RF Preamplifier

W1FB 6M RF Preamp

Discussion

VE7GC Popcorn RF Preamp

Discussion

Here is an easy RF preamp by Dick Pattinson, VE7GC. It uses a single tuned circuit at the front end and can connect directly to a mixer or product detector in a simple receiver project. Note how Dick provided adjustable RF gain control for this circuit in his Wee Willy project on this website.
If you can not find Tak Lee green 10.7 MHz IF coils, probably any other brand of 10.7 MHz slug tuned IF transformer would work. The Mouser catalog number is 421F123 . If your 10.7 MHz IF coil has a built in capacitor at the base , remove it. A fixed inductor may also be wound using a powdered iron toroid core and then all or a portion of the C1 capacity would be made variable.
The input impedance is 50 ohms and the output impedance is low due to the Q2 emitter follower stage.

A Low Noise, High Dynamic Range Broadband RF Amp

The above schematic is a version of a circuit developed and patented by David Norton and Allen Podell in June 1974. This variation was described by Joe Reisert, W1JR in the now defunct Ham Radio Magazine. The Norton design uses transformer coupling to achieve "noiseless negative feedback" and is really outstanding. A great article utilizing and augmenting on this technique receivers is by Jacob Makhinson, N6NWP in QST magazine for Feb 1993 with "A High Dynamic Range MF/HF Receiver Front End". Makhinson arranged 2 in push-pull to obtain excellent results. Obtain a back-issue of QST for closer study. If you are building a contest-grade receiver and need a good RF preamp and/or post mixer amplifier, the Norton type is quite suitable. An amp built using a 2N5109 can have a noise figure in the 2.5 - 3dB range. I have also built them with 2N3866, MRF517, MRF581 and a 2N5179 although the last transistor would be a somewhat poorer choice. This schematic with a 2N5109 is good from 1.8 to 150 MHz with a 1.2:1 VSWR or less according to Joe Reisert. I have even put one in a friends CB radio and he was delighted.

Winding and Construction Hints

Making the Norton amps requires some planning to keep all component leads as short as possible. The transistor leads and any connecting components should be trimmed as short as practical to promote stability. Sketch the component layout on a piece of paper and modify it until you are satisfied you have designed a good layout. I usually use a terminal strip piece to support the transformer and to attached the positive voltage wire to. The ferrite beads on the transistor collector aid in stability and should be used to preserve the noise figure by squashing any oscillations should they develop. The 22 uH choke can be the little epoxy coated units that are color coded and look somewhat like resistors. Do not use a choke less than 22 uH.
Before winding, the builder must first decide how much gain is needed from the amp. For an RF preamp, the stage should have gain equal to or greater than the passive stages after it. Also there will be losses in the transformer, so the theoretical gain of the Norton amp maybe 1 dB off and will need to be factored in.
For the purposes of discussion, a 9.5 dB amp is desired , so N = 5 and M = 3. The first step is to mark one side of the core with a dab of liquid paper, paint or a small piece of tape. This will allow you to keep track of the transformer later. To mark, hold the core so that both channels are parallel to the floor, one on top of the other. Apply your dab of paint to the top of the core and use the marked top to denote the A windings. 1a, Ma and Na will all start from the top channel in the balun core.
Using 32 AWG wire for all three windings, start with winding 1 and wind the single turn from point 1a to 1b. Cut off the leads so thay are shorter than 2 inches.
Next, wind Ma to Mb three complete turns through the binocular core and trim the leads if needed. Tie a small knot in the wire at both ends. This will clearly mark this M winding. Both windings should look like the diagram under the schematic. 1a to 1b are on the left of the balun core and winding Ma to Mb are on the right side of the core. Mb has a distingishing knot at the tip of both wire ends. Ma starts from the top of the core which you have marked with a dab of paint or something.
Finally, wind Na to Nb five complete turns through the core in the same direction as the previous winding M.
Strip wires Na and Mb (Mb has the knot) , twist together and solder.
Strip the leads VERY gently with sandpaper or your favorite other method. Insert the transformer in your circuit and cut the leads to their proper length and then solder away. It maybe preferable to prestrip the leads on winding 1 as it is hard to strip the enamel off a fine wire that has only one turn and it may accidentally pull out of the core. If it does, just re-insert it into the balun core on the correct side. Once you have soldered Na and Mb you can always identify the windings later because you have marked the top of the balun core which denotes the A windings. Try and make your windings gently tight as if there is too much slack you may have difficulty getting the last few windings thru the core channels. A 14 dB gain amp maybe impossible to wind with 32 AWG wire, it may best to use 34 AWG for that amplifier. I have never built one for greater than 12 dB. The transformers are a bit tedious to wind, however persevere and the results will be well worth it. For HF, you can substitute 0.1 uF caps for the 0.01 caps shown if you like.

Experimental Norton Amp

The amp shown in the above schematic experimentally uses a ferrite toroid for the transformer and has ~10 dB gain. Incidently, it can also be wound on a balun core using 2 windings, a one turn loop and then a second wire with a tap as shown in the above schematic. A balun core would not be experimental and this amp is very stable with a balun core.
To wind the toroidal version of this amp, wind 13 turns of AWG #26 on a FT37-43 core. Next wind the 14th loop but leave a generous loop for tapping into. Then wind 3 more close turns on the coil to finish. You should have 17 turns of wire with a tap 4 from the end, thus creating the 13 - 4 inductor as shown in the schematic. To complete the coil, wind 1 turn of wire over the cold end as shown in the schematic. It is tricky, but try to keep the one turn link as short as possible. A ferrite bead over the transistor collector is also helpful, but not mandatory. You can try increasing the turns (1:21:5 etc ) to experimentally obtain more gain from this amp. The toroid version of this amp is good for understanding how the Norton amp works and maybe an option for builders who do not have balun core ferrites in their junkbox.