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Okay there are literally a thousand ways to make an oscillator. There are RC Oscillators, LC Oscillators, Oscillators that don't use any passive component, but use the resonant frequency of a cavity, negative resistance Oscillators, ring Oscillators, and that's just for sinewave, that can be rearranged at many many ways. I chose to represent only the 3 basic transistor configurations, and how they can be used as LC sinewave Oscillators, and I'll add a few words on the specifics of each topology.
- The first one is a common base based oscillator. That means the transistor should be working very near the cutoff region, and the signal is being fed to the emitter, while the base is AC blocked. For this topology the usual specifics are low input impedance, medium high output impedance, very high voltage gain, maybe some current gain, if it's constructed to have it, and very broad bandwidth, that makes it suitable for high frequency applications.
- The second topology is based around a common emitter configuration. For such Topologies, high input impedance, low to medium high output impedance, high voltage and current gain, and mediocre bandwidth properties are to be expected.
- The third type is a common collector e.g. emitter follower based oscillator. This one is a little different than the rest because the transistor in this configuration has no voltage gain, just some current gain, ranging from very high, to medium high, but it almost never has voltage gain, due to emitter degeneration (sometimes this rule can be bent). The current gain is directly proportional to the hFe of the transistor. The specifics are high input impedance and low output impedance. The other specificity is that it doesn't have any phase shift. In the precious Oscillators there was always a need to shift the phase with another 180degrees to add it to the 180dgs shift of the collector and make a full 360dgs positive feedback. Here you needn't shift the phase, because it already coincides with the output, you just need some voltage gain. You can get that by using a step up transformer. The slightest boost will be enough, no need for advanced phase shift circuitry, just a little voltage boost.
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