The designation of a composite transistor, made of two separate transistors connected according to a Darlington circuit, is indicated in Figure No. 1. The first of the mentioned transistors is connected according to the emitter follower circuit; the signal from the emitter of the first transistor goes to the base of the second transistor. The advantage of this circuit is its exceptionally high gain. The overall current gain p for this circuit is equal to the product of the current gain coefficients of individual transistors: p = pgr2.
For example, if the input transistor of a Darlington pair has a gain of 120, and the gain of the second transistor is 50, then the total p is 6000. In fact, the gain will be even slightly greater, since the total collector current of the composite transistor is equal to the sum of the collector currents of the pair entering him transistors.
The complete circuit of a composite transistor is shown in Figure 2. In this circuit, resistors R 1 and R 2 form a voltage divider that creates a bias at the base of the first transistor. Resistor Rн connected to the emitter of the composite transistor forms an output circuit. Such a device is widely used in practice, especially in cases where a large current gain is required. The circuit has a high sensitivity to the input signal and is characterized by a high level of output collector current, which allows this current to be used as a control current (especially at low supply voltage). The use of the Darlington circuit helps to reduce the number of components in circuits.
The Darlington circuit is used in low-frequency amplifiers, oscillators and switching devices. The output impedance of a Darlington circuit is many times lower than the input impedance. In this sense, its characteristics are similar to those of a step-down transformer. However, unlike a transformer, the Darlington circuit allows for high power amplification. The input resistance of the circuit is approximately equal to $²Rn, and its output resistance is usually less than Rн. In switching devices, the Darlington circuit is used in the frequency range up to 25 kHz.
Literature: Matthew Mandl. 200 SELECTED ELECTRONICS DIAGRAMS. Editorial office of literature on computer science and electronics. © 1978 Prentice-Hall, Inc. © translation into Russian, “Mir”, 1985, 1980
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The stereo volume, balance and tone control on the TCA5550 has the following parameters: Low nonlinear distortion no more than 0.1% Supply voltage 10-16V (12V nominal) Current consumption 15...30mA Input voltage 0.5V (gain at a supply voltage of 12V unit) Tone adjustment range -14...+14dB Balance adjustment range 3dB Difference between channels 45dB Signal to noise ratio...
Darlington), are often components of amateur radio designs. As is known, with such a connection, the current gain, as a rule, increases tens of times. However, it is not always possible to achieve a significant operating capacity margin for the voltage acting on the cascade. Amplifiers consisting of two bipolar transistors (Fig. 1.23) often fail when exposed to pulse voltage, even if it does not exceed the value of the electrical parameters specified in the reference literature.
This unpleasant effect can be dealt with in different ways. One of them - the simplest - is the presence in a pair of a transistor with a large (several times) resource reserve in terms of collector-emitter voltage. The relatively high cost of such “high-voltage” transistors leads to an increase in the cost of the design. You can, of course, purchase special composite silicon devices in one package, for example: KT712, KT829, KT834, KT848, KT852, KT853, KT894, KT897, KT898, KT973, etc. This list includes high-power and medium-power devices designed for almost the entire spectrum radio engineering devices. Or you can use the classic one - with two field-effect transistors of the KP501V type connected in parallel - or use devices KP501A...V, KP540 and others with similar electrical characteristics (Fig. 1.24). In this case, the gate output is connected instead of the base VT1, the source output - instead of the emitter VT2, the drain output - instead of the combined collectors VT1, VT2.
Rice. 1.24. Replacement of a composite transistor with field-effect transistors
After such a simple modification, i.e. replacement of components in electrical circuits, universal application, current on transistors VT1, VT2 does not fail even with 10 times or more voltage overload. Moreover, the limiting resistor in the gate circuit VT1 also increases several times. This leads to the fact that they have a higher input and, as a result, withstand overloads due to the pulsed nature of control of this electronic unit.
The current gain of the resulting cascade is at least 50. It increases in direct proportion to the increase in the node supply voltage.
VT1, VT2. In the absence of discrete transistors of the KP501A...B type, you can use the 1014KT1V microcircuit without losing the quality of the device. Unlike, for example, 1014KT1A and 1014KT1B, this one can withstand higher overloads of applied pulse voltage - up to 200 V DC voltage. The pinout for switching on the transistors of the 1014KT1A…1014K1V microcircuit is shown in Fig. 1.25.
Just as in the previous version (Fig. 1.24), they are switched on in parallel.
Pinout of field-effect transistors in the 1014KT1A…V microcircuit
The author has tested dozens of electronic components enabled by . Such nodes are used in amateur radio designs as current switches in the same way as composite transistors switched on. To the above-listed features of field-effect transistors, we can add their energy efficiency, since in the closed state, due to the high input, they consume practically no current. As for the cost of such transistors, today it is almost the same as the cost of medium-power transistors of the type (and similar ones), which are usually used as a current amplifier to control load devices.
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.
The amplifier is called exactly that, not because its author is DARLINGTON, but because the output stage of the power amplifier is built on Darlington (composite) transistors.
For reference
: Two transistors of the same structure are connected in a special way for high gain. This connection of transistors forms a composite transistor, or Darlington transistor - named after the inventor of this circuit design. Such a transistor is used in circuits operating 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. 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 composite transistor is ≈1000 for high-power transistors and ≈50,000 for low-power transistors.
Advantages of the Darlington transistor
High current gain.
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.
Disadvantages of a compound transistor
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.
The forward voltage drop across the base-emitter junction in a Darlington circuit is almost twice as large as in a conventional transistor, and is about 1.2 - 1.4 V for silicon transistors.
High collector-emitter saturation voltage, for a silicon transistor about 0.9 V for low-power transistors and about 2 V for high-power transistors.
Schematic diagram of ULF
The amplifier can be called the cheapest option for building a subwoofer amplifier yourself. The most valuable thing in the circuit is the output transistors, the price of which does not exceed $1. In theory, such an amplifier can be assembled for $3-5 without a power supply. Let's make a small comparison: which microcircuit can provide 100-200 watts of power into a 4 ohm load? Famous people immediately come to mind. But if you compare prices, the Darlington circuit is both cheaper and more powerful than the TDA7294!
The microcircuit itself, without components, costs at least $3, and the price of the active components of a Darlington circuit is no more than $2-2.5! Moreover, the Darlington circuit is 50-70 watts more powerful than the TDA7294!
With a 4 ohm load, the amplifier delivers 150 watts; this is the cheapest and best option for a subwoofer amplifier. The amplifier circuit uses inexpensive rectifier diodes, which can be found in any electronic device.
The amplifier can provide such power due to the fact that composite transistors are used at the output, but if desired, they can be replaced with conventional ones. It is convenient to use the complementary pair KT827/25, but of course the amplifier power will drop to 50-70 watts. In the differential cascade, you can use domestic KT361 or KT3107.
A complete analogue of the TIP41 transistor is our KT819A. This transistor serves to amplify the signal from the differential stages and drive the outputs. Emitter resistors can be used with a power of 2-5 watts; they protect the output stage. Read more about the technical characteristics of the TIP41C transistor. Datasheet for TIP41 and TIP42.
PN Junction Material: Si
Transistor structure: NPN
Limit constant collector power dissipation (Pc) of the transistor: 65 W
Limit constant collector-base voltage (Ucb): 140 V
Limit constant collector-emitter voltage (Uce) of the transistor: 100 V
Limit constant emitter-base voltage (Ueb): 5 V
Limit constant transistor collector current (Ic max): 6 A
Limit temperature of p-n junction (Tj): 150 C
Cutoff frequency of current transfer coefficient (Ft) of the transistor: 3 MHz
- Collector junction capacitance (Cc): pF
Static current transfer coefficient in a common emitter circuit (Hfe), min: 20
Such an amplifier can be used both as a subwoofer and for wideband acoustics. The amplifier's performance is also quite good. With a load of 4 ohms, the output power of the amplifier is about 150 watts, with a load of 8 ohms the power is 100 watts, the maximum power of the amplifier can reach up to 200 watts with a power supply of +/-50 volts.
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
.
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,
