Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Resistive opto-isolator wikipedia , lookup
Power MOSFET wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Thermal runaway wikipedia , lookup
Rectiverter wikipedia , lookup
Operational amplifier wikipedia , lookup
Invention of the integrated circuit wikipedia , lookup
Opto-isolator wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Integrated circuit wikipedia , lookup
Transistors Part 4 Transistors Part 4 • Transistors Part 4 - Silicon Controlled Rectifiers (SCRs) - Class of Operation (Load-Line Analysis) - Class A - Class B - Class AB - Class C Transistors Part 4 • What is an SCR? - An SCR is a Silicon Controlled Rectifier. It is a thyristor which are switching devices that have the characteristics of either being fully turned on or being fully turned off. Once the SCR is turned on, it conducts current like a regular diode and will remain on as long as the current level does not drop below a predetermined value. If it does then the SCR turns off. Transistors Part 4 Silicon-Controlled Rectifiers • Construction – Four alternately doped silicon layers of P- and Ntype semiconductor materials • Operation – Current flows only in the forward-biased condition and is blocked when reverse biased • Equivalent circuit – Consists of both an npn and a PNP transistor Transistors Part 4 • SCR Transistors Part 4 Silicon-Controlled Rectifiers (cont’d.) • Current–voltage characteristics of an SCR – Similar to that of standard diodes when they are reverse biased – Reverse breakdown voltage • Voltage level that causes conduction to occur • Triggering methods of an SCR – Turning on by applying a positive voltage to the gate terminal Transistors Part 4 • Current-Voltage characteristics for an SCR Transistors Part 4 Silicon-Controlled Rectifiers (cont’d.) • DC triggering – Unidirectional device • AC triggering – Connect an AC voltage across its anode and cathode leads Figure of a properly biased SCR circuit Transistors Part 4 • Phase control SCR circuit – Used to vary the SCR’s conduction angle – Time relationship between the occurrence of the trigger pulse and the point at which the conduction alternation begin • Temperature-operated alarms – Used as automatic overheat alarms or underheat alarms Transistors Part 4 • Over-temperature alarm Transistors Part 4 • Under-temperature alarm Transistors Part 4 • Classification by Class of Operation - Cut-Off - Saturation When VCE ≈ VCC then the transistor is in Cut-Off When the forward Base Current increases to a point where further increase if base current causes no additional increase in collector current then the transistor is in Saturation. Transistors Part 4 • Class A Amplifier-By definition Class A amplifier is one where the collector current flows throughout the entire input cycle. The transistor never goes into cut-off or saturation. • This class of amplifier is used where it is desirable to keep signal distortion as low as possible thus they are good for small-signal amplifiers for radio, TV, and audio-equipment. Transistors Part 4 • Class B Amplifier-Collector current flows only during the positive half cycles of the input signal since the other half is at cut-off. The transistor is turn-off during the times no signal is applied or during cut-off. • Higher Efficiency than Class A since it is not continuously operating but Class B distorts the signal since half of the input signal is clipped off. Used with another transistor in push-pull configuration. Transistors Part 4 • Class AB Amplifier-is a compromise between Class A – low distortion, low efficiency, and Class B – high distortion and high efficiency. • More efficient than Class A but not efficient as Class C. • Used to avoid distortions in audio amplifiers Transistors Part 4 • Class C amplifiers-biased below cut-off and the collector current flows less than 180⁰ • Class C operation is used in RF-Radio frequency-circuits. • Class C has high efficiency and large-signal handling capability. Transistors Part 4 The End