There were no changes in this section, except for a very minor wording change to the answer of question G7B02.
Digital circuits are circuits whose output are one of two voltages—either “on” or “off,” “high” or “low,” “one” or “zero.” Digital circuits use the binary system to represent numbers because each of the digits in a binary number is either a 1 or a 0. An advantage of using the binary system when processing digital signals is that binary “ones” and “zeros” are easy to represent by an “on” or “off” state. (G7B02)
We use digital circuits to implement logic functions and there are many integrated circuits that implement specific logic functions, such as AND and NOR. For a two input AND gate, the output is high only when both inputs are high. (G7B03) For a two input NOR gate, the output is low when either or both inputs are high. (G7B04)
Integrated circuits that provide more complex logic functions, such as counters and shift registers, are also available. A 3-bit binary counter has 8 states. (G7B05) A shift register is a clocked array of circuits that passes data in steps along the array. (G7B06)
Complex digital circuitry can often be replaced by a microcontroller. (G7B01) Microcontrollers can be programmed in much the same way that you program a personal computer. The advantage to this approach is that instead of rewiring a circuit, you simply modify the microcontroller’s program. Microcontrollers, such as the PIC line of microcontrollers, are available for less than a dollar each.
An oscillator is a circuit that generates an AC output signal. The basic components of virtually all sine wave oscillators are a filter and an amplifier operating in a feedback loop. (G7B07)
An “LC” oscillator uses an inductor and a capacitor connected so that they form what’s called a tank circuit to provide feedback. The inductance and capacitance in the tank circuit determines the frequency of an LC oscillator. (G7B09)
There are many different types of amplifiers. An amplifier in which the output preserves the input waveform is called a linear amplifier. (G7B14) Linear amplifiers are usually Class A amplifiers. Low distortion is a characteristic of a Class A amplifier. (G7B10) They are, therefore, most appropriate for amplifying phone signals.
The Class C amplifier is not linear. A Class C power stage is appropriate for amplifying a CW modulated signal. (G7B11) Class C amplifiers have the highest efficiency. (G7B12)To determine the efficiency of an RF power amplifier, divide the RF output power by the DC input power. (G7B08)
High-power amplifiers are often prone to self-oscillation due to stray capacitive feedback. To prevent this from happening, you induce some feedback that is out of phase with the stray capacitive feedback to neutralize it. The reason for neutralizing the final amplifier stage of a transmitter is to eliminate self-oscillations. (G7B13)
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