21.20(a) is usually called the Pierce crystal. The ensuing circuit frequency of oscillations is about with the collection resonant frequency of the crystal. Versions in source voltage, transistor parameters, and so forth. don't have any impact on the circuit operating frequency which happens to be held stabilized by the crystal. https://electronics-crystal-oscil60371.dm-blog.com/34984734/5-simple-techniques-for-consumer-electronics-crystal-oscillators