Contents
When to use a Darlington pair in a circuit?
When designing a circuit using a Darlington pair, exactly the same rules are used as for designing a circuit using a standard transistor. The Darlington pair can be treated as a form of transistor with the differences of the very much higher current gain, and the higher base emitter voltage.
Why is the leakage current high in the Darlington pair?
The overall leakage current is high because the leakage current of the first transistor is amplified by the next transistor. That’s why the three or more stages of Darlington is impossible. Hence, the Darlington pair is very useful in most of the application because it provides high current gain at low base currents.
Who is the inventor of the Darlington pair transistor?
A Darlington transistor or simply Darlington pair is mainly used to offer a very high current gain even with low base current. Darlington configuration was invented by Sidney Darlington in the year of 1953.
How does the NPN configuration of Darlington transistor work?
Consider the NPN configuration of Darlington transistor. In this a very small base current causes a large emitter current to flow which is then applied to the base of the next transistor. Amplified current in the first transistor is again amplified by the current gain of the second transistor.
How to calculate the current gain of a Darlington transistor?
Darlington Transistor Pair Current Gain Calculation: In the below image we can see two PNP or two NPN transistors are connected together. The overall current gain of the Darlington pair will be-Current gain (hFE) = First transistor gain (hFE 1) * Second transistor gain (hFE 2)
Why is a 100K resistor used in a Darlington transistor?
A 100K resistor is used at the signal input side to reduce the current to a few millamps. Normally with such low current at the base, the 2N3055 alone can never illuminate a high current load such as a 12V 2 amp bulb. This is because the current gain of 2N3055 is very low to process the low base current into high collector current.
How to choose the bias point for a Darlington?
Choose the bias point for the Darlington base: This is the emitter voltage plus the overall base-emitter voltage for the Darlington (normally 1.2 to 1.4 volts). Choose bias current for the bias potential divide: This is normally chosen to be approximately ten times the base current.
How to calculate the current gain of a Darlington connection?
If the Darlington connection comprises of two individual BJTs with current gains β 1 and β 2 the combined current gain can be calculated using the formula: β D = β 1β 2 ——– (12.7) When matched transistors are used in a Darlington connection such that β 1 = β 2 = β , the above formula for the current gain gets simplified as:
What kind of transistors are used in Darlington circuit?
This kind of packaged Darlington transistors have external features similar to a normal transistor but have very high and enhanced current gain output, compared to the normal single transistors. The following figure shows a common Darlington circuit using transistors with a very high current gain β D.
Which is an example of a Darlington pair transistor?
There are many ways in which a circuit using a Darlington pair transistor can be implemented. The design example below provides a simple example showing some of the basic principles behind them. The design example with its calculations is based around an AC coupled emitter follower using a Darlington transistor pair.