This article originally appeared in the Australian, June, issue of Amateur Radio magazine. The design will be useful for constructors more comfortable working with larger size mosFETs. The BF can still be sourced via eBay. On a performance for dollar basis it was probably the best device as of available. However, circuits published in some overseas magazines have failed to perform as well as expected. Curves are provided for determining the source admittance necessary to obtain these figures.
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The finished amplifier. Mechanical design The amplifier is build on a double sided board measuring 25x50 mm. The upper side functions as a ground plane and the traces are on the lower side. The board can either be build into the radio or mounted in a small metal box with BNC connectors.
The amplifier is build on a small PCB. Electrical design The diagram is shown below. The input circuit is a series transformation, the output circuit a "normal" parallel circuit with a capacitive output tap. As this amplifer has high gain, there is provision for a pi-attenuator on the output. This circuit tightly controls the DC parameters of the transistor. L2, L3: 5. Circuit details The input circuit is a bit unusual as it uses a series inductor L2 to step the 50 ohm generator impedance up to ohm, which is the optimum gate impedance for noise matching.
L2 resonates with C4 in parallel with the gate1 input capacitance. This minimises the loaded Q of the input circuit - important to keep insertion loss down - and the correct impedance noise match is fixed by design.
The output circuit is a parallel circuit with with L3, C6 and C8. This also determines the gain. Component layout.
Tune up When powering up for the first time, start by checking the DC current consumption of abt. Then adjust L2 and L3 for maximum signal. For best noise match use a weak unmodulated signal and a receiver in FM mode.
Then tune only L2 for best noise suppression. Any resulting variation of the preamplifier gain will be nullified by the limiter in the FM receiver. Attenuator The can be taylored to your specific needs - which should be "not more than enough" - by having an attenuator at the output of the amplifier.
The following table shows resistor values for different attenuation. PI - attenuator values. I have build and measured many of these amplifiers, and they have almost identical performance. The typical result with a 3-dB output attenuator is shown here: Noise figure and gain from to MHz. NF is 1.
N-Channel MOSFET low noise amplifier
By adding the following VHF pre-amp as 1st stage, the improving of sensitivity is spectacular. The pre-amp including also a Band-Pass input filter which rejects any undesirable signal outside of 2 m. There is nothing unusual on this VHF pre-amplifier design. The shield point "A" as shown in FIG.
BF961 / BF981 / BF199 Part # substitutes ???