Telecommunication Circuits and Technology pptx

[...]... feedback is used in amplifier circuits in order to give stability and reduced gain Bandwidth is generally increased, noise reduced and input and output resistances altered These are all desirable parameters for an amplifier, but if the feedback is overdone then the amplifier becomes unstable and will produce a ringing effect In order to understand stability, instability and its causes must be considered... in the other as the magnetic and electric fields of the coil and capacitor interchange their energies A constant frequency is therefore generated The RC network is a time-constant network and as such responds to the charge and discharge times of a capacitor The frequency of this network is determined by the values of R and C The capacitor and resistor cause phase shift and produce positive feedback... Splitting a single coil causes similar effects and mutual inductance exists between the two parts As can be seen from equations (1.14), (1.15) and (1.16), the gain and frequency are dependent on the mutual inductance, and these parameters may be difficult to achieve as the tapping point has to be precise Two practical circuits are shown in Fig 1.15 In both circuits the frequency is given by 1 (1.14) f=... oscillation A= C1 C2 (1.13) In practice, A > C1/C2 for start up conditions Two practical circuits are shown in Fig 1.13 Input and output resistances have an effect on the Q factor and hence the stability of these circuits Figure 1.13(a) has the input resistance (hie) of the transistor in parallel with the tuned load and this will reduce the Q factor substantially Some further points should be noted concerning... responses of the low and high-pass filters will pass all other frequencies in their flat passbands This type of oscillator provides good frequency stability due to the notch filter effect There are two feedback paths, the negative feedback path of the twin-T network and the positive feedback path caused by the voltage divider R5 and R4 One of the T-networks is low-pass (R, 2C) and the other is high-pass... power-handling capabilities The high-frequency part of the spectrum generally uses the 23 24 Oscillators curvature of the earth for transmission and this requires high-power circuits at the transmission end However, because of the transformer size and cost, it is not as common as the other oscillators discussed in this chapter 1.6 Crystal oscillators These are amongst the most stable of all oscillators and. .. vibrations occur in the X direction and in the Y direction The vibration frequency in each direction is determined by the dimensions of the crystal in that direction and is dependent on the width and thickness of the slice in that direction Hence the terms width vibration and thickness vibration are used The frequency temperature coefficient is the same for both of these vibrations and the crystal can be made... mechanical energy is changed to electrical energy The crystal has a large Q factor and this means that it is highly selective and stable The amplifying device may be a bipolar transistor, a field-effect transistor (FET) or operational amplifier This block is responsible for maintaining amplitude and frequency stability and the correct d.c bias conditions must apply, as in any simple discrete amplifier,... constitute the Barkhausen criterion and an oscillating amplifier selfadjusts to meet them The gain must initially provide βAV > 1 with a switching surge at the input to start operation An output voltage resulting from this input pulse propagates back to the input and appears as an amplified output The process repeats at greater amplitude and as the signal reaches saturation and cut-off the average gain is... commercially and is a highly accurate instrument containing a powerful processor capable of determining resistance, capacitance, self-inductance and mutual inductance values It can also select batches of components having exactly the same value, which is useful in such circuits as the Wien bridge oscillator where similar component values are used The twin-T oscillator This oscillator is shown in Fig 1.10(a) and . alt="" Telecommunication Circuits and Technology Telecommunication Circuits and Technology Andrew Leven BSc (Hons), MSc, CEng, MIEE, MIP OXFORD AUCKLAND. is used in amplifier circuits in order to give stability and reduced gain. Bandwidth is generally increased, noise reduced and input and output resistances