Darlington pair

In electronics, the Darlington transistor (often called a Darlington pair) is a compound structure made by two bipolar transistors connected in such a way that the current amplified by the first transistor is amplified further by the second one. This configuration gives a much higher current gain than each transistor taken separately.

The Darlington configuration was invented by Bell Laboratories engineer Sidney Darlington in 1953. He patented the invention of having two or three transistors on a single chip sharing a collector.

A Darlington pair behaves like a single transistor with a high current gain (approximately the product of the gains of the two transistors). In fact, integrated devices have three leads (B, C, and E), broadly analogous to those of a standard transistor.

A general relation between the compound current gain and the individual gains is given by:

If β₁ and β₂ are high enough (hundreds), this relation can be approximated with:

A typical Darlington transistor has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on.

One drawback is an approximate doubling of the base–emitter voltage. Since there are two junctions between the base and emitter of the Darlington transistor, the equivalent base–emitter voltage is the sum of both base–emitter voltages:

For silicon-based technology, where each V_BEi is about 0.65 V when the device is operating in the active or saturated region, the necessary base–emitter voltage of the pair is 1.3 V.

Another drawback of the Darlington pair is its increased "saturation" voltage. The output transistor is not allowed to saturate (i.e. its base–collector junction must remain reverse-biased) because the first transistor, when saturated, establishes full (100%) parallel negative feedback between the collector and the base of the second transistor. Since collector–emitter voltage is equal to the sum of its own base–emitter voltage and the collector-emitter voltage of the first transistor, both positive quantities in normal operation, it always exceeds the base-emitter voltage. (In symbols, ${\displaystyle \mathrm {V_{CE2}=V_{CE1}+V_{BE2}>V_{BE2}} \Rightarrow \mathrm {V_{C2}>V_{B2}} }$ always.) Thus the "saturation" voltage of a Darlington transistor is one V_BE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon. For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor. The increased low output level can cause troubles when TTL logic circuits are driven.

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