Analog Electronic Circuit
eBook - ePub

Analog Electronic Circuit

Beijia Ning, Beijia Ning

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eBook - ePub

Analog Electronic Circuit

Beijia Ning, Beijia Ning

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A comprehensive collection of fundamental principles and applications of analog electronic circuits, including semiconductor diodes, bipolar junction transistors, field-effect transistors (FETs), operational amplifiers, power amplifiers, and feedback circuits. With abundant practical examples, it is an essential reference for researchers, students and engineers in electronical engineering and information processing.

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1The analog world and semiconductors


1.1.1Definition of analog signals

An analog (or analogue) electric signal is defined as a time-continuous electric signal for which the time-varying feature (amplitude) is a representation of some other time-varying quantity. In other words, the amplitude of analog electric signal looks like other time-varying phenomena. For example, the air temperature in some place can be measured with a thermometer for 24 h and recorded as shown in Fig. 1.1 (a). It can be seen that the curve is time continuous and the amplitude ranges between roughly 19 and 30 °C. On the other hand, the air temperature can also be measured by a temperature sensor and recorded as its output voltages in the form of an analog electric signal, as shown in Fig. 1.1 (b). It is obvious that the instantaneous voltage of the signal changes continuously with the temperature of the surrounding air. So, the shape of the voltage curve looks like that of the temperature. Moreover, the voltage can be used to represent the temperature [1].
An analog signal is a measured response to changes in physical phenomena, such as force, energy, torque, pressure, sound, light, temperature, position and motion. Normally, an analog signal is achieved using a transducer or a sensor. Analog electric circuits are those that generate, receive and process analog signals with general electric components, such as resistors, inductors and capacitors, or analog devices, such as diodes and transistors. Normally, a practical electric circuit contains both analog and digital parts.
Fig. 1.1: Air temperatures and their measurements, (a) Temperature record during 24 hours, (b) Sensor output voltage during 24 hours.

1.1.2The existence of analog signals

In recent years, there has inevitably been a “digitization” trend, that is, to perform signal processing in digital domains due to the convenience of device use and flexibility of design. The influence of analog circuit analysis and design appears to have diminished. However, this is an illusion. More and more signal processing can be done digitally, but analog circuits will not disappear [2]. The main reason for this is detailed below.
The physical world is an analog place and the analog circuit is the first step for humans to interact with the world. For example, in the real world, as shown in Fig. 1.2, when we speak, the sound wave is a continuous vibration of air. The analog electric signal is first generated by a sensor from the sound wave. Here, the sensor is a microphone. Then, the analog signal is fed into an analog-to-digital converter (A/D) to generate a digital signal. Actually, a digital signal is a sequence of 1 s and 0 s, a numerical representation of the analog signal. It may be easier and more cost efficient to process signals in the digital world. This is called digital signal processing (DSP), a mathematical technique to perform transformations or extract information. Then, if necessary, a digital signal can be brought back out to the analog world through the digital-to-analog converter (D/A). Now, we can hear the voice from a loud-speaker. It is again an analog signal. When we contact the real world, we use analog signals.
Fig. 1.2: The analog signal and digital signal processing.
Moreover, there are many mature analog building blocks such as operational amplifiers, transistor amplifiers, comparators, A/ D and D/A converters, phase-locked loops (PLL) and voltage references (to name just a few) that are still in use and will be used far into the future.
Another reason that makes some users ignore analog circuits is that many latest type DSP integrated circuits (ICs) contain an analog circuit part; in other words, analog circuits are integrated peripherals of a digital IC to serve some interface functions.
More than this, some sensors also contain A/ D convertors to output digital signals for convenience.
No matter what kind of designer one wants to be, analog or digital, analog signals and circuits will always exist. Without sufficient knowledge of analog signals and circuits, no one can be a qualified circuit designer.

1.1.3Analog vs. digital

Analog and digital signals are two different ways to describe the world, the former directly, while the latter indirectly [2]. The differences between them is obvious. For analog signals, as shown in Fig. 1.1 (b), we can see the following:
(1)It may appear at any time instant. The time is dense, from 0 o’clock to 24 o’clock.
(2)The values are real numbers and they are uncountable. Here, from 0 to 5 V.
(3)Small fluctuations in analog signals are meaningful, for example, 3.99 V ≠ 4.00 V.
(4)It is easily affected by noise. A noise contaminated signal is shown in Fig. 1.3. The noise makes the signal fluctuate, away from original values.
Fig. 1.3: Noise contaminated temperature record.
The digital signal, on the other hand, is a mathematical representation of the analog signal. Here, we assume that the sampling interval of time is 1 h and the amplitude is quantized as unsigned 8-bit binary number. As shown in Fig. 1.4 (a), the 24 sampled original values are 0.8 V, 1.0 V, 1.2 V, 1.5 V, 1.8 V, 2.2 V, 2.8 V, 3.1 V, 3.5 V, 4.0 V, 4.4 V, 4.6 V, 4.8 V, 4.7 V, 4.5 V, 4.1 V, 3.8 V, 3.3 V, 2.8 V, 2.2 V, 1.8 V, 1.2 V, 0.9 V and 0.4 V. Then the unsigned 2-bit hexadecimal digital values (the same as the unsigned 8-bit binary numbers) are 29, 33, 3D, 4D, 5C, 70, 8F, 9E, B3, CC, E0, EB, F5, F0, EA, D1, C2, A8, 8F, 70, 5C, 3D, 2E and 14. As long as the difference between two analog values is less than the quantization level, that is 5/255 = 0.0196 V, the digital values are same. So, 3.99 V and 4.00 V both correspond to digital values of CC. This explains why small fluctuations of values can be ignored by the digital signal. This is also the reason that the digital signal cannot be easily affected by noise, as shown in Fig. 1.4 (b).
Fig. 1.4: Digitalization of temperature recordings, (a) Digitalized temperature signal, (b) Noise contaminated digital signal.
The properties of digital signal are as follows:
(1)It is meaningful only within sampling intervals.
(2)The values are integers and they are countable.
(3)Small fluctuations in the digital signal are ignored.
(4)It is robust to noise.
To be exact, the digital sign...

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