Darlington Transistors
Sometimes the DC current gain of the bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors are used. Here, one small input transistor is used to switch "ON" or "OFF" a much larger current handling output transistor. To maximise the signal gain the two transistors are connected in a "Complementary Gain Compounding Configuration" or what is generally called a "Darlington Configuration" where the amplification factor is the product of the two individual transistors.
Darlington Transistors simply contain two individual bipolar NPN or PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor with a very high current gain. The overall current gain Beta (β) or Hfe value of a Darlington device is the product of the two individual gains of the transistors and is given as:
So Darlington Transistors with very high β values and high Collector currents are possible compared to a single transistor. An example of the two basic types of Darlington transistor are given below.Darlington Transistor Configurations
Sometimes the DC current gain of the bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors are used. Here, one small input transistor is used to switch "ON" or "OFF" a much larger current handling output transistor. To maximise the signal gain the two transistors are connected in a "Complementary Gain Compounding Configuration" or what is generally called a "Darlington Configuration" where the amplification factor is the product of the two individual transistors.
Darlington Transistors simply contain two individual bipolar NPN or PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor with a very high current gain. The overall current gain Beta (β) or Hfe value of a Darlington device is the product of the two individual gains of the transistors and is given as:
So Darlington Transistors with very high β values and high Collector currents are possible compared to a single transistor. An example of the two basic types of Darlington transistor are given below.Darlington Transistor Configurations
The above NPN Darlington transistor configuration shows the Collectors of the two transistors connected together with the Emitter of the first transistor connected to the Base of the second transistor therefore, the Emitter current of the first transistor becomes the Base current of the second transistor. The first or "input" transistor receives an input signal, amplifies it and uses it to drive the second or "output" transistors which amplifies it again resulting in a very high current gain. As well as its high increased current and voltage switching capabilities, another advantage of a Darlington transistor is in its high switching speeds making them ideal for use in Inverter circuits and DC motor or stepper motor controlapplications.
One difference to consider when using Darlington transistors over the conventional single bipolar transistor type is that the Base-Emitter input voltage Vbe needs to be higher at approx 1.4v for silicon devices, due to the series connection of the two PN junctions.
Then to summarise when using a Transistor as a Switch.
One difference to consider when using Darlington transistors over the conventional single bipolar transistor type is that the Base-Emitter input voltage Vbe needs to be higher at approx 1.4v for silicon devices, due to the series connection of the two PN junctions.
Then to summarise when using a Transistor as a Switch.
· Transistor switches can be used to switch and control lamps, relays or even motors.
· When using bipolar transistors as switches they must be fully "OFF" or fully "ON".
· Transistors that are fully "ON" are said to be in their Saturation region.
· Transistors that are fully "OFF" are said to be in their Cut-off region.
· In a transistor switch a small Base current controls a much larger Collector current.
· When using transistors to switch inductive relay loads a "Flywheel Diode" is required.
· When large currents or voltages need to be controlled, Darlington Transistors are used
· When using bipolar transistors as switches they must be fully "OFF" or fully "ON".
· Transistors that are fully "ON" are said to be in their Saturation region.
· Transistors that are fully "OFF" are said to be in their Cut-off region.
· In a transistor switch a small Base current controls a much larger Collector current.
· When using transistors to switch inductive relay loads a "Flywheel Diode" is required.
· When large currents or voltages need to be controlled, Darlington Transistors are used
Posted By 225-╰»мαηιкυмαя Y
SHARE A JOB AND HELP YOUR FRIENDS. JOB OPENINGS, WALKIN UPDATES DAILY FOR FRESH AND EXPERIENCE PEOPLE. *** SEND AN EMAIL TO indiancareerportal@gmail.com contact me
Thank You
