An oscillator can be described as a source of alternating voltage. An amplifier delivers an amplified version of input signal while oscillator generates an output waveform without an input signal. The additional power content in the output signal is supplied by an external DC power source.

The oscillator requires no external signal to initiate or maintain the energy conversion process. Instead, an output signal is produced as long as a DC power source is connected. Figure 12.23 depicts the comparison between amplifier and oscillator.

Principle of oscillators

Oscillators are amplifiers with positive feedback. Consider a feedback amplifier with an input signal Vin and output V◦ as shown in Figure 12.24.

Universal gates

It is possible to implement any Boolean expression using only NAND gates. In a similar manner, it can be shown that NOR gates alone can be used to implement any Boolean operation. Therefore NAND and NOR gates are called universal gates.

Figure 13.12 illustrates that NOT, AND and OR gates can be implemented using NAND gates.

Figure 13.12 illustrates that NOT, OR and AND gates can also be implemented using NOR gates.