06-12-2012, 01:16 PM
Tubes, Transistors and Amplifiers
1Tubes, Transistors.ppt (Size: 1.67 MB / Downloads: 39)
Interest
In 1947, Bardeen & Brattain at Bell Laboratories created the first amplifier! Shockley (boss), came near to canceling the project. The three shared a Nobel Prize. Bardeen and Brattain continued in research (and Bardeen later won another Nobel). Shockley quit to start a semiconductor company in Palo Alto. It folded, but its staff went on to invent the integrated circuit (the "chip") & to found the Intel Corporation.
Cathode Ray Tube (CRT)
The cathode is a heated filament (like light bulb filament) in a vacuum inside a glass tube. The ray is a stream of electrons that naturally pour off a heated cathode into the vacuum.
The + anode attracts the electrons pouring off the cathode. In a TV's CRT, the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.
Bipolar Transistors
History
Created in 1948 in the AT&T Bell Laboratories.
Scientists were performing doping experiments on semiconductor material (diodes) and developed a semiconductor device having three (3) PN junctions.
Bipolar Transistor Operation (PNP)
90% of the current carriers pass through the reverse biased base - collector PN junction and enter the collector of the transistor.
10% of the current carriers exit transistor through the base.
The opposite is true for a NPN transistor.
Amplifier Electric Switch Operation
When the input signal is large enough, the transistor can be driven into saturation & cutoff which will make the transistor act as an electronic switch.
Saturation - The region of transistor operation where a further increase in the input signal causes no further increase in the output signal.
Cutoff - Region of transistor operation where the input signal is reduced to a point where minimum transistor biasing cannot be maintained => the transistor is no longer biased to conduct. (no current flows)
Transistor Maintenance
When troubleshooting transistors, do the following:
Remove the transistor from the circuit, if possible.
Use a transistor tester, if available, or use a digital multimeter set for resistance on the diode scale.
Test each PN junction separately. ( A “front to back” ratio of at least 10:1 indicates a good transistor).