Field Effect transistors can be classified between the enhancement mode and

Tag : inductance
In the November 1995 issue of Amateur Radio, I discussed negativeresistance and oscillator circuits which made use of the negative resistance characteristic. One typeof circuit discussed used the tunnel diode and, as a furtherapplication of this diode, many radio amateurs will remember thetunnel diode dip meter kit available in past years from the HeathCompany.
These days the tunnel diode is a scarce item, probably unobtainablefrom the normal electronic suppliers. An alternative solid statenegative resistance circuit can be achieved by interconnecting an Nchannel junction field effect transistor (JFET) with a P channelJFET. This has been called the Lambda circuit because itscharacteristic curve looks something like the Greek upper caselambda (an upside down V) .
There have been various dip meter circuits published in radioamateur handbooks and in past issues of Amateur Radio. However, wehaven't had one in Amateur Radio for some time and I thought Iwould introduce one around the concept of the lambda circuit. Anadvantage in using the negative resistance type of circuit,compared to one such as the Hartley, is that two terminal plug-incoils can he used.
I will first discuss the operation of the lambda circuit, then leadup to how an arrangement for the dip meter was devised.
As assembled, and using a range of six plug-in coils, the dip meteroperates over a frequency range of 1.6 to 150 MHz. It can also beswitched to operate as an absorption meter.
Lambda Negative Resistance Circuit
Field Effect transistors can be classified between those whichoperate in the enhancement mode and those which operate in thedepletion mode.
Enhancement mode means that the FET must be biased on to set theoperating point for use as an amplifier (much like biasing abipolar transistor). Depletion mode means that it must be reversebiased, or biased off, to set the operating point (as in a valveamplifier). The Junction FET or JFET operates in the depletion modeand to reverse bias a JFET stage, it is only necessary to insert anappropriate value of resistance in series with the source electrode(much like cathode bias in a valve stage).
Fig 1 shows a P channel JFET and an N channel JFET, each with asource bias resistor Rs. The only difference between the twocircuits is the polarity in connecting to the supply rail. Voltageis developed across Rs and applied across the gate-to-sourcejunction in reverse or depletion polarity. Due to the reversefeedback, the drain current is stabilised at a value determined bythe value of Rs.

Now, instead of Rs in the P channel JFET, let's replace it with thesource and drain of the N channel JFET and instead of Rs in Nchannel JFET, let's replace it with the source and drain of the Pchannel JFET. We now get the circuit of Fig 2, and this is ourlambda circuit. Connected in this way, the two transistors interactwith each other and produce an interesting characteristic.
The curve of Fig 3 shows the drain current versus drain-to-sourcevoltage, which I plotted for an N channel MPFI02 transistor and a Pchannel 2N4342 transistor connected in the lambda circuit. Up topoint A, the drain current increases as the voltage is increased.Beyond point A, the current then decreases, with further voltageincrease creating a negative slope and the negative resistanceregion A-C . The amount of resistance can be scaled off by takingthe ratio of voltage change to current change along curve A-C andthis is a value of around minus 1700 ohms.


To make a negative resistance oscillator, we simply connect a tunedcircuit in series with the lambda circuit, and the drain-to-sourcesupply and set the supply voltage at, say, point B, around fourvolts. Provided the parallel resistance of the tuned circuit atresonance is somewhat greater than 1700 ohms, the circuit willoscillate and the oscillator circuit formed becomes the basis forour dip meter.
In principle, it is similar to the tunnel diode dip meter, butdifferent because it requires around four volts as compared to thetunnel diode voltage of somewhat less than one. For more detail onthe theory of negative resistance oscillators, I refer you to myarticle in November 1995 Amateur Radio.
Dip Meter Circuit
The circuit of the dip meter is shown in Fig 4. In my circuit Ihave used an N channel MPF102 (V1) and a P channel 2N4342 (V2). Iwas hoping to make my unit work well up into the VHF region, andthere were a number of readily available and suitable N channelJFET transistors which could have been used. I selected the MPFI02because I happened to have these. P channel JFETs seem to be morescarce and the only one I could find in the catalogues of the usualretail outlets was the 2N4342.
NOTES: SK1 and PL1 are RCA type concentric plug and socket. The trimpot RV1 is set to 4V.

Figure 4 Dip Meter - Circuit Diagram


I was a bit dubious about using the 2N4342 as it was shown in mydata sheets as a general purpose transistor and there was nothingto indicate how it might perform at high frequencies. With littleelse to choose, I bought some of these and gave them a try in thelambda oscillator circuit. As it turned out, I was able to make thecircuit work at frequencies as high as 200 MHz.
To cover the tuning range in conjunction with plug in coils, a 100pF variable capacitor (C2) is used. The only limitation is that onthe top VHF band, the maximum setting of this capacitance must belimited to about 45 pF. At these frequencies, the circuit will stoposcillation if too much capacitance is used.
The Dip meter operates from a 9 V battery and the supply to thelambda circuit is stabilised by 5.1 V zener diode ZD1. To set thecorrect operating point, the lambda circuit supply is adjusted to 4V with trimpot RVI. A switch, SW1, can be used to disconnect V1 -V2so that the unit can operate in an absorption mode. Components C1,SW1, V 1, V2, and C2 are all part of the oscillator circuit and asit operates up to VHF, the lead lengths to these components must beshort and earthing carefully commoned. Interconnecting the MPFI02and 2N4342 works out quite well. Turn one 180 degrees to the otherand the three leads on one connect directly across to the three onthe other.
The idea of the dip meter is as follows: When the Dip Meteroscillator coil is placed near another tuned circuit, that circuitabsorbs some of the energy from the oscillator. This causes a dipin oscillation level when resonance of the other circuit is found.Monitoring of the DC load current to detect this dip is often usedwith class C oscillators. However, the lambda oscillator worksessentially in a class A mode and its load current does not varygreatly with change in level of oscillation.
To detect a dip in the oscillation level, the output voltage acrossthe tuned circuit is monitored using a detector circuit whichconverts the RF voltage to a direct current to actuate a micro-ampmeter or milli-amp meter. To prevent the detector loading the tunedcircuit, it is coupled via a source follower stage V3, anotherMPF102 FET.
Two detector circuits are shown. If a 50 or 100 micro-amp meter isavailable, circuit A does the job. For a 1 or 2 milliamp meter, anadditional current amplifier, V4, is needed and circuit B is used.In each case, the signal is rectified and filtered by voltagedoubler C5, DI, D2, C6 and the following load resistance. For V4,almost any small signal silicon bipolar NPN transistor is suitable.
In explanation of one part of circuit B, the voltage developedacross diode D3 forward biases the base of V4 to compensate for theresidual voltage step set by its base-emitter junction. RV2 adjuststhe sensitivity of the meter circuit so that it can he set at asuitable reading level.
Load current from the 9 V battery is approximately 14 mA.
Dip
As pointed out earlier, the oscillator will work provided the shuntresistance of the resonant tuned circuit is somewhat greater thanthe negative resistance value of 600 ohms. Tuned circuits of evenquite low Q factor have a shunt resistance of much higher than thisand, hence, almost any practical inductor can produce oscillation.In effect, the feedback is greater than need be but the AC voltagedeveloped is controlled because the voltage swing is limited by theextremities A and C on the curve in Fig 3.
As an oscillator source this is good, but it is not so good iflooking for a dip in output level when energy is absorbed by acircuit being measured. With so much feedback, the circuit is stillable to deliver the full signal swing when energy is absorbed and,hence there is little dip to be seen.
To produce a good dip, the shunt resistance of the tuned circuit islowered to a point where the circuit just oscillates nicely and alittle above the value which would stop oscillation. To achievethis condition, a resistor is shunted across the tuned circuit. Asthe optimum value of the resistor was found to be different foreach band, the appropriate resistor for each coil is fitted at itsbase as part of its plug-in module. The selected value varies from1.6 k.ohm to 4.7 k.ohm and no resistance at all for the top VHFband.
Coils

In making up the coils, I was influenced by the style ofconstruction used in the Heathkit meter. A long length of smalldiameter tube of some form of insulating material is used. At oneend, a concentric RCA type plug is fitted which mates with an RCAsocket mounted on the dip meter case. At the other end, the coil iswound and the coil leads are wired back to the plug. The long thinform of module makes it convenient to poke in close to the coil tobe dip tested.
Finding a source of supply of small diameter insulating tube seemedto be a problem. Eventually I found a source of 0.5 inch (12.7 mm)diameter polystyrene tube at a local hobby train shop and this wasideal for the job. For the four coils which covered 1.6 to 34 MHz,I cut the tubing to a 57 mm length. For the 32 to 85 MHz coil,where the length of the wire from coil to plug was an accountablepart of the total inductance, I reduced the tube length to 46 mm.For the highest VHF band, there is no former and the inductor isjust a wire loop.
Coils are wound on the tube with the active end 3 mm from the tubeend. The winding detail is shown in Chart 1. The ends of the coilsare passed to the inside of the tube through holes drilled in thetube. A small amount of fast Araldite (trade name of Ciba-GeigyCorp) fixes the end turns in place.
I stress the small amount as I tried smothering one in Araldite andfound that the stray capacitance increased sufficiently to reducethe tunable range.