tailieunhanh - Mechanical Engineer's Reference Book 2011 Part 2

Tham khảo tài liệu 'mechanical engineer's reference book 2011 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Therefore ou _ R vin R i L - 1 C Using the complex conjugate and calculating the modulus of the voltage ratio gives fl2 L 1 oC 2 1 2 2 59 lite phase angle r oL - 1 wC b - tan 1 -------------- The voltage ratio will have a maximum value of unity when the frequency 7777 2-61 V LC Equation defines the resonance- condition at which the inductive and capacitive reactances are equal and selfcancelling. The resonant frequency is usually denoted OJ J and it is the frequency at which the power transferred through the circuit IS maximum. At any other frequency above or below u 0 the power transferred is reduced. The impedance of the circuit is given by z fl j XL - xc At the resonant frequency the total reactance is zero and the circuit behaves as if only the resistive element were present. The general variation of the voltage ratio or amplitude ratio and phase angle with frequency is illustrated in Figure . Also shown in the figure are the two frequencies ojj and OJ at which the amplitude ratio is 3 dB. The 3 dB amplitude ratio is chosen because it corresponds to a halving in the power transmitted. The bandwidth is the frequency range between ỜJ and OJ2. A quality parameter used with respect to resonant circuits is the so-called Q factor which is defined as the ratio of the resonant frequency to the bandwidth. Semiconductors The materials commonly used for semiconductors are germanium and silicon. In recent times silicon has all but replaced germanium as a semiconductor material. These materials have a crystalline structure such that each atom is surrounded by equally spaced neighbours. The basic structure can be visualized as a two-dimensional grid where the node points represent the central nucleus and the inner shell electrons while the connecting lines of the grid represent the four valence electrons associated with each nucleus. This grid concept is adequate to describe an intrinsic or pure semiconductor. At absolute zero temperature the .