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Bipolar Junction Transistors

2016-05-08 00:11  
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Bipolar Junction Transistors
VVC diodes may not be in your junk box. For popcorn VFOs, if you really want to save money on a parts order, bipolar transistors such as the 2N3904 may be used as voltage variable capacitors for a popcorn receiver project. Only two leads on the transistor are ever used and the redundant lead maybe cut off. Voltage variable capacitors using one, two or four BJTs may be used depending on the tuning range and popcorn factor you desire. It is obvious that the Q of a BJT tuning diode will not be as high as those seen in better VVC diodes, but they are practical alternatives for very low-cost, simple receiver projects. I would probably not use a BJT-tuned VFO to tune a transmitter project as the potential for drift is high, however reasonable stability is possible with careful design and a bit of luck. Consider Figure 3. For brevity sake, the VFO buffer stages will not be shown any more but were used in all of the Figures to follow. A single 2N3904 is used as the variable tuning capacitor in this experiment. Note that there are two voltage regulators used on this schematic. If you were really going frugal, only one could also be used. The voltage regulator on the nJFET of the Hartley oscillator is used to lower its operating voltage to minimize the effects of internal heating on this device which can cause drift. Also keeping the RF amplitude down in a PN-junction turned circuit is probably a good idea as large AC voltage swings may cause the PN junction of our tuning device to conduct and reduce Q and waveform purity. You can always build up the oscillator output voltage with buffer/amplifiers. The second zener diode voltage regulator is used on the voltage control circuit of the tuning circuit. Unregulated voltage for the tuning device whether it is a transistor or diode is not recommended as fluctuations in DC supply voltage will change the frequency of the VFO. Additionally, your DC tuning voltage must be free of any ripple, hum or noise riding along the DC voltage. AC noise if present, can sweep the oscillator frequency at a rate consistent with the ripple frequency. Clean DC power may be achieved by decoupling your VFO power supply from RF with RF chokes, resistors and capacitors. It is also good practice to ensure that correct polarity to your VFO circuit is present at all times. You can not eliminate the series 100 pF NP0 ceramic capacitor connecting the 2N3904 to the tank circuit or the VFO will not oscillate. You can however, experiment with this capacitor value. Figure 3 contains the minimum tuning voltage and frequency measured in the test VFO. The capacitance of the 2N3904 will be at its maximum level at the minimum control voltage. The frequency and voltage with the pot turned all the way to the left is 7.036 MHz and 5.82 volts respectively. The minimum control voltage is set by the 47K resistor connecting the 50K pot to ground. Raising this value will raise the minimum control voltage and lowering or eliminating this resistor will lower the control voltage. When the 47K resistor was removed, the measured voltage was 3.42 volts which gave a minimum frequency of 6.977 MHz. From Figure 2 we learned that the minimum voltage should be 7 volts for maximum linearity in the VFO tuning response. I decided to accept 5.82 volts as a trade off to illustrate that in many cases you may compromise the minimum tuning voltage to suit the standard value resistors you have on hand or to attain a bigger tuning range for your VFO. In many design scenarios, compromises may be made to suit your needs and parts collections! The maximum control voltage is largely set by the 11.7 volt zener diode. If you wished to lower this value series resistors or a voltage divider could be added as well. The 11.7 volt zener was chosen as I may want to use this VFO with a 12 volt battery and this zener diode would facilitate such operation. The maximum control voltage and frequency with the pot turned all the way to the right is 7.053 MHz and 11.88 volts. The 2N3904's capacitance would be at its minimum value for this circuit at 11.88 volts. The Figure 3 VFO tuning range is 17 KHz, with the control voltage going from 5.82 to 11.88 volts. The stability of this VFO is fair. I have a commercial kit receiver that has worse drift, but again I would hesitate to use this VFO design for a transmitter. One of the drawbacks of using just a single tuning diode or BJT is that if the device is forward biased by the RF signal during part of the AC cycle, it's reverse leakage will increase momentarily and it's Q will be reduced. Also, harmonic energy is produced as the tuning diode or BJT is alternately biased positive and negative which results in reduced VFO output waveform linearity. The solution is to connect two tuning diodes or BJTs back to back with the reverse DC voltage applied to both devices simultaneously. The two tuning diodes will be driven alternately into high and low capacitance and the net capacitance will remain constant and not be affected by the AC signal amplitude. The circuit in Figure 3 could be improved by adding another 2N3904 back-to-back with the existing one, but the tuning range will be reduced. This occurs because you have now connected two capacitors in series and the total capacitance has been reduced and may be analyzed by the classic capacitors in series equation we all learned in radio school.