Be ready to retell the topic.

Combine words into word combinations and make sentences.

Unit 19

AMPLIFIERS AND SIGNAL CONDITIONERS

Practice reading the following words.

Wheatstone, amplifier, trigger, encoders, oscillator, divider, attenuated, modulating, thermocouples, photovoltaic, simultaneously, opposite, narrowband, further.

2. Answer the following questions after reading the text.

- Name the role of “ideal” and “practical” amplifiers.

- What does receiver require?

- Which sensors reproduce a current?

TEXT A

Signals from sensors do not usually have suitable characteristics for display, recording, transmission, or further processing. For example, they may lack the amplitude, power, level, or bandwidth required, or they may carry superimposed interference that masks the desired information. Signal conditioners, including amplifiers (The ideal amplifier would have any required gain for all signal frequencies. A practical amplifier has a gain that rolls off at high frequency because of parasitic capacitances. In order to reduce noise and reject interference, it is common to add reactive components to reduce the gain for out-of-band frequencies further), adapt sensor signals to the requirements of the receiver (circuit or equipment) to which they are to be connected. The functions to be performed by the signal conditioner derive from the nature of both the signal and the receiver. Commonly, the receiver requires a single-ended, low-frequency (dc) voltage with low output impedance and amplitude range close to its power-supply voltage(s). A typical receiver here is an analog-to-digital converter (ADC). Signals from sensors can be analog or digital. Digital signals come from position encoders, switches, or oscillator-based sensors connected to frequency counters. The amplitude for digital signals must be compatible with logic levels for the digital receiver, and their edges must be fast enough to prevent any false triggering. Large voltages can be attenuated by a voltage divider and slow edges can be accelerated by a Schmitt trigger. Analog sensors are either self-generating or modulating. Self-generating sensors yield a voltage (thermocouples, photovoltaic, and electrochemical sensors) or current (piezo- and pyroelectric sensors) whose bandwidth equals that of the measurand. Modulating sensors yield a variation in resistance, capacitance, self-inductance or mutual inductance, or other electrical quantities. Modulating sensors need to be excited or biased (semiconductor junction-based sensors) in order to provide an output voltage or current. Impedance variation-based sensors are normally placed in voltage dividers, or in Wheatstone bridges (resistive sensors) or ac bridges (resistive and reactance-variation sensors). The bandwidth for signals from modulating sensors equals that of the measured in dc-excited or biased sensors, and is twice that of the measure and in ac-excited sensors (sidebands about the carrier frequency). Capacitive and inductive sensors require an ac excitation, whose frequency must be at least ten times higher than the maximal frequency variation of the measurand. Current signals can be converted into voltage signals by inserting a series resistor into the circuit.

Date: 2016-04-22; view: 885