content_image/587116.jpg content_image/587117.jpg content_image/587118.jpg
<img src='content_image/587116.jpg'>

to flow from the collector to the emitter during both cycles of the AC. A circuit for AC amplification is illustrated in Figure 11-53  with a graph of the output voltage showing the large increase produced. The common-base amplifier is limited in its use since it does not increase current flow. This makes it the least used configuration. However, it is used in radio frequency amplification because of the low input Z. Figure 11-54  summarizes the characteristics of the bipolar amplifier circuits discussed above. 

NOTE: There are many variations in circuit design. JFETs and MOSFETS are also used in amplifier circuits, usually in small signal amplifiers due to their low noise outputs. 

## Oscillator Circuits 

Oscillators function to make AC from DC. They can produce various waveforms as required by electronic circuits. There are many different types of oscillators and oscillator circuits. Some of the most common types are discussed below. 

A sine wave is produced by generators when a conductor is rotated in a uniform magnetic field. The typical AC sine wave is characterized by a gradual build-up and decline of voltage in one direction, followed by a similar smooth build- up to peak voltage and decline to zero again in the opposite direction. The value of the voltage at any given time in the cycle can be calculated by taking the peak voltage and multiplying it by the sine of the angle through which the conductor has rotated. [Figure 11-55] 

A square wave is produce when there is a flow of electrons for a set period that stops for a set amount of time and 

<img src='content_image/587117.jpg'>

then repeats. In DC current, this is simply pulsing DC. [Figure 11-56]  This same wave form can be of opposite polarities when passed through a transformer to produce AC. Certain oscillators produce square waves. 

An oscillator known as a relaxation oscillator produces another kind of wave form, a sawtooth wave. A slow rise from zero to peak voltage is followed by a rapid drop-off of voltage back to almost zero. Then it repeats. [Figure 11-57] In the circuit, a capacitor slowly charges through a resistor. A neon bulb is wired across the capacitor. When its ignition voltage is reached, the bulb conducts. This short-circuits the charged capacitor, which causes the voltage to drop to nearly zero and the bulb goes out. Then, the voltage rises again as the cycle repeats. 

<img src='content_image/587118.jpg'>

