A JFET preamplifer for 50 MHz

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Glen Gardner

AA8C





Recently, I began looking for a suitable replacement for the MPF-102 JFET (which is now obsolete and not in production). Eventually I came across the MMBF5458 JFET and decided to test it in a couple of applications that would have been typical for the MPF-102. One of these test builds promised to be useful to Ham Radio operators, so I thought that it might be a nice thing to share with others.

The Transistor

The MMBF5458/2N5458 JFET has a number of properties that make it useful;

1) It has characteristics very close to that of the MPF-105 and MPF-102, and should be usable as a drop-in replacement in most applications with good results.

2) It is symmetrical (drain and source are interchangeable).

3) It is a good small signal amplifier for AF and RF well into VHF frequencies.

4) Low noise (less than 2 nV/√Hz above 100 KHz).

5) It is currently in production and is available in TO-92 through-hole as well as SOT-23 SMT packaging. Some vendors do not stock this device in TO-92 packaging due to low demand.

6) It is low cost (about forty-five cents each).



The Preamplifier

The circuit is simple, designed with a low component count and an inherent simplicity in order to keep the cost of building the preamplifier low, and to assure low noise operation.

Please refer to the schematic shown below.

The JFET (Q1) is operated in a zero-bias configuration having no source or drain resistor. With a +8VDC Vdd the drain current is typically less than 6 mA at idle. The input circuit is a simple L network comprised of C9, L1 and L2, which provides a reasonable transition from a 50 ohm source to the much higher input impedance of the JFET without undue complexity. The Q of the input circuit is modest and allows for up to 4 MHz bandwidth at the -6 dB points after C9 is adjusted properly. The output circuit is a 4:1 impedance ratio step-down RF autotransformer. The transformer (T1) is bifilar wound on a small mix 43 binocular core. The preamplifier provides about 15 dB gain after tuning C9. The on-board 8VDC voltage regulator (U2) provides a clean, regulated source of +8VDC for the preamplifier. A DC blocking diode (D1) is provided to reduce the chance of damage in the unlikely event that the power supply connections are accidentally reversed.



Construction

You want to read everything in this section carefully before building this project.

The preamplifier is build on a double sided printed circuit board with an ample ground plane on the bottom side of the board. All components are surface mount , with the exception of the RF transformer (T1) , the trimmer capacitor (C9), and the SMA connectors (J2, J3). The layout is open with ample room to make easy soldering of components. The use of extremely small SMT components was avoided to make for easy assembly. The board was laid out using the free software provided by Express PCB. The CAD file for the board is available HERE. You can use the Express PCB software to order boards from expresspcb.com. NOTE: The cad file has a layout for two preamplifiers on the board, so when you order boards from Express PCB, you will get the usual three boards, each with two preamplifier boards on it, for a total of six bare preamplifier boards. You will have to cut the boards out with either a shear or a nibbling tool to separate them.

The build goes a little easier if parts are installed in the following order;

1) resistors, capacitors, inductors

2) semiconductors (D1, Q1, U2)

3) the SMA connectors

4) the trimmer capacitor (C9)

5) the RF transformer (T1).

I usually prefer to install aluminum standoffs on the board to protect the board from scuffing and damage during assembly. This is optional and you might have other preferences.

The design makes use of tantalum electrolytic capacitors which ARE polarized. Unfortunately, the silk screening on the board does not indicate polarity for these capacitors. When assembling the board you should be very sure that the banded end of the tantalum capacitors are connect to V+ and not to the grounded pad. If you install a capacitor backwards, it will fail and may cause quite a lot of drama.

Be sure you are always connecting the non-banded end of the tantalum capacitors to a pad that is connected to the ground plane on the circuit board. You can verify this easily with an ohmmeter. You may wish to refer to the picture of the completed board at the top of this page for correct component placement.

The RF transformer is tiny and needs to be made carefully. You need to use either 32 or 36 gauge enameled wire for this. Twist two conductors of the wire together loosely to form a crude transmission line... one turn per inch twist or less, then wind 6 turns of the bifilar pair onto the core. If you twist the wires too tightly the transformer will not work very well. Once the transformer is correctly wound, connect the two wires as shown in the schematic, making sure you get the phasing right.

Once you have the transformer wound you will have four wires coming from it. Connect opposite ends of the two conductors together such that both conductors are wired in series and are correctly phased. The connected pair goes into the center hole in the circuit board at T1. The other two wires that make up the free ends of the transformer go into the two adjacent holes. Keep the leads coming from T1 short, and dress the core of the transformer down onto the board to keep things neat and to reduce any tendency to pickup stray noise. You may wish to refer to the picture of the board at the top of this page for correct placement of T1.

This is simple, it is just an RF autotransformer with a 2:1 turns ratio. If you find this difficult refer to the ARRL handbook for info on how to make an RF autotranformer.

Getting Parts

Nearly all of the parts are available from Digi-Key electronics. The one exception is the ferrite core for the RF transformer, which is available from Amidon Associates.

The bill of Materials, with part numbers, is HERE.

As far as substituting parts goes, you are on your own with this. Do something different, and you can expect different results.



Don't fear the Raptor

People who are new to working with surface mount components might find this project to be a little intimidating at first. But the components chosen were chosen to be a little larger than absolutely needed in order to make it a bit easier to build. If you are new to working with surface mount electronics you might want to enlist the help of someone who is experienced with it in order to start out on the right track. Working with SMT is easy and this project is readily assembled with an iron, a pair of tweezers, and a little patience. Most experienced SMT technicians can assemble this board in a few minutes.



Caveats

The preamplifier has no output filtering and while the input circuitry offers some selectivity, the purpose of the input circuit is more about impedance matching than rejecting out of band signals. Since the useful dynamic range and noise figure of this preamplifier are expected to be reasonably good, a suitable bandpass filter could be added to the output (not on the input) to satisfy those who wish to have strong out-of-band signal rejection without spoiling the noise figure of the preamplifier.

Don't let the magic smoke out! This preamplifier is operating with zero bias and has no drain or source resistors to limit current. Also, the on-board voltage regulator will readily supply100 mA current (that comes out to 0.8 watt!) to the JFET before it starts to limit. This makes for good linearity and really good dynamic range, but it also means that the preamp will happily continue to amplify signals until either current demand exceeds 100 mA, or until an output of nearly 4V p-p is reached at the output connector (8V p-p at the drain). Since the JFET has a maximum allowable power dissipation of 350 mW, it is possible to drive the preamplifier to self-destruction long before it begins to distort the signal. Be sure you avoid doing this and all will be good.

Adjust the variable capacitor (C9) using a nonmetallic tool for best signal to noise ratio. Do not simply adjust C9 for maximum noise.

A 10M Option ?

The preamplifier can be made to work well on the 28 MHz band if you do the following:

1) install an 18 pF capacitor in parallel with the variable capacitor (C9). You can place this on the bottom of the board.

2) Replace T1 with a 2:1 transformer consisting of 12 turns bifilar wound on a small mix 77 ferrite core.



73

Glen

AA8C