Domestic Darlington transistors. Features of operation and circuit of the Darlington transistor. Advantages and disadvantages of composite transistors

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).

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

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,

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,

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 equal to at least 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 using 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 overall 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 to a 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! Throughout my 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

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To obtain the main parameters of the CT, one should set the model of the bipolar transistor (BT) itself for low frequencies in Fig. 1a.

Rice. 1. BT equivalent circuit options n-p-n

There are only two primary design parameters: current gain and transistor input resistance. Having received them, for a specific circuit, using known formulas, you can calculate the voltage gain, input and output resistance of the cascade.

The equivalent circuits of composite Darlington (STD) and Szyklai (STSh) transistors are shown in Fig. 2, ready-made formulas for calculating parameters are in table. 1.

Table 1 - Formulas for calculating CT parameters

Here re is the emitter resistance, calculated by the formula:

Rice. 2 Options for composite transistors

It is known that b depends on the collector current (the dependence graph is indicated in the datasheet). If the base current VT2 (also known as the emitter or collector current VT1) turns out to be too small, the actual parameters of the CT will be much lower than the calculated ones. Therefore, to maintain the initial collector current VT1, it is enough to plug an additional resistor Radd into the circuit (Fig. 2c). For example, if the STD uses KT315 as VT1 with the minimum required current Ik.min, then the additional resistance will be equal to

you can put a resistor with a nominal value of 680 ohms.

The shunting effect of Radd reduces the parameters of the CT, so in microcircuits and other sophisticated circuits it is replaced by a current source.

As can be seen from the formulas in table. 1, the gain and input impedance of the STD are greater than those of the STS. However, the latter has its advantages:

at the STS input the voltage drops less than that of the STD (Ube versus 2Ube);
the VT2 collector is connected to the common wire, i.e. in a circuit with OE for cooling, VT2 can be placed directly on the metal body of the device.

Practice of compound transistor operation

In Fig. Figure 3 shows three options for constructing an output stage (emitter follower). When selecting transistors, you should strive for b1~b2 and b3~b4. The difference can be compensated by selecting pairs based on the equality of the ST gain factors b13~b24 (see Table 1).

Scheme in Fig. 3a has the highest input resistance, but this is the worst of the given circuits: it requires insulation of the flanges of powerful transistors (or separate radiators) and provides the smallest voltage swing, since ~2 V must drop between the bases of the CT, otherwise “step” distortion will appear strongly.
Scheme in Fig. 3b was inherited from those times when complementary pairs of powerful transistors were not yet produced. The only advantage compared to the previous version is a lower voltage drop of ~1.8 V and a larger swing without distortion.
Scheme in Fig. 3c clearly demonstrates the advantages of STS: a minimum voltage drops between the ST bases, and powerful transistors can be placed on a common radiator without insulating spacers.

In Fig. Figure 4 shows two parametric stabilizers. The output voltage for the version with STD is:

Since Ube varies depending on temperature and collector current, the output voltage spread of a circuit with STD will be greater, and therefore the option with STS is preferable.

Rice. 3. Options for output emitter followers on ST

Rice. 4. Application of CT as a regulator in a linear stabilizer

Any suitable combination of transistors can be used in linear circuits. The author has encountered Soviet household appliances that used STS in pairs KT315+KT814 and KT3107+KT815 (although /KT361 and KT3102/KT3107 are accepted). As a complementary pair, you can take C945 and A733, often found in old computer power supplies.

Discuss the article THEORY AND PRACTICE OF COMPOSITE TRANSISTOR

When designing radio-electronic circuits, there are often situations when it is desirable to have transistors with parameters better than those offered by manufacturers of radio elements. In some cases, we may need a higher current gain h 21 , in others a higher value of input resistance h 11 , and in others a lower value of output conductance h 22 . To solve these problems, the option of using an electronic component, which we will discuss below, is excellent.

The structure of a composite transistor and designation on the diagrams

The circuit below is equivalent to a single n-p-n semiconductor. In this circuit, the emitter current VT1 is the base current VT2. The collector current of the composite transistor is determined mainly by the current VT2.

These are two separate bipolar transistors made on the same chip and in the same package. The load resistor is also located there in the emitter circuit of the first bipolar transistor. A Darlington transistor has the same terminals as a standard bipolar transistor - base, collector and emitter.

As we can see from the figure above, a standard compound transistor is a combination of several transistors. Depending on the level of complexity and power dissipation, there may be more than two Darlington transistors.

The main advantage of a composite transistor is a significantly higher current gain h 21, which can be approximately calculated using the formula as the product of the parameters h 21 of the transistors included in the circuit.

h 21 =h 21vt1 × h21vt2 (1)

So if the gain of the first is 120, and the second is 60, then the total gain of the Darlington circuit is equal to the product of these values - 7200.

But keep in mind that parameter h21 depends quite strongly on the collector current. In the case when the base current of transistor VT2 is low enough, the collector VT1 may not be enough to provide the required value of the current gain h 21. Then by increasing h21 and, accordingly, decreasing the base current of the composite transistor, it is possible to achieve an increase in the collector current VT1. To do this, additional resistance is included between the emitter and the base of VT2, as shown in the diagram below.

Let's calculate the elements for a Darlington circuit assembled, for example, on BC846A bipolar transistors; the current VT2 is 1 mA. Then we determine its base current from the expression:

i kvt1 =i bvt2 =i kvt2 / h 21vt2 = 1×10 -3 A / 200 =5×10 -6 A

With such a low current of 5 μA, the coefficient h 21 decreases sharply and the overall coefficient may be an order of magnitude less than the calculated one. By increasing the collector current of the first transistor using an additional resistor, you can significantly gain in the value of the general parameter h 21. Since the voltage at the base is a constant (for a typical silicon three-lead semiconductor u be = 0.7 V), the resistance can be calculated from:

R = u bevt2 / i evt1 - i bvt2 = 0.7 Volt / 0.1 mA - 0.005mA = 7 kOhm

In this case, we can count on a current gain of up to 40,000. Many superbetta transistors are built according to this circuit.

Adding to the ointment, I will mention that this Darlington circuit has such a significant drawback as increased voltage Uke. If in conventional transistors the voltage is 0.2 V, then in a composite transistor it increases to a level of 0.9 V. This is due to the need to open VT1, and for this it is necessary to apply a voltage level of up to 0.7 V to its base (if during manufacture semiconductor used silicon).

As a result, in order to eliminate the mentioned drawback, minor changes were made to the classical circuit and a complementary Darlington transistor was obtained. Such a composite transistor is made up of bipolar devices, but with different conductivities: p-n-p and n-p-n.

Russian and many foreign radio amateurs call this connection the Szyklai scheme, although this scheme was called a paradoxical pair.

A typical disadvantage of composite transistors that limits their use is their low performance, so they are widely used only in low-frequency circuits. They work great in the output stages of powerful ULFs, in control circuits for engines and automation devices, and in car ignition circuits.

In circuit diagrams, a composite transistor is designated as an ordinary bipolar one. Although, rarely, such a conventionally graphical representation of a composite transistor on a circuit is used.

One of the most common is the L293D integrated assembly - these are four current amplifiers in one housing. In addition, the L293 microassembly can be defined as four transistor electronic switches.

The output stage of the microcircuit consists of a combination of Darlington and Sziklai circuits.

In addition, specialized micro-assemblies based on the Darlington circuit have also received respect from radio amateurs. For example . This integrated circuit is essentially a matrix of seven Darlington transistors. Such universal assemblies perfectly decorate amateur radio circuits and make them more functional.

The microcircuit is a seven-channel switch of powerful loads based on composite Darlington transistors with an open collector. The switches contain protection diodes, which allow switching inductive loads, such as relay coils. The ULN2004 switch is required when connecting powerful loads to CMOS logic chips.

The charging current through the battery, depending on the voltage on it (applied to the B-E junction VT1), is regulated by transistor VT1, the collector voltage of which controls the charge indicator on the LED (as charging the charge current decreases and the LED gradually goes out) and a powerful composite transistor containing VT2, VT3, VT4.

The signal requiring amplification through the preliminary ULF is fed to a preliminary differential amplifier stage built on composite VT1 and VT2. The use of a differential circuit in the amplifier stage reduces noise effects and ensures negative feedback. The OS voltage is supplied to the base of transistor VT2 from the output of the power amplifier. DC feedback is implemented through resistor R6.

When the generator is turned on, capacitor C1 begins to charge, then the zener diode opens and relay K1 operates. The capacitor begins to discharge through the resistor and the composite transistor. After a short period of time, the relay turns off and a new generator cycle begins.