RF Electronics Chapter 6: Oscillators Page 184
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The gain of the common emitter amplifiers of figures 6.17 and 6.18 are stabilised by using
a tuneable gain constant Kg and including a resistor Rg = Zr/Kg in the circuits. The
amplifier gain is then close to -Kg at resonance. The resonators themselves can have gain,
depending on the tapping ratio. The tapping constants Khe and Kht include the gain
constant Kg as shown in the equations in figure 6.16. Changing the amplifier gain by
changing Kg, will then also change the resonator tapping-ratio, to keep the step 4 linear
oscillator gain, shown in figure 6.9, nearly constant. Figure 6.16 shows that to obtain an
oscillating frequency of 100 MHz, the designed resonator frequency for the non-coupled
coils is 107.14 MHz and for the coupled coils is 88.66 MHz. The coupling will thus cause
a significant frequency shift that must be corrected.
A common collector transistor configuration can be used, similar to figure 6.10 with the
resonating inductor Lc replaced by a capacitor and the capacitors Cr1 and Cr2 replaced
by inductors. The resulting Hartley oscillator circuit is shown in figure 6.19. To facilitate
a comparison between the common collector Colpitts and the Hartley oscillators, the bias
voltages for the Hartley oscillator in figures 6.19, is similar to those for the Colpitts
oscillator in figure 6.10.
Figure 6.19. Hartley CC (Common Collector) oscillator with non-coupled inductors.
Figure 6.20. Output spectrum of Hartley oscillators.
RF Electronics: Design and Simulation
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