100M-500M低噪放设计方案
Thus, by forcing the emitter voltage (VE) of transistor Q1 equal to Vds, this circuit regulates the drain current in a manner similar to a current mirror. As long as Q2 oper-ates in the forward active mode, this holds true. In other words, the collector-base junction of Q2 must be kept reverse biased.
An evaluation board was designed for the feedback ampli-fier network. This single-layer board (see Figures 4 and 5) is 0.031-inch thickness FR-4 material with a dielectric constant of 4.2. The feedback network should be made as short as possible, since introducing inductance into the feedback network causes instability in the 5-6 GHz region. The RC feedback uses 40 x 20 mil components that are soldered close together with a small solder pad in between.
The ATF-54143 is conditionally stable below 3.5 GHz, having 29-26 dB gain in the 100-500 MHz region. The RC feedback reduces low frequency gain and increases the stability factor to >1 below 2 GHz. The amplifier uses a high-pass impedance matching network, consisting of C1 and L1, for the noise match. The circuit loss will directly relate to noise figure, thus the Q of L1 is extremely important. The Toko LL1608-FSR15 is a small multi-layer chip inductor with a rated Q of 19 at 50 MHz. The shunt inductor (L1) provides low frequency gain reduction, which can minimize the amplifier’s susceptibility to overload from nearby low frequency transmitters. It is also part of the input match-ing network along with C1. C1 also doubles as a DC block, while L1 also provides a means of inserting gate voltage for the PHEMT. This requires a good bypass capacitor in the form of C2.
Figure 4. RF Layout for Demo Board.
Figure 5. Assembly Drawing for Amplifier.
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