indeed, a particularly poor RF layout may, in rare cases, demand a completely new PCB design and board fabrication due to electromagnetic, magnetic, or capacitive coupling from the PA s output port to its input port This can cause positive feedback, resulting in oscillations (as can a transistor s own interelectrode capacitance feeding an in-phase signal back from its collector to its base) A power amplifier, when properly biased and matched, must be stable across all output impedances that it expects to see, especially if connected to an antenna that is directly interfacing with the outside world Testing this type of stability is fairly rapid with the use of an automated load-pull station However, if an automated load-pull station is not available, a quick and dirty method of impedance-based stability analysis of a low power (< 3 W) PA is by attaching the appropriate antenna to the amplifier s output port and, over various supply voltages and temperatures, place your hand near, around, and at various distances from the side and top of the antenna If the amplifier does not oscillate during this procedure, as checked with a sniffer probe or a 20-dB coupler attached to a spectrum analyzer set for a wide sweep, then the stage can be considered sufficiently stable, unless component-to-component tolerances are excessively poor with succeeding PA builds Stability testing over temperature is critical, as the stage may only oscillate when thermally hot due to the typical changes in transistor parameters with temperature This is referred to as thermal feedback Class C power amplifiers, too, must remain stable under any load or VCC, since instability can destroy the transistor because of the increased collector currents and high voltages created while oscillating One way to rapidly test Class C amplifier stability is to place its output port into a slide-stub tuner, and drive the stage s input at its design frequency If, while varying the amplifier s RF input drive power level, the
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