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.
"There is safety in numbers". This is how we can symbolically characterize single-transistor switches. Naturally, it is much easier to solve problems when paired with others like yourself. The introduction of a second transistor makes it possible to reduce the requirements for the spread and the magnitude of the transmission coefficient A 2 1e. Two-transistor switches are widely used for switching high voltages, as well as for passing a large current through the load.
In Fig. 2.68, a...y shows diagrams for connecting two-transistor switches on bipolar transistors to MK.
Rice. 2.68. Connection diagrams for two-transistor switches on bipolar transistors (beginning):
a) transistor VT1 serves as an emitter follower. It amplifies the current and, through limiting resistor R2, supplies it to the base of transistor VT2, which directly controls the load RH;
b) transistors K77, VT2 are connected according to the Darlington circuit (another name is “composite transistor”). The total gain is equal to the product of the transmission coefficients L 21E of both transistors. Transistor VT1 is usually installed with low power and higher frequency than VT2. Resistor R1 determines the degree of saturation of the “pair”. The resistance of resistor R2 is selected in inverse proportion to the current in the load: from several hundred ohms to tens of kiloohms;
c) D. Boxtel’s scheme. The Schottky diode VD1 accelerates the turn-off of the powerful transistor VT2, increasing by 2...3 times the steepness of the signal edges at a frequency of 100 kHz. This eliminates the main disadvantage of circuits with Darlington transistors - low performance;
d) similar to Fig. 2.68, a, but transistor VT1 opens when the MK line is switched to the input mode with a Z-state or an input with an internal “pull-up” resistor. In this regard, the current load on the port line is reduced, but the efficiency is reduced due to the dissipation of additional power on resistor R1 at a LOW level at the MK output;
e) “self-protected switch” on the power transistor VT2 and the limiting transistor VT1 As soon as the current in the load Ln exceeds a certain threshold, for example, due to an accident or short circuit, a voltage sufficient to open the transistor VT1 is released on the resistor R3. It shunts the base junction transistor VT2, causing output current limitation;
f) push-pull pulse amplifier using transistors of different structures; ABOUT
g) transistor I72 opens with a relatively short time delay (R2, VD1, C7), and closes with a relatively large time delay (C7, R3, VT1)\
h) a high-voltage switch providing pulse edges of 0.1 MK s at a repetition rate of up to 1 MHz. In the initial state, transistor VT1 is open and GT2 is closed. During the pulse, transistor VT1 opens and load capacitance 7 is quickly discharged through it? n. Diode VD1 prevents the flow of through currents through transistors VT1, VT2\
i) the composite emitter follower on transistors VT1, GT2 has an extremely high current gain. Resistor 7?2 is guaranteed to close the transistors at a LOW level at the MK output;
j) transistor VT1 in the open state blocks transistor VT2. Resistor R1 serves as a collector load for transistor VT1 and a base current limiter for transistor VT2\ l) a powerful push-pull cascade with a buffer logic chip 7)7)7, which has open-collector outputs. The signals from the two MK lines must be out of phase. Resistors R5, 7?6 limit the currents in the load connected to the 6-out circuit; ABOUT
m) key for load Ln, which is connected to a negative voltage source. Transistor VT1 serves as an emitter follower, and transistor VT2 serves as an amplifier with a common base. The maximum load current is determined by the formula / n [mA] = 3.7 / L, [kOhm]. Diode VDJ protects transistor VT2 from power reversal.
n) a switch on transistors of different structures. Resistor R1 determines the current in the load RH, but it must be selected carefully so as not to exceed the base current of transistor VT2 when transistor VT1 is fully open. The circuit is critical to the transfer coefficients of both transistors;
o) similar to Fig. 2.68, n, but transistor VT1 is used as a switch, and not as a variable resistance. The load current is set by resistor R4. Resistor R5 limits the initial starting current of transistor VT2 with a large capacitive component of the load RH. The circuit is not critical to the transmission coefficients of the transistors. If a KT825 “superba” transistor is used as K72, then the resistance of R4 should be increased to 5.1 ... 10 kOhm;
n) a practical example of switching a high-voltage voltage of 170 V at a low load current with a resistance RH of at least 27 kOhm;
p) similar to Fig. 2.68, n, but with an active LOW level at the MK output; ABOUT
About Fig. 2.68. Connection diagrams for two-transistor switches on bipolar transistors (end):
c) transistors VT1 and kT2 operate in antiphase. Voltage is supplied to the load Ln through transistor VT2 and diode VD1, while transistor VT1 must be closed at a HIGH level from the upper output of MK. To remove voltage from the load, transistor G72 is closed at a HIGH level from the lower output of MK, after which transistor VT1 opens and through diode VD2 rapidly discharges the load capacitance. The advantage is high performance, the ability to quickly re-apply voltage to the load;
t) the MK is supplied with “weighted” and filtered power in the range of 4...4.5 V. This is provided by the damping zener diode VD1 and the noise suppression capacitor C1. At a HIGH level at the output of the MK, transistors K77, G72 are closed, at a LOW level they are open. The maximum permissible current of the zener diode VD1 must be such that it is greater than the sum of the current consumption of MK, the current through resistor R1 at a LOW level at the output of MK and the current of external circuits if they are connected to MK via other port lines;
y) video amplifier on transistors VT1 and VT2, which are connected according to the Sziklai circuit. This is a type of Darlington circuit, but with transistors of different conductivities. This “pair” is equivalent to one transistor of the p-p-p structure with an ultra-high gain L21E. Diodes VD1, KD2 protect transistors from voltage surges penetrating from the outside along the OUT circuit. Resistor R1 limits the current in the event of an accidental short circuit in the cable connected to an external remote load of 75 Ohms.
The basic logical element of the series is the AND-NOT logical element. In Fig. Figure 2.3 shows diagrams of the three initial NAND TTL elements. All circuits contain three main stages: transistor input VT1, implementing the logical AND function; phase separating transistor VT2 and a push-pull output stage.
Fig 2.3.a. Schematic diagram of the basic element of the K131 series
The operating principle of the logical element of the K131 series (Fig. 2.3.a) is as follows: when a low-level signal (0 - 0.4V) is received at any of the inputs, the base-emitter junction of the multi-emitter transistor VT1 is forward-biased (unlocked), and almost the entire the current flowing through resistor R1 is branched to ground, as a result of which VT2 closes and operates in cutoff mode. The current flowing through resistor R2 saturates the base of transistor VT3. Transistors VT3 and VT4 connected according to the Darlington circuit form a composite transistor, which is an emitter follower. It functions as an output stage to amplify the signal power. A high logic level signal is generated at the output of the circuit.
If a high-level signal is supplied to all inputs, the base-emitter junction of the multi-emitter transistor VT1 is in closed mode. The current flowing through resistor R1 saturates the base of transistor VT1, as a result of which transistor VT5 is unlocked and a logical zero level is set at the output of the circuit.
Since at the moment of switching transistors VT4 and VT5 are open and a large current flows through them, a limiting resistor R5 is introduced into the circuit.
VT2, R2 and R3 form a phase separating cascade. It is necessary to turn on the output n-p-n transistors one by one. The cascade has two outputs: collector and emitter, the signals on which are antiphase.
Diodes VD1 - VD3 are protection against negative impulses.
Fig 2.3.b, c. Schematic diagrams of the basic elements of the K155 and K134 series
In microcircuits of the K155 and K134 series, the output stage is built on a non-composite repeater (only a transistor VT3) and a saturable transistor VT5 with the introduction of a level shift diode VD4(Fig. 2.3, b, c). The last two stages form a complex inverter that implements the logical NOT operation. If you introduce two phase separating stages, then the OR-NOT function is implemented.
In Fig. 2.3, and shows the basic logical element of the K131 series (foreign analogue - 74N). The basic element of the K155 series (foreign analogue - 74) is shown in Fig. 2.3, b, a in Fig. 2.3, c - element of the K134 series (foreign analogue - 74L). Now these series are practically not developed.
TTL microcircuits of the initial development began to be actively replaced by TTLSh microcircuits, which have junctions with a Schottky barrier in their internal structure. The Schottky junction transistor (Schottky transistor) is based on the well-known circuit of an unsaturated transistor switch (Fig. 2.4.a).
Figure 2.4. Explanation of the principle of obtaining a structure with a Schottky transition:
a - unsaturated transistor switch; b - transistor with a Schottky diode; c - symbol of the Schottky transistor.
To prevent the transistor from entering saturation, a diode is connected between the collector and the base. The use of a feedback diode to eliminate transistor saturation was first proposed by B. N. Kononov. However, in this case it can increase to 1 V. The ideal diode is a Schottky barrier diode. It is a contact formed between a metal and a lightly doped n-semiconductor. In a metal, only some of the electrons are free (those outside the valence zone). In a semiconductor, free electrons exist at the conduction boundary created by the addition of impurity atoms. In the absence of bias voltage, the number of electrons crossing the barrier on both sides is the same, i.e., there is no current. When forward biased, electrons have the energy to cross the potential barrier and pass into the metal. As the bias voltage increases, the barrier width decreases and the forward current increases rapidly.
When reverse biased, electrons in a semiconductor require more energy to overcome the potential barrier. For electrons in a metal, the potential barrier does not depend on the bias voltage, so a small reverse current flows, which remains practically constant until an avalanche breakdown occurs.
The current in Schottky diodes is determined by the majority carriers, so it is greater at the same forward bias and, therefore, the forward voltage drop across the Schottky diode is less than at a conventional p-n junction at a given current. Thus, the Schottky diode has a threshold opening voltage of the order of (0.2-0.3) V, in contrast to the threshold voltage of a conventional silicon diode of 0.7 V, and significantly reduces the lifetime of minority carriers in the semiconductor.
In the diagram of Fig. 2.4, b transistor VT1 is kept from going into saturation by a Shatky diode with a low opening threshold (0.2...0.3) V, so the voltage will increase slightly compared to a saturated transistor VT1. In Fig. 2.4, c shows a circuit with a “Schottky transistor”. Based on Schottky transistors, microcircuits of two main TTLSh series were produced (Fig. 2.5)
In Fig. 2.5, and shows a diagram of a high-speed logic element used as the basis of microcircuits of the K531 series (foreign analogue - 74S), (S is the initial letter of the surname of the German physicist Schottky). In this element, the emitter circuit of a phase separating cascade made on a transistor VT2, the current generator is turned on - transistor VT6 with resistors R4 And R5. This allows you to increase the performance of the logic element. Otherwise, this logical element is similar to the basic element of the K131 series. However, the introduction of Schottky transistors made it possible to reduce tzd.r doubled.
In Fig. 2.5, b shows a diagram of the basic logical element of the K555 series (foreign analogue - 74LS). In this circuit, instead of a multi-emitter transistor, a matrix of Schottky diodes is used at the input. The introduction of Shatky diodes eliminates the accumulation of excess base charges, which increase the turn-off time of the transistor, and ensures the stability of the switching time over a temperature range.
Resistor R6 of the upper arm of the output stage creates the necessary voltage at the base of the transistor VT3 to open it. To reduce power consumption when the gate is closed (), a resistor R6 connect not to the common bus, but to the output of the element.
Diode VD7, connected in series with R6 and parallel to the collector load resistor of the phase separating cascade R2, allows you to reduce the turn-on delay of the circuit by using part of the energy stored in the load capacitance to increase the transistor collector current VT1 in transition mode.
Transistor VT3 is implemented without Schottky diodes, since it operates in active mode (emitter follower).
In Fig. Figure 2.16 shows a diagram of a logic element with an induced channel of type n (the so-called n MIS technology). The main transistors VT 1 and VT 2 are connected in series, transistor VT 3 acts as a load. In the case when a high voltage U 1 is applied at both inputs of the element (x 1 = 1, x 2 = 1), both transistors VT 1 and VT 2 are open and a low voltage U 0 is set at the output. In all other cases, at least one of the transistors VT 1 or VT 2 is closed and the voltage U 1 is set at the output. Thus, the element performs the logical AND-NOT function.
In Fig. Figure 2.17 shows a diagram of the OR-NOT element. A low voltage U 0 is set at its output if at least one of the inputs has a high voltage U 1 , opening one of the main transistors VT 1 and VT 2 .
Shown in Fig. 2.18 diagram is a diagram of the NOR-NOT element of the KMDP technology. In it, transistors VT 1 and VT 2 are the main ones, transistors VT 3 and VT 4 are the load ones. Let high voltage U 1. In this case, transistor VT 2 is open, transistor VT 4 is closed and, regardless of the voltage level at the other input and the state of the remaining transistors, a low voltage U 0 is set at the output. The element implements the logical OR-NOT operation.
The CMPD circuit is characterized by very low current consumption (and therefore power) from power supplies.
Logic elements of integral injection logic
In Fig. Figure 2.19 shows the topology of the logical element of the integral injection logic (I 2 L). To create such a structure, two phases of diffusion in silicon with n-type conductivity are required: during the first phase, regions p 1 and p 2 are formed, and during the second phase, regions n 2 are formed.
The element has the structure p 1 -n 1 -p 2 -n 1 . It is convenient to consider such a four-layer structure by imagining it as a connection of two conventional three-layer transistor structures:
p 1 -n 1 -p 2 n 1 -p 2 -n 1
The diagram corresponding to this representation is shown in Fig. 2.20, a. Let's consider the operation of the element according to this scheme.
Transistor VT 2 with a structure of type n 1 -p 2 -n 1 performs the functions of an inverter with several outputs (each collector forms a separate output of an element according to an open collector circuit).
Transistor VT 2, called injector, has a structure like p 1 -n 1 -p 2 . Since the area n 1 of these transistors is common, the emitter of transistor VT 2 must be connected to the base of transistor VT 1; the presence of a common area p 2 leads to the need to connect the base of transistor VT 2 with the collector of transistor VT 1. This creates a connection between transistors VT 1 and VT 2, shown in Fig. 2.20a.
Since the emitter of transistor VT 1 has a positive potential and the base is at zero potential, the emitter junction is forward biased and the transistor is open.
The collector current of this transistor can be closed either through transistor VT 3 (inverter of the previous element) or through the emitter junction of transistor VT 2.
If the previous logical element is in the open state (transistor VT 3 is open), then at the input of this element there is a low voltage level, which, acting on the basis of VT 2, keeps this transistor in the closed state. The injector current VT 1 is closed through the transistor VT 3. When the previous logic element is closed (transistor VT 3 is closed), the collector current of the injector VT 1 flows into the base of the transistor VT 2, and this transistor is set to the open state.
Thus, when VT 3 is closed, transistor VT 2 is open and, conversely, when VT 3 is open, transistor VT 2 is closed. The open state of the element corresponds to the log.0 state, and the closed state corresponds to the log.1 state.
The injector is a source of direct current (which can be common to a group of elements). Often they use the conventional graphic designation of an element, presented in Fig. 2.21, b.
In Fig. Figure 2.21a shows a circuit that implements the OR-NOT operation. The connection of element collectors corresponds to the operation of the so-called installation I. Indeed, it is enough that at least one of the elements is in the open state (log.0 state), then the injector current of the next element will be closed through the open inverter and a low log.0 level will be established at the combined output of the elements. Consequently, at this output a value is formed corresponding to the logical expression x 1 · x 2. Applying the de Morgan transformation to it leads to the expression x 1 · x 2 = . Therefore, this connection of elements really implements the OR-NOT operation.
Logic elements AND 2 L have the following advantages:
provide a high degree of integration; in the manufacture of I 2 L circuits, the same technological processes are used as in the production of integrated circuits on bipolar transistors, but the number of technological operations and the necessary photomasks is smaller;
a reduced voltage is used (about 1V);
provide the ability to exchange power over a wide range of performance (power consumption can be changed by several orders of magnitude, which will correspondingly lead to a change in performance);
are in good agreement with TTL elements.
In Fig. Figure 2.21b shows a diagram of the transition from the I 2 L elements to the TTL element.
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.
