If you connect the transistors as shown in Fig. 2.60, then the resulting circuit will operate as one transistor, and its coefficient β will be equal to the product of the β coefficients of the component transistors. This technique is useful for circuits that handle high currents (such as voltage regulators or power amplifier output stages) or for amplifier input stages where high input impedance must be provided.
Rice. 2.60. Composite Darlington transistor.
In a Darlington transistor, the voltage drop between base and emitter is twice as large as normal, and the saturation voltage is at least equal to the voltage drop across the diode (since the emitter potential of transistor T 1 must exceed the emitter potential of transistor T 2 by the amount of voltage drop across the diode). In addition, transistors connected in this way behave like one transistor with a fairly low speed, since transistor T 1 cannot quickly turn off transistor T 2. Taking this property into account, a resistor is usually included between the base and emitter of transistor T 2 (Fig. 2.61). Resistor R prevents transistor T 2 from moving into the conduction region due to leakage currents of transistors T 1 and T 2. The resistance of the resistor is chosen so that the leakage currents (measured in nanoamps for small-signal transistors and in hundreds of microamps for high-power transistors) create a voltage drop across it that does not exceed the voltage drop across the diode, and at the same time, current flows through it. small compared to the base current of transistor T 2. Typically, the resistance R is several hundred ohms in a high-power Darlington transistor and several thousand ohms in a small-signal Darlington transistor.
Rice. 2.61. Increasing the turn-off speed in a composite Darlington transistor.
The industry produces Darlington transistors in the form of complete modules, which usually include an emitter resistor. An example of such a standard circuit is the 2N6282 type Darlington power npn transistor, which has a current gain of 4000 (typical) for a collector current of 10 A.
Connecting transistors according to the Sziklai circuit. The connection of transistors according to the Sziklai circuit is a circuit similar to that. which we just looked at. It also provides an increase in the β coefficient. Sometimes such a connection is called a complementary Darlington transistor (Fig. 2.62). The circuit behaves like an n-p-n transistor with a large β coefficient. The circuit has a single voltage between base and emitter, and the saturation voltage, as in the previous circuit, is at least equal to the voltage drop across the diode. It is recommended to include a resistor with low resistance between the base and emitter of transistor T2. Designers use this circuit in high-power push-pull output stages when they want to use output transistors of only one polarity. An example of such a circuit is shown in Fig. 2.63. As before, the resistor is the collector resistor of transistor T 1 Darlington transistor formed by transistors T 2 and T 3 . behaves like a single n-p-n transistor. with high current gain. Transistors T 4 and T 5, connected according to the Sziklai circuit, behave like a powerful p-n-p transistor. with high gain. As before, resistors R 3 and R 4 have a small resistance. This circuit is sometimes called a push-pull repeater with quasi-complementary symmetry. In a real cascade with additional symmetry (complementary), transistors T 4 and T 5 would be connected according to a Darlington circuit.
Rice. 2.62. Connecting transistors according to the Sziklai circuit (“complementary Darlington transistor”).
Rice. 2.63. A powerful push-pull cascade that uses only n-p-n type output transistors.
Transistor with ultra-high current gain. Composite transistors - the Darlington transistor and the like - should not be confused with transistors with an ultra-high current gain, in which a very large h21e coefficient is obtained during the manufacturing process of the element. An example of such an element is a 2N5962 type transistor. for which a minimum current gain of 450 is guaranteed when the collector current changes in the range from 10 μA to 10 mA; this transistor belongs to the 2N5961-2N5963 series of elements, which is characterized by a maximum voltage range Uke from 30 to 60 V (if the collector voltage should be higher, then the value of C should be reduced). The industry produces matched pairs of transistors with ultra-high coefficient β. They are used in low-signal amplifiers for which the transistors must have matched characteristics; Section is devoted to this issue. 2.18. Examples of such standard circuits are circuits such as LM394 and MAT-01; they are transistor pairs with a high gain, in which the voltage U be matched to fractions of a millivolt (in the best circuits, matching is provided up to 50 μV), and the coefficient h 21e is up to 1%. The MAT-03 type circuit is a matched pair of p-n-p transistors.
Transistors with an extremely high β coefficient can be combined using a Darlington circuit. In this case, the base bias current can be made equal to only 50 pA (examples of such circuits are operational amplifiers such as LM111 and LM316.
When designing circuits for radio-electronic devices, it is often desirable to have transistors with parameters better than those models offered by manufacturers of radio-electronic components (or better than what is possible with the available transistor manufacturing technology). This situation is most often encountered in the design of integrated circuits. We usually require higher current gain h 21, higher input resistance value h 11 or less output conductance value h 22 .
Various circuits of composite transistors can improve the parameters of transistors. There are many opportunities to implement a composite transistor from field-effect or bipolar transistors of different conductivities, while improving its parameters. The most widespread is the Darlington scheme. In the simplest case, this is the connection of two transistors of the same polarity. An example of a Darlington circuit using npn transistors is shown in Figure 1.
Figure 1 Darlington circuit using NPN transistors
The above circuit is equivalent to a single NPN transistor. In this circuit, the emitter current of transistor VT1 is the base current of transistor VT2. The collector current of the composite transistor is determined mainly by the current of transistor VT2. The main advantage of the Darlington circuit is the high current gain h 21, which can be approximately defined as the product h 21 transistors included in the circuit:
(1)However, it should be kept in mind that the coefficient h 21 depends quite strongly on the collector current. Therefore, at low values of the collector current of transistor VT1, its value can decrease significantly. Dependency example h 21 from the collector current for different transistors is shown in Figure 2
Figure 2 Dependence of transistor gain on collector current
As can be seen from these graphs, the coefficient h 21e practically does not change for only two transistors: the domestic KT361V and the foreign BC846A. For other transistors, the current gain depends significantly on the collector current.
In the case when the base current of transistor VT2 is sufficiently small, the collector current of transistor VT1 may be insufficient to provide the required current gain value h 21. In this case, increasing the coefficient h 21 and, accordingly, a decrease in the base current of the composite transistor can be achieved by increasing the collector current of transistor VT1. To do this, an additional resistor is connected between the base and emitter of transistor VT2, as shown in Figure 3.
Figure 3 Composite Darlington transistor with an additional resistor in the emitter circuit of the first transistor
For example, let's define the elements for a Darlington circuit assembled on BC846A transistors. Let the current of transistor VT2 be equal to 1 mA. Then its base current will be equal to:
(2)
At this current, the current gain h 21 drops sharply and the total current gain may be significantly less than the calculated one. By increasing the collector current of transistor VT1 using a resistor, you can significantly gain in the value of the overall gain h 21. Since the voltage at the base of the transistor is a constant (for a silicon transistor u be = 0.7 V), then we calculate according to Ohm’s law:
(3)In this case, we can expect a current gain of up to 40,000. This is how many domestic and foreign superbetta transistors are made, such as KT972, KT973 or KT825, TIP41C, TIP42C. The Darlington circuit is widely used in the output stages of low frequency amplifiers (), operational amplifiers and even digital ones, for example.
It should be noted that the Darlington circuit has the disadvantage of increased voltage U ke. If in ordinary transistors U ke is 0.2 V, then in a composite transistor this voltage increases to 0.9 V. This is due to the need to open transistor VT1, and for this a voltage of 0.7 V should be applied to its base (if we are considering silicon transistors).
In order to eliminate this drawback, a compound transistor circuit using complementary transistors was developed. On the Russian Internet it was called the Siklai scheme. This name comes from the book by Tietze and Schenk, although this scheme previously had a different name. For example, in Soviet literature it was called a paradoxical pair. In the book by W.E. Helein and W.H. Holmes, a compound transistor based on complementary transistors is called a White circuit, so we will simply call it a compound transistor. The circuit of a composite pnp transistor using complementary transistors is shown in Figure 4.
Figure 4 Composite pnp transistor based on complementary transistors
An NPN transistor is formed in exactly the same way. The circuit of a composite npn transistor using complementary transistors is shown in Figure 5.
Figure 5 Composite npn transistor based on complementary transistors
In the list of references, the first place is given by the book published in 1974, but there are BOOKS and other publications. There are basics that do not become outdated for a long time and a huge number of authors who simply repeat these basics. You must be able to tell things clearly! During my entire professional career, I have come across less than ten BOOKS. I always recommend learning analog circuit design from this book.
Last file update date: 06/18/2018
Literature:
Along with the article "Composite transistor (Darlington circuit)" read:
http://site/Sxemoteh/ShVklTrz/kaskod/
http://site/Sxemoteh/ShVklTrz/OE/
If we take, for example, a transistor MJE3055T it has a maximum current of 10A, and the gain is only about 50; accordingly, in order for it to open completely, it needs to pump about two hundred milliamps of current into the base. A regular MK output won’t handle that much, but if you connect a weaker transistor between them (some kind of BC337) capable of pulling this 200mA, then it’s easy. But this is so that he knows. What if you have to make a control system out of improvised rubbish - it will come in handy.
In practice, ready-made transistor assemblies. Externally, it is no different from a conventional transistor. Same body, same three legs. It’s just that it has a lot of power, and the control current is microscopic :) In price lists they usually don’t bother and write simply - a Darlington transistor or a composite transistor.
For example a couple BDW93C(NPN) and BDW94С(PNP) Here is their internal structure from the datasheet.
Moreover, there are Darlington assemblies. When several are packed into one package at once. An indispensable thing when you need to steer some powerful LED display or stepper motor (). An excellent example of such a build - very popular and easily available ULN2003, capable of dragging up to 500 mA for each of its seven assemblies. Outputs are possible include in parallel to increase the current limit. In total, one ULN can carry as much as 3.5A through itself if all its inputs and outputs are parallelized. What makes me happy about it is that the exit is opposite the entrance, it is very convenient to route the board under it. Directly.
The datasheet shows the internal structure of this chip. As you can see, there are also protective diodes here. Despite the fact that they are drawn as if they were operational amplifiers, the output here is an open collector type. That is, he can only short circuit to the ground. What becomes clear from the same datasheet if you look at the structure of one valve.
Composite transistor (Darlington transistor) - combining two or more bipolar transistors to increase the current gain. Such a transistor is used in circuits that operate with high currents (for example, in voltage stabilizer circuits, output stages of power amplifiers) and in the input stages of amplifiers if it is necessary to provide a high input impedance.
Symbol for a composite transistor
A compound transistor has three terminals (base, emitter and collector), which are equivalent to the terminals of a conventional single transistor. The current gain of a typical compound transistor (sometimes erroneously called "superbeta") is ≈ 1000 for high-power transistors and ≈ 50,000 for low-power transistors. This means that a small base current is enough to turn on the compound transistor.
Unlike bipolar transistors, field-effect transistors are not used in a composite connection. There is no need to combine field-effect transistors, since they already have an extremely low input current. However, there are circuits (for example, an insulated gate bipolar transistor) where field-effect and bipolar transistors are used together. In a sense, such circuits can also be considered composite transistors. Same for a composite transistorIt is possible to increase the gain value by reducing the thickness of the base, but this presents certain technological difficulties.
Example superbeta (super-β)transistors can be used in the KT3102, KT3107 series. However, they can also be combined using the Darlington scheme. In this case, the base bias current can be made equal to only 50 pA (examples of such circuits are operational amplifiers such as LM111 and LM316).
Photo of a typical amplifier using composite transistors
Darlington circuit
One type of such transistor was invented by electrical engineer Sidney Darlington.
Schematic diagram of a composite transistor
A compound transistor is a cascade connection of several transistors connected in such a way that the load in the emitter of the previous stage is the base-emitter transition of the transistor of the next stage, that is, the transistors are connected by collectors, and the emitter of the input transistor is connected to the base of the output transistor. In addition, a resistive load of the first transistor can be used as part of the circuit to accelerate closing. Such a connection as a whole is considered as one transistor, the current gain of which, when the transistors are operating in the active mode, is approximately equal to the product of the gains of the first and second transistors:
β с = β 1 ∙ β 2
Let us show that a composite transistor actually has a coefficientβ , significantly larger than both of its components. Setting the incrementdlb=dlb1, we get:
dle1 = (1 + β 1) ∙ dlb=dlb2
dlTo=dlk1+dlk2= β 1 ∙ dlb+ β 2 ∙ ((1 + β 1) ∙ dlb)
Sharing dl to on dlb, we find the resulting differential transmission coefficient:
β Σ = β 1 + β 2 + β 1 ∙ β 2
Because alwaysβ >1 , it could be considered:
β Σ = β 1 ∙ β 1
It should be emphasized that the coefficientsβ 1 And β 1 may differ even in the case of transistors of the same type, since the emitter currentI e2 V 1 + β 2times the emitter currentI e1(this follows from the obvious equalityI b2 = I e1).
Siklai scheme
The Darlington pair is similar to the Sziklai transistor connection, named after its inventor George Sziklai, and is also sometimes called a complementary Darlington transistor. Unlike the Darlington circuit, which consists of two transistors of the same conductivity type, the Sziklai circuit contains transistors of different polarities ( p–n–p and n–p–n ). The Siklai couple behaves like n–p–n -transistor with high gain. The input voltage is the voltage between the base and emitter of transistor Q1, and the saturation voltage is equal to at least the voltage drop across the diode. It is recommended to include a low resistance resistor between the base and emitter of transistor Q2. This circuit is used in powerful push-pull output stages when using output transistors of the same polarity.
Sziklai cascade, similar to a transistor with n – p – n transition
Cascode circuit
A composite transistor, made according to the so-called cascode circuit, is characterized by the fact that transistor VT1 is connected in a circuit with a common emitter, and transistor VT2 is connected in a circuit with a common base. Such a composite transistor is equivalent to a single transistor connected in a common-emitter circuit, but it has much better frequency properties and greater undistorted power in the load, and can also significantly reduce the Miller effect (an increase in the equivalent capacitance of the inverting amplifier element due to feedback from the output to the input of this element when it is turned off).
Advantages and disadvantages of composite transistors
High gain values in composite transistors are realized only in static mode, so composite transistors are widely used in the input stages of operational amplifiers. In circuits at high frequencies, composite transistors no longer have such advantages - the limiting frequency of current amplification and the speed of operation of composite transistors is less than the same parameters for each of the transistors VT1 and VT2.
Advantages:
A)High current gain.
b)The Darlington circuit is manufactured in the form of integrated circuits and, at the same current, the working surface of the silicon is smaller than that of bipolar transistors. These circuits are of great interest at high voltages.
Flaws:
A)Low performance, especially the transition from open to closed state. For this reason, composite transistors are used primarily in low-frequency key and amplifier circuits; at high frequencies, their parameters are worse than those of a single transistor.
b)The forward voltage drop across the base-emitter junction in a Darlington circuit is almost twice as large as in a conventional transistor, and for silicon transistors it is about 1.2 - 1.4 V (cannot be less than twice the voltage drop at the p-n junction) .
V)High collector-emitter saturation voltage, for a silicon transistor about 0.9 V (compared to 0.2 V for conventional transistors) for low-power transistors and about 2 V for high-power transistors (cannot be less than the voltage drop across the p-n junction plus voltage drop across the saturated input transistor).
The use of load resistor R1 allows you to improve some characteristics of the composite transistor. The resistor value is selected in such a way that the collector-emitter current of transistor VT1 in the closed state creates a voltage drop across the resistor that is insufficient to open transistor VT2. Thus, the leakage current of transistor VT1 is not amplified by transistor VT2, thereby reducing the total collector-emitter current of the composite transistor in the off state. In addition, the use of resistor R1 helps to increase the speed of the composite transistor by forcing the closing of transistor VT2. Typically, the resistance of R1 is hundreds of ohms in a high-power Darlington transistor and several kOhms in a small-signal Darlington transistor. An example of a circuit with an emitter resistor is a powerful npn Darlington transistor type KT825, its current gain is 10000 (typical value) for a collector current of 10 A.
In integrated circuits and discrete electronics, two types of composite transistors have become widespread: the Darlington and Sziklai circuits. In micropower circuits, such as op-amp input stages, compound transistors provide high input impedance and low input currents. In devices operating with high currents (for example, for voltage stabilizers or output stages of power amplifiers), to increase efficiency it is necessary to ensure a high current gain of power transistors.
Siklai's scheme implements a powerful p-n-p high gain transistor using low power p-n-p transistor with small IN and powerful n-p-n transistor ( Figure 7.51). In integrated circuits, this inclusion is implemented by a high-beta p-n-p transistor based horizontal p-n-p transistor and vertical n-p-n transistor. This circuit is also used in powerful push-pull output stages, when output transistors of the same polarity are used ( n-p-n).
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Figure 7.51 - Composite p-n-p transistor Figure 7.52 - Composite n-p-n according to the Szyklai circuit, transistor according to the Darlington circuit
Sziklai circuit or complementary Darlington transistor behaves like a transistor p-n-p type ( Figure 7.51) with a large current gain,
The input voltage is identical to a single transistor. The saturation voltage is higher than that of a single transistor by the amount of voltage drop across the emitter junction n-p-n transistor. For silicon transistors, this voltage is on the order of one volt, as opposed to fractions of a volt for a single transistor. Between base and emitter n-p-n transistor (VT2), it is recommended to include a resistor with a small resistance to suppress uncontrolled current and increase thermal stability.
The Darlington transistor is implemented using unipolar transistors ( Figure 7.52). The current gain is determined by the product of the coefficients of the component transistors.
The input voltage of a Darlington transistor is twice that of a single transistor. The saturation voltage exceeds the output transistor. Input impedance of the operational amplifier at
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The Darlington circuit is used in discrete monolithic switching transistors. Two transistors, two shunt resistors and a protective diode ( Figure 7.53). Resistors R 1 and R 2 suppress the gain in low current mode, ( Figure 7.38), which ensures a low value of the uncontrolled current and an increase in the operating voltage of the closed transistor,
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Figure 7.53 - Electrical circuit of a monolithic Darlington pulse transistor
Resistor R2 (about 100 Ohms) is formed in the form of a technological shunt, similar to the cathode junction shunts of thyristors. For this purpose, when forming the emitter using photolithography, an oxide mask in the form of a circle is left in certain local areas. These local masks do not allow the donor impurity to diffuse, and they remain p- columns ( Figure 7.54). After metallization over the entire area of the emitter, these columns represent a distributed resistance R2 and a protective diode D ( Figure 7.53). A protective diode protects the emitter junctions from breakdown when the collector voltage is reversed. The input power consumption of a transistor using a Darlington circuit is one and a half to two orders of magnitude lower than that of a single transistor. The maximum switching frequency depends on the limiting voltage and collector current. Current transistors operate successfully in pulse converters up to frequencies of the order of 100 kHz. A distinctive feature of the monolithic Darlington transistor is its quadratic transfer characteristic, since IN- the ampere characteristic increases linearly with increasing collector current to the maximum value,
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