Download Transistors IV 6-7-11

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