High Performance Tunable
This is a further progression in the application of the J310 JFET in high performance preamplifiers. This one is designed to amplify the 20KHz to >30 MHz range, uses the premium J310 FET and 2N5109 CATV bipolar transistors, giving a good combination of low thermal noise, gain and IMD resistance.
This pre-amp departs from previous units that I've designed and built in that, rather than a series tuned circuit to match the antenna to the FET, it uses a parallel tuned circuit with a tap for the antenna connection. This overcomes a "Q" problem I found the series circuit at HF frequencies. That precluded good selectivity when using it as a Shortwave pre-amp. This Parallel tuned circuit works very well from <100KHz through 30 MHz (the range I've tested it over). With suitable coils and careful construction it should perform well from <20KHz to over 40MHz.
A side benefit of this change in tuned circuits is that the unit seems to be far less susceptible to IMD products in the LF range from strong Broadcast Band stations. All previous amplifier designs required EITHER a 510KHz Low Pass Filter OR a BCB band reject filter be installed between the antenna and the preamp to eliminate IMD. I did build a switchable BCB reject filter into this pre-amp but have found that it isn't required in normal operation. (but it's there in case I should need it). I suspect this is because the antenna is INDUCTIVELY coupled into the parallel tuned circuit, while a series tuned circuit can behave as a relatively inefficient 2 pole low pass filter. In any event, I'm very pleased with the overall performance of the Pre-amp.
Attenuation: If you examine the schematic you will notice that there appears to be TWO attenuators in the circuit, and the variable gain resistor is in an "unusual" position for a RF preamp. The reasoning is very simple. While I don't have the test gear to test absolute gain, the prototype proved to have more gain than I felt any reasonable need for, right up through 30MHz.
So the two following attenuators were added.
1. 50 Ohm, 6dB pad at the output. This to facilitate use with old (boatanchor) receivers, many of which have a lot of capacitive or inductive reactance at the antenna connection. This may occasionally cause a pre-amp to become unstable and oscillate. This unit appears to be unconditionally stable, but the attenuator insures a good 50 Ohm load to the last amplifier stage. And to add insult to injury, I built my pre-amp in my Multicoupler cabinet (which is ALWAYS active). The 6dB gain of the multicoupler would overcome the attenuator's loss anyway.
2. Resistive variable attenuation between the first and second stages. This allows optimization of the gain to just enough for the desired application. By putting it between the two stages, lowering the gain doesn't effect the input tuned circuit "Q" (selectivity) OR the noise figure of the first stage. Often a "gain" control is placed at the OUTPUT of the last stage. This may reduce overload of the following receiver, but it also opens the door to overload of the second preamp stage in the presence of strong signals. Putting the gain control between the two stages eliminates both problems (this control can be left out if you want MAXIMUM gain all the time). IF you mount the gain control on the front panel of the pre-amp as I did, you should use small, shielded coax cable between the PC board and the control to eliminate any instability from feedback.
A FEW CONSTRUCTION NOTES:
1. The output transistor bias is adjustable via R9. To properly adjust it, adjust the bias control so that the collector (case) of the transistor has EXACTLY HALF as much Voltage as the junction of resistors R16 & R11 has. I.E., if there is 12.6VDC at the junction, adjust the bias for 6.3VDC on the case of the transistor.
2. In normal operation the transistor runs quite warm and "should" have a small, finned heatsink attached. (remember, the case is at collector potential, DON'T ground it).
3. Diode D1 isn't essential, it is there for reverse polarity protection of the transistors. Diode type isn't critical either. Any small rectifier type diode will suffice.
4. Unit should be mounted in a shielded metal box, and you can use any type of RF connectors you desire. (BNC, SO-239, RCA etc). I used BNC connectors because of their high quality and ease of installing (single hole mount).
5. Resistors R4 and R10 are there for RF stability and should be mounted right AT the base of the transistor (short leads).
6. Resistor R12 is there for temperature stability and to provide some degenerative feedback, DON'T bypass it for more gain.
7. Resistor R11 actually consists of TWO 390 Ohm, 1/2 Watt resistors in parallel. They run warm, don't use smaller Wattages (you may substitute single 220 Ohm, 1 Watt resistor).
SOME NOTES ABOUT COILS:
IF you are able to obtain them, high quality Pot core coils are by far the best, toroids are second best with conventional slug tuned coils being the "easier" choice for many builders. I chose conventional coils as I had a "complete set" covering 100KHz through 45MHz from a scrapped antique "Genometer" signal generator that had smoked it's power transformer. IF you decide to do like I did and use the coils from an old generator or whatever, keep the coil taps in mind. The optimum tap point is at approximately 25-33% of turns from the ground end. Generators that used a "Hartley" oscillator are good candidates for donor coils. Some generators like many Heathkit units were actually tapped at the 50% point. They'll work, but you will have less selectivity due to antenna loading.
For your convenience, a table of Coil Values is included at the end of the Parts List. This will make it easy for you to select the electrical value of a coil for whatever frequency range you wish to cover. If you want to use switchable coils, you will need a rotary switch with however many positions you will need. EXAMPLE: My unit tunes <100KHz through >30MHz in 6 ranges and uses a 6P2P (6 Position, 2 Pole) switch to select the 6 coils. Radio Shack does/did sell a 6P2P switch that is suitable.
Melt Some Solder!