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Bipolar Junction Transistors 2

2016-05-07 19:47  
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Bipolar Junction Transistors 2
Experiments were conducted on this circuit and the minimum and maximum frequency and corresponding measured DC voltage on the BJT emitters for each was recorded. In Figure 4 as shown, the lower frequency/DC voltage was 7.054 MHz and 1.08 volts and the upper frequency/voltage was 7.062 MHz and 6.52 volts. In the red insets are a schematic showing a four BJT version of this circuit and a plot of the reverse voltage versus frequency of the Figure 4 VFO without the series 22K and grounding 10K resistor ( This accounts for the frequency differences between the plot and the final VFO ). As shown, the best tuning linearity is above 3 volts, however, that would limit the tuning range too much and I chose components to give a voltage swing from 1.08 volts to 6.52 volts. With this particular arrangement the maximum voltage was limited with the 9.1 volt zener and series 22K resistor as the zener (breakdown) voltage on the EB junction of a 2N3904 is close to 8 or 9 V and you never want to lose the reverse bias and go into breakdown.