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.
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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. The noise figures claimed for some circuits published in European magazines are from 1. The reason for this probably lies in the type of input circuit used. The European circuits use the standard coil and parallel capacitor combination with the input tapped down the coil.
It is difficult to adjust this combination to the point where the input gate sees the source admittance required for optimum noise figures. Input circuit Any component placed in circuit between the source of signal and the control electrode of the first amplifier will cause a reduction in noise figures.
This fact must be weighed against the necessity to provide that impedance match demanded by the amplifier for best noise figure and any requirement to guard against strong adjacent channel interference by limiting the input bandwidth. The latter requirement has not been considered in the amplifier described. Reference to the circuit diagram will indicate that the number of components is minimal.
The sizes of L1 and C2 have been calculated to enable G1 to see that source admittance which will give optimum noise figure at MHz. This value was interpolated from the circles of typical constant noise figures for MHz and MHz given on the data sheets. Circuit Description C1 should be a low loss, low inductance capacitor. RFC must be low loss and preferably self resonant just above the two metre band, ie, it must exhibit high impedance.
L1 is most important. C2 must be a high Q trimmer. Best noise figure will be obtained with about 1. The source and gate, two bypass capacitors, must present an impedance which is to all intents zero at MHz. The leads should be as short as possible and soldered as close to G2 and S as possible. If the lead length is such that the capacitor can be removed from the circuit and used again, it is too long. The same applies to the decoupling capacitor at the cold end of the drain coil L2 although these are not as critical.
The effectiveness of the source and gate two bypass capacitors may be tested by running the amplifier while observing the noise figure or by observing the level of a very weak signal and placing the metal end of a screwdriver on S or G2.
If the noise, noise figure or signal changes, the bypass is ineffective. In other words the element should be "dead".
No attempt has been made to match the output for optimum gain. With the circuit shown, the author obtained Attempts to obtain more gain may result in instability. The values of resistors R2, R3, and R4 are not criical. The requirement is to be able to adjust the voltage on G2 to volts. A seperate source of voltage with appropriate adjustment may be used if convenient. If a noise figure meter is available adjust L1 and C2 for best noise figure. Adjust C10 for maximum gain.
Note that the gain is not as important as the noise figure. For example, a typical second stage noise figure may be, say 5 dB, optimistic for most transceivers and you have a preamp for 1 dB noise figure with 20 dB gain.
The overall system noise figure would be 1. If your preamp noise figure now deteriorated to say 1. Thus in this example, 0. Next try adjusting the current by setting R3, thus G2 voltage for best noise figure.
Up to milliamps maximum would not be unreasonable. Readjust L1 and C2 afterwards. If a noise source is not available, set the current to 10 mA, the slug in L1 flush with the top of the former and C2 just in mesh. You could then try readjusting each for best signal to noise ratio using the weakest steady signal available, ie S1 or S2. The frustration caused by this method should make your next project clear. Build a noise source.
Results Obtained The author obtained an indicated noise figure of 0. As these results and settings are almost exactly as predicted by the data sheets, there is no reason to believe that they are not repeatable. Good luck and best of DX. Caution Some earlier BFs were sold in a symmetrical X pack. With these it was difficult to tell which way was up. Parts list S1-S2 Coaxial sockets to suit your system. C1 pF Ceramic or leadless mica capacitor. C2 pF Trimmer - see text. Parallel Combination to make pF.
Say one pF and one pF. See text. C7 pF Ceramic capacitor. C10 pF trimmer. C11 pF Ceramic capacitor. No screening Can is used on the former. Space the turns by winding the winding wire on double and then removing one lot.
Note that F29 is the only suitable slug. L2 4 turns 22 guage air spaced 1 cm long on an 8 mm mandrel. RFC 0. R1 33 ohm.
BF961 / BF981 / BF199 Part # substitutes ???
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. Looking for a "single-side" PCB, which is easier contruction than a double-side PCB, I found after experiments that singularity with opposite GND-tracks connected by a "shield-plate" over T1 is "shot of" unwanted Ground-paths, offering an easy construction without problems and feedback during the operation. Repeating the construction many times, the result was always perfect, without problems. That is very important on a project, especially if that working on VHF area.
N-Channel MOSFET low noise amplifier