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 n-p-n Darlington transistor type KT825, its current gain is 10,000 (typical value) for a collector current of 10 A.
If you connect the transistors as shown in Fig. 2.60, then the resulting circuit will work as one transistor, and its coefficient β will be equal to the product of the coefficients β components of transistors.
Rice. 2.60. Composite transistor Darlington .
This technique is useful for circuits that handle high currents (such as voltage regulators or power amplifier output stages) or for amplifier input stages that require high input impedance.
In a Darlington transistor, the voltage drop between base and emitter is twice the normal voltage, and the saturation voltage is at least equal to the voltage drop across the diode (since the transistor's emitter potential T 1 must exceed the transistor emitter potential T 2 by the voltage drop across the diode). In addition, transistors connected in this way behave like one transistor with a fairly low speed, since the transistor T 1 cannot quickly turn off the transistor T 2. Given this property, it is usually between the base and emitter of the transistor T 2 turn on the resistor (Fig. 2.61).
Rice. 2.61. Increasing the turn-off speed in a composite Darlington transistor.
Resistor R prevents transistor bias T 2 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 so that a current flows through it that is small compared to base current of the transistor T 2. Usually resistance R is several hundred ohms in a high-power Darlington transistor and several thousand ohms in a small-signal 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 scheme is the powerful n‑р‑n The Darlington transistor is a 2N6282 type, its current gain is 4000 (typical) for a collector current of 10 A.
Connecting transistors according to the Sziklai scheme (Sziklai). The connection of transistors according to the Sziklai circuit is a circuit similar to the one we just looked at. It also provides an increase in the coefficient β . Sometimes such a connection is called a complementary Darlington transistor (Fig. 2.62).
Rice. 2.62 . Connecting transistors according to the diagram Siklai(“complementary Darlington transistor”).
The circuit behaves like a transistor n‑р‑n‑ type with a large coefficient β . 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. Between the base and emitter of the transistor T 2 It is recommended to include a resistor with a small resistance. 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.
Rice. 2.63. A powerful push-pull cascade that uses only output transistors n‑р‑n-type.
As before, the resistor is the collector resistor of the transistor T 1. Darlington transistor formed by transistors T 2 And T 3, behaves like a single transistor n‑р‑n‑type, with a large current gain. Transistors T 4 And T 5, connected according to the Sziklai circuit, behave like a powerful transistor p‑n‑p‑ type with high gain. As before, resistors R 3 And R 4 have little 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 the Darlington circuit.
Transistor with ultra-high current gain. Composite transistors - Darlington transistors and the like - should not be confused with ultra-high current gain transistors, which have a very high gain h 21E obtained during the technological process of manufacturing an element. An example of such an element is the 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 range of maximum voltages U CE from 30 to 60 V (if the collector voltage should be higher, then you should reduce the value β ). The industry produces matched pairs of transistors with ultra-high coefficient values β . They are used in low-signal amplifiers for which the transistors must have matched characteristics; dedicated to this issue section 2.18. Examples of such standard circuits are circuits such as LM394 and MAT-01; they are high-gain transistor pairs in which the voltage U BE matched to fractions of a millivolt (the best circuits provide matching up to 50 μV), and the coefficient h 21E– up to 1%. The MAT-03 type circuit is a matched pair p‑n‑p- transistors.
Ultra-high ratio transistors β can be combined according to 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.
Tracking link
When setting the bias voltage, for example in an emitter follower, the divider resistors in the base circuit are selected so that the divider in relation to the base acts as a hard voltage source, that is, so that the resistance of parallel-connected resistors is significantly less than the input resistance of the circuit on the side bases. In this regard, the input resistance of the entire circuit is determined by the voltage divider - for a signal arriving at its input, the input resistance turns out to be much less than is really necessary. In Fig. Figure 2.64 shows a corresponding example.
Rice. 2.64.
The input impedance of the circuit is approximately 9 kΩ, and the voltage divider resistance for the input signal is 10 kΩ. It is desirable that the input resistance be always high, and in any case it is unwise to load the input signal source of the circuit with a divider, which is ultimately needed only to provide bias to the transistor. The tracking communication method allows you to get out of this difficulty (Fig. 2.65).
Rice. 2.65. Increasing the input impedance of the emitter follower at signal frequencies by including a divider in the tracking circuit, which provides a base bias.
Transistor bias is provided by resistors R1, R2, R3. Capacitor C 2 is chosen such that its total resistance at signal frequencies is small compared to the resistance of the bias resistors. As always, the bias will be stable if the DC resistance of its source given in the base (in this case 9.7 kOhm) is significantly less than the DC resistance from the base (in this case ~ 100 kOhm). But here the input resistance for signal frequencies is not equal to the DC resistance.
Consider the signal path: input signal U in generates a signal at the emitter u E ~= u in, so the increment of current flowing through the bias resistor R 3, will be i = (u in – u E)/R 3~= 0, i.e. Z in = u in /i input) ~=
We found that the input (shunt) resistance of the bias circuit is very high for signal frequencies .
Another approach to circuit analysis is based on the fact that the voltage drop across a resistor R 3 for all frequencies of the signal is the same (since the voltage between its terminals changes equally), i.e. it is a current source. But the resistance of the current source is infinite. In fact, the actual value of the resistance is not infinite, since the follower gain is slightly less than 1. This is caused by the fact that the voltage drop between base and emitter depends on the collector current, which changes as the signal level changes. The same result can be obtained if we consider the divider formed by the output resistance on the emitter side [ r E = 25/I K(mA) Ohm] and emitter resistor. If the voltage gain of the repeater is denoted A (A~= 1), then the effective resistance value R 3 at signal frequencies equals R 3 /(1 – A). In practice, the effective value of resistance R 3 is approximately 100 times larger than its nominal value, and the input resistance is dominated by the input resistance of the transistor on the base side. In a common emitter inverting amplifier, a similar tracking connection can be made, since the signal at the emitter follows the signal at the base. Note that the bias voltage divider circuit is AC powered (at signal frequencies) from the low-impedance emitter output, so the input signal does not have to do this.
Servo connection in collector load. The servo coupling principle can be used to increase the effective resistance of the collector load resistor if the cascade is loaded onto a repeater. In this case, the voltage gain of the cascade will significantly increase [recall that K U = – g m R K, A g m = 1/(R 3 + r E)]·
In Fig. Figure 2.66 shows an example of a push-pull output stage with a servo link, built similar to the push-pull repeater circuit discussed above.
Rice. 2.66. Servo coupling in the collector load of a power amplifier, which is a loading stage.
Since the output repeats the signal based on the transistor T 2, capacitor WITH creates a tracking connection into the collector load of the transistor T 1 and maintains a constant voltage drop across the resistor R 2 in the presence of a signal (capacitor impedance WITH should be small compared to R 1 And R 2 over the entire signal frequency band). Thanks to this, the resistor R 2 becomes similar to a current source, the gain of the transistor increases T 1 voltage and maintains sufficient voltage at the base of the transistor T 2 even at peak signal values. When the signal gets close to the supply voltage U QC potential at the resistor connection point R 1 And R 2 becomes more than U QC, thanks to the charge accumulated by the capacitor WITH. Moreover, if R 1 = R 2(a good option for choosing resistors), then the potential at the point of their connection will exceed U QC 1.5 times at the moment when the output signal becomes equal U QC. This circuit has become very popular in the design of low-frequency household amplifiers, although a simple current source has advantages over a servo circuit in that it eliminates the need for an undesirable element - an electrolytic capacitor - and provides better low-frequency performance.
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.
If you open any book on electronic technology, you will immediately see how many elements are named after their creators: Schottky diode, Zener diode (also known as a zener diode), Gunn diode, Darlington transistor.
Electrical engineer Sidney Darlington experimented with brushed DC motors and their control circuits. The circuits used current amplifiers.
Engineer Darlington invented and patented a transistor consisting of two bipolar ones and made on a single silicon crystal with diffused n(negative) and p(positive) transitions. A new semiconductor device was named after him.
In the domestic technical literature, a Darlington transistor is called composite. So, let's get to know him better!
The device of a composite transistor.
As already mentioned, these are two or more transistors manufactured on one semiconductor chip and packaged in one common package. There is also a load resistor in the emitter circuit of the first transistor.
The Darlington transistor has the same terminals as the familiar bipolar transistor: Base, Emitter and Collector.
Darlington circuit
As you can see, such a transistor is a combination of several. Depending on the power, it may contain more than two bipolar transistors. It is worth noting that in high-voltage electronics a transistor consisting of a bipolar and a field-effect transistor is also used. This is an IGBT transistor. It can also be classified as a composite, hybrid semiconductor device.
Main features of the Darlington transistor.
The main advantage of a composite transistor is its high current gain.
It is worth recalling one of the main parameters of a bipolar transistor. This is the gain ( h 21). It is also denoted by the letter β (“beta”) of the Greek alphabet. It is always greater than or equal to 1. If the gain of the first transistor is 120, and the second is 60, then the gain of the composite is already equal to the product of these values, that is, 7200, and this is very good. As a result, a very small base current is enough to turn the transistor on.
Engineer Sziklai slightly modified the Darlington connection and obtained a transistor, which was called a complementary Darlington transistor. Let us remember that a complementary pair is two elements with absolutely identical electrical parameters, but different conductivities. Such a pair at one time were KT315 and KT361. Unlike the Darlington transistor, a composite transistor according to the Sziklai circuit is assembled from bipolar ones of different conductivities: p-n-p And n-p-n. Here is an example of a compound transistor according to the Sziklai circuit, which works like an npn transistor, although it consists of two different structures.
Siklai scheme
The disadvantages of composite transistors include low performance, therefore they are widely used only in low-frequency circuits. Such transistors have proven themselves to be excellent in the output stages of powerful low-frequency amplifiers, in electric motor control circuits, and in switches of electronic car ignition circuits.
Main electrical parameters:
Collector – emitter voltage 500 V;
Emitter – base voltage 5 V;
Collector current – 15 A;
Maximum collector current – 30 A;
Power dissipation at 25 0 C – 135 W;
Crystal (transition) temperature – 175 0 C.
On the circuit diagrams there is no special symbol to indicate composite transistors. In the vast majority of cases, it is designated on the diagram as a regular transistor. Although there are exceptions. Here is one of its possible designations on a circuit diagram.
Let me remind you that a Darlington assembly can have either a p-n-p structure or an n-p-n structure. In this regard, manufacturers of electronic components produce complementary pairs. These include the TIP120-127 and MJ11028-33 series. For example, transistors TIP120, TIP121, TIP122 have the structure n-p-n, and TIP125, TIP126, TIP127 - p-n-p.
You can also find this designation on circuit diagrams.
Examples of applications of a composite transistor.
Let's consider a control circuit for a commutator motor using a Darlington transistor.
When a current of about 1 mA is supplied to the base of the first transistor, a current of 1000 times more, that is, 1000 mA, will flow through its collector. It turns out that the simple circuit has a decent gain. Instead of a motor, you can connect an electric light bulb or a relay, with which you can switch powerful loads.
If instead of the Darlington assembly we use the Sziklai assembly, then the load is connected to the emitter circuit of the second transistor and is connected not to the plus, but to the minus of the power supply.
If you combine a Darlington transistor and a Sziklai assembly, you get a push-pull current amplifier. It is called push-pull because at a particular moment in time only one of the two transistors, the upper or the lower, can be open. This circuit inverts the input signal, that is, the output voltage will be the opposite of the input voltage.
This is not always convenient, and therefore another inverter is added at the input of the push-pull current amplifier. In this case, the output signal exactly repeats the input signal.
Application of Darlington assembly in microcircuits.
Integrated circuits containing several composite transistors are widely used. One of the most common is the L293D integrated assembly. It is often used by robotics enthusiasts in their homemade projects. The L293D microcircuit is four current amplifiers in a common housing. Since in the push-pull amplifier discussed above only one transistor is always open, the output of the amplifier is alternately connected to either the plus or minus of the power source. This depends on the input voltage. In essence, we have an electronic key. That is, the L293 chip can be defined as four electronic keys.
Here is a “piece” of the output stage diagram of the L293D microcircuit, taken from its datasheet (reference sheet).
As you can see, the output stage consists of a combination of Darlington and Szyklai circuits. The upper part of the circuit is a composite transistor according to the Sziklai circuit, and the lower part is made according to the Darlington circuit.
Many people remember the times when there were VCRs instead of DVD players. And with the help of the L293 chip, two electric motors of the VCR were controlled, and in full-function mode. For each motor, it was possible to control not only the direction of rotation, but by sending signals from the PWM controller, it was possible to control the rotation speed within large limits.
Specialized microcircuits based on the Darlington circuit have also been widely used. An example is the ULN2003A microcircuit (analogous to K1109KT22). This integrated circuit is an array of seven Darlington transistors. Such universal assemblies can be easily used in amateur radio circuits, for example, radio-controlled relays. This is what I'm talking about.
Literally immediately after the appearance of semiconductor devices, say, transistors, they rapidly began to displace electric vacuum devices and, in particular, triodes. Currently, transistors occupy a leading position in circuit technology.
A beginner, and sometimes even an experienced amateur radio designer, does not immediately manage to find the desired circuit solution or understand the purpose of certain elements in the circuit. Having at hand a set of “bricks” with known properties, it is much easier to build the “building” of one or another device.
Without dwelling in detail on the parameters of the transistor (enough has been written about this in modern literature, for example, in), we will consider only individual properties and ways to improve them.
One of the first problems that a developer faces is increasing the power of the transistor. It can be solved by connecting transistors in parallel (). Current equalizing resistors in the emitter circuits help distribute the load evenly.
It turns out that connecting transistors in parallel is useful not only for increasing power when amplifying large signals, but also for reducing noise when amplifying weak ones. The noise level decreases in proportion to the square root of the number of transistors connected in parallel.
Overcurrent protection is most easily solved by introducing an additional transistor (). The disadvantage of such a self-protecting transistor is a decrease in efficiency due to the presence of a current sensor R. A possible improvement option is shown in. Thanks to the introduction of a germanium diode or Schottky diode, it is possible to reduce the value of the resistor R several times, and therefore the power dissipated on it.
To protect against reverse voltage, a diode is usually connected parallel to the emitter-collector terminals, as, for example, in composite transistors such as KT825, KT827.
When the transistor is operating in switching mode, when it is required to quickly switch from open to closed state and back, sometimes a forcing RC circuit () is used. At the moment the transistor opens, the capacitor charge increases its base current, which helps reduce the turn-on time. The voltage across the capacitor reaches the voltage drop across the base resistor caused by the base current. At the moment the transistor closes, the capacitor, discharging, promotes the resorption of minority carriers in the base, reducing the turn-off time.
You can increase the transconductance of the transistor (the ratio of the change in the collector (drain) current to the change in voltage at the base (gate) that caused it at a constant Uke Usi)) using a Darlington circuit (). A resistor in the base circuit of the second transistor (may be missing) is used to set the collector current of the first transistor. A similar composite transistor with high input resistance (due to the use of a field-effect transistor) is presented in. Composite transistors shown in Fig. and , are assembled on transistors of different conductivity according to the Szyklai circuit.
Introduction of additional transistors into Darlington and Sziklai circuits, as shown in Fig. and, increases the input resistance of the second stage for alternating current and, accordingly, the transmission coefficient. Application of a similar solution in transistors Fig. and gives the circuits and respectively, linearizing the transconductance of the transistor.
A high-speed wideband transistor is presented at. Increased performance was achieved as a result of reducing the Miller effect in a similar way.
The "diamond" transistor according to the German patent is presented at. Possible options for enabling it are shown on. A characteristic feature of this transistor is the absence of inversion at the collector. Hence the doubling of the circuit's load capacity.
A powerful composite transistor with a saturation voltage of about 1.5 V is shown in Fig. 24. The power of the transistor can be significantly increased by replacing the VT3 transistor with a composite transistor ().
Similar reasoning can be made for a p-n-p type transistor, as well as a field-effect transistor with a p-type channel. When using a transistor as a regulating element or in switching mode, two options are possible for connecting the load: in the collector circuit () or in the emitter circuit ().
As can be seen from the above formulas, the lowest voltage drop, and accordingly the minimum power dissipation, is on a simple transistor with a load in the collector circuit. The use of a composite Darlington and Szyklai transistor with a load in the collector circuit is equivalent. A Darlington transistor may have an advantage if the collectors of the transistors are not combined. When a load is connected to the emitter circuit, the advantage of the Siklai transistor is obvious.
Literature:
1. Stepanenko I. Fundamentals of the theory of transistors and transistor circuits. - M.: Energy, 1977.
2. US Patent 4633100: Publ. 20-133-83.
3. A.s. 810093.
4. US Patent 4,730,124: Pub. 22-133-88. - P.47.
1. Increasing the transistor power.
Resistors in the emitter circuits are needed to distribute the load evenly; The noise level decreases in proportion to the square root of the number of transistors connected in parallel.
2. Overcurrent protection.
The disadvantage is a decrease in efficiency due to the presence of a current sensor R.
Another option is that thanks to the introduction of a germanium diode or a Schottky diode, the value of the resistor R can be reduced several times, and less power will be dissipated on it.
3. Composite transistor with high output resistance.
Due to the cascode connection of transistors, the Miller effect is significantly reduced.
Another circuit - due to the complete decoupling of the second transistor from the input and supplying the drain of the first transistor with a voltage proportional to the input, the composite transistor has even higher dynamic characteristics (the only condition is that the second transistor must have a higher cutoff voltage). The input transistor can be replaced with a bipolar one.
4. Protection of the transistor from deep saturation.
Preventing forward bias of the base-collector junction using a Schottky diode.
A more complex option is the Baker scheme. When the transistor collector voltage reaches the base voltage, the “excess” base current is dumped through the collector junction, preventing saturation.
5. Saturation limitation circuit for relatively low-voltage switches.
With base current sensor.
With collector current sensor.
6. Reducing the on/off time of the transistor by using a forcing RC chain.
7. Composite transistor.
Darlington diagram.
Siklai scheme.
