Electronic key on a transistor - operating principle and circuit. Transistor switch circuit and operation.

Electrical measuring instruments
07.07.2019

The transistor switch is the main component in pulse converter technology. In everyone's schemes pulse sources nutrition, which almost completely replaced transformer sources power supply, transistor switches are used. An example of such power sources are computer blocks power supplies, chargers for phones, laptops, tablets, etc. Transistor keys have replaced electromagnetic relays because they have the main advantage of the absence of mechanical moving parts, which results in increased reliability and durability of the key. In addition, the speed of switching on and off electronic semiconductor switches is much higher than the speed of electromagnetic relays.

Also, a transistor switch is often used to turn on/off (switch) a load of significant power based on a signal from the microcontroller.

The essence of the electronic key is to control it with high power using a low power signal.

There are semiconductor switches based on transistors, thyristors, and triacs. However, this article discusses the operation of an electronic switch based on a bipolar transistor. In subsequent articles, other types of semiconductor switches will be discussed.

Depending on the semiconductor structure, bipolar transistors are divided into two types: p — n — p And n — p — n type ( rice. 1 ).

Rice. 1 – Structures of bipolar transistors

In circuit diagrams, bipolar transistors are designated as shown in rice. 2 . The middle pin is called the base, the pin with the “arrow” is the emitter, and the remaining pin is the collector.


Rice. 2 – Designation of transistors in circuits

Also, transistors can be conventionally depicted as two diodes that are connected back-to-back; the place where they are connected will always be the base ( Fig.3 ).

Rice. 3 – Diagrams for replacing transistors with diodes

Transistor switch. Switching schemes.

Connection circuits for transistors of different semiconductor structures are shown in rice. 4 . The junction between base and emitter is called emitter junction, and the junction between base and collector is called collector junction. To turn on (open) the transistor, it is necessary that the collector junction be biased in the opposite direction, and the emitter – in the forward direction.


Rice. 4 – Transistor switch. Connection schemes

Power supply voltage U SP applied to the collector and emitter terminals U ke via load resistor R To (cm. rice. 4 ). Control voltage (control signal) is applied between base and emitter U bae through a current limiting resistor R b .

When a transistor operates in switching mode, it can be in two states. The first is the cutoff mode. In this mode, the transistor is completely closed, and the voltage between the collector and emitter is equal to the voltage of the power source. The second state is the saturation mode. In this mode, the transistor is fully open, and the voltage between the collector and emitter is equal to the voltage drop across p — n – transitions and for various transistors is in the range from hundredths to tenths of volts.

On the load line of the input static characteristic of the transistor ( rice. 5 ) the saturation region is on the segment 1-2 , and the cutoff area on the segment 3-4 . The intermediate area between these segments is the area 2-3 called the active region. It is used when the transistor operates in amplifier mode.


Rice. 5 – Input static characteristic of the transistor

To make it easier to remember the polarity of the power supply connection and the control signal voltage, you should pay attention to the emitter arrow. It indicates the direction of current flow ( Fig.6 ).


Rice. 6 – Path of current flow through the transistor switch

Calculation of transistor switch parameters

For an example of how a key works, we will use an LED as a load. Its connection diagram is shown in rice. 7 . Pay attention to the polarity of connecting power supplies and LEDs in transistors of different semiconductor structures.


Rice. 7 – Diagrams for connecting LEDs to transistor switches

Let's calculate the main parameters of a transistor switch made on a transistor n — p — n type. Let us have the following initial data:

- voltage drop across the LED Δ UVD = 2 V;

— rated current of the LED IVD= 10 mA;

- power supply voltage USP(indicated on the diagram by Uke) = 9 V;

— input signal voltage USun= 1.6 V.

Now let's take another look at the diagram shown in rice. 7 . As we can see, it remains to determine the resistance of the resistors in the base and collector circuits. The transistor can be selected from any bipolar corresponding semiconductor structure. Let's take the Soviet transistor as an example. n — p — n type MP111B.

Calculation of resistance in the transistor collector circuit

The resistance in the collector circuit is designed to limit the current that flows through the LED VD , as well as to protect the transistor itself from overload. Because when the transistor opens, the current in its circuit will be limited only by the resistance of the LED VD and resistor R To .

Let's determine the resistance R To . It is equal to the voltage drop across it Δ U R To divided by the current in the collector circuit I To :


So we initially set the collector - this is the rated current of the LED. It should not exceed I k=10mA .

Now let's find the voltage drop across the resistor R To . It is equal to the voltage of the power supply U SP (U ke ) minus the voltage drop across the LED Δ U VD and minus the voltage drop across the transistor ΔU ke :

The voltage drop across the LED, as well as the voltage of the power supply, are initially set and equal to 0.2V and 9V, respectively. The voltage drop for the MP111B transistor, as well as for other Soviet transistors, is taken to be about 0.2 V. For modern transistors (for example BC547, BC549, N2222 and others), the voltage drop is about 0.05 V and lower.

The voltage drop across the transistor can be measured when it is fully open, between the collector and emitter terminals, and the calculation can be adjusted later. But, as we will see later, the collector resistance can be selected using a simpler method.

The resistance in the collector circuit is:

Calculation of resistance in the transistor base circuit

Now we just need to determine the base resistance R b . It is equal to the voltage drop across the resistance itself ΔURb divided by base current I b :


The voltage drop across the base of the transistor is equal to the input signal voltage Uvs minus the voltage drop across the base-emitter junction ΔUbe . The input signal voltage is specified in the source data and is equal to 1.6 V. The voltage drop between the base and emitter is about 0.6 V.

Next we find the base current Ib . It is equal to the collector current Ib divided by the transistor current gain β . The gain for each transistor is given in datasheets or reference books. It's even easier to find out the meaning β You can use a multimeter. Even the simplest multimeter has this function. For a given transistor β=30 . With modern transistors β equal to about 300...600 units.

Now we can find the required base resistance.

Thus, using the above method, you can easily determine the required resistor values ​​in the base and collector circuits. However, you must remember that calculated data does not always allow you to accurately determine resistor values. Therefore, it is better to fine-tune the key experimentally, and calculations are necessary only for the initial estimate, that is, they help narrow the range of choice of resistor values.

To determine the resistor values, you need to turn on a variable resistor in series with the base and collector resistors and, by changing its value, obtain the required values ​​of the base and collector currents ( rice. 8 ).


Rice. 8 – Scheme for connecting variable resistors

Recommendations for choosing transistors for electronic keys

    The nominal voltage between collector and emitter, which is specified by the manufacturer, must be higher than the voltage of the power supply.

    The rated collector current, which is also indicated by the manufacturer, must be greater than the load current.

    It is necessary to ensure that the current and voltage of the base of the transistor do not exceed the permissible values.

  1. Also, the voltage at the base in saturation mode should not be lower than the minimum value, otherwise the transistor switch will operate unstable.

What kind of load are we talking about? Yes, about any - relays, light bulbs, solenoids, motors, several LEDs at once or a heavy-duty power LED spotlight. In short, anything that consumes more than 15mA and/or requires a supply voltage of more than 5 volts.

Take, for example, a relay. Let it be BS-115C. The winding current is about 80mA, the winding voltage is 12 volts. Maximum contact voltage 250V and 10A.

Connecting a relay to a microcontroller is a task that has arisen for almost everyone. One problem is that the microcontroller cannot provide the power necessary for normal operation of the coil. The maximum current that the controller output can pass through rarely exceeds 20mA, and this is still considered cool - a powerful output. Usually no more than 10mA. Yes, our voltage here is no higher than 5 volts, and the relay requires as much as 12. There are, of course, relays with five volts, but they consume more than twice the current. In general, wherever you kiss a relay, it’s an ass. What to do?

The first thing that comes to mind is to install a transistor. The right solution is that the transistor can be selected for hundreds of milliamps, or even amperes. If one transistor is missing, then they can be switched on in cascades, when the weak one opens the stronger one.

Since we have accepted that 1 is on and 0 is off (this is logical, although it contradicts my long-standing habit that came from the AT89C51 architecture), then 1 will supply power, and 0 will remove the load. Let's take a bipolar transistor. The relay requires 80mA, so we are looking for a transistor with a collector current greater than 80mA. In imported datasheets this parameter is called Ic, in ours Ic. The first thing that came to mind was KT315 - a masterpiece Soviet transistor that was used almost everywhere :) Such an orange one. It costs no more than one ruble. It will also rent KT3107 with any letter index or imported BC546 (as well as BC547, BC548, BC549). For a transistor, first of all, it is necessary to determine the purpose of the terminals. Where is the collector, where is the base, and where is the emitter. This is best done using a datasheet or reference book. Here, for example, is a piece from the datasheet:

If you look at its front side, the one with the inscriptions, and hold it with its legs down, then the conclusions, from left to right: Emitter, Collector, Base.

We take the transistor and connect it according to this diagram:

The collector to the load, the emitter, the one with the arrow, to the ground. And the base to the controller output.

A transistor is a current amplifier, that is, if we pass a current through the Base-Emitter circuit, then a current equal to the input can pass through the Collector-Emitter circuit, multiplied by the gain h fe.
h fe for this transistor is several hundred. Something like 300, I don’t remember exactly.

The maximum output voltage of the microcontroller when supplied to the unit port = 5 volts (the voltage drop of 0.7 volts at the Base-Emitter junction can be neglected here). The resistance in the base circuit is 10,000 ohms. This means that the current, according to Ohm’s law, will be equal to 5/10000 = 0.0005A or 0.5mA - a completely insignificant current from which the controller will not even sweat. And the output at this moment in time will be I c =I be *h fe =0.0005*300 = 0.150A. 150mA is more than 100mA, but this just means that the transistor will open wide and produce the maximum it can. This means our relyuha will receive full nutrition.

Is everyone happy, is everyone satisfied? But no, there is a bummer here. In a relay, a coil is used as an actuator. And the coil has a strong inductance, so it is impossible to abruptly cut off the current in it. If you try to do this, then the potential energy accumulated in the electromagnetic field will come out in another place. At zero break current, this place will be the voltage - with a sharp interruption of the current, there will be a powerful surge of voltage across the coil, hundreds of volts. If the current is interrupted by a mechanical contact, there will be an air breakdown - a spark. And if you cut it off with a transistor, it will simply be destroyed.

We need to do something, somewhere to put the energy of the coil. No problem, we’ll close it to ourselves by installing a diode. During normal operation, the diode is switched on against the voltage and no current flows through it. And when turned off, the voltage across the inductance will be in the other direction and will pass through the diode.

True, these games with voltage surges have a nasty effect on the stability of the device’s power supply network, so it makes sense to screw in an electrolytic capacitor of a hundred or more microfarads near the coils between the plus and minus of the power supply. It will take on most of the pulsation.

Beauty! But you can do even better - reduce your consumption. The relay has enough high current breaking away, but the armature holding current is three times less. It depends on you, but I’m pressured by the toad to feed the reel more than it deserves. This means heating and energy consumption and much more. We also take and insert into the circuit a polar capacitor of another ten microfarads with a resistor. What happens now:

When the transistor opens, capacitor C2 is not yet charged, which means that at the moment of its charging it represents almost a short circuit and the current flows through the coil without restrictions. Not for long, but this is enough to break the relay armature from its place. Then the capacitor will charge and turn into an open circuit. And the relay will be powered through a current limiting resistor. The resistor and capacitor should be selected in such a way that the relay operates clearly.
After the transistor closes, the capacitor discharges through the resistor. This leads to the opposite problem - if you immediately try to turn on the relay when the capacitor has not yet discharged, then there may not be enough current for the jerk. So here we need to think at what speed the relay will click. The Conder, of course, will discharge in a split second, but sometimes that’s too much.

Let's add one more upgrade.
When the relay opens, the energy of the magnetic field is released through the diode, only at the same time current continues to flow in the coil, which means it continues to hold the armature. The time between the removal of the control signal and the loss of the contact group increases. Zapadlo. It is necessary to make an obstacle to the flow of current, but such that it does not kill the transistor. Let's plug in a zener diode with an opening voltage below the limiting breakdown voltage of the transistor.
From a piece of datasheet it can be seen that the maximum Collector-Base voltage for BC549 is 30 volts. We screw in the zener diode for 27 volts - Profit!

As a result, we provide a voltage surge on the coil, but it is controlled and below the critical breakdown point. Thus, we significantly (by several times!) reduce the shutdown delay.

Now you can stretch out and start scratching your head painfully about how to place all this rubbish on a printed circuit board... You have to look for compromises and leave only what is needed in a given circuit. But this is an engineering instinct and comes with experience.

Of course, instead of a relay, you can plug in a light bulb and a solenoid, and even a motor, if the current carries it. The relay is taken as an example. Well, of course, the light bulb does not require the entire diode-capacitor kit.

Enough for now. Next time I’ll tell you about Darlington assemblies and MOSFET switches.

In pulsed devices you can often find transistor switches. Transistor switches are present in flip-flops, switches, multivibrators, blocking oscillators and other electronic circuits. In each circuit, the transistor switch performs its own function, and depending on the operating mode of the transistor, the switch circuit as a whole may change, but the main circuit diagram transistor switch is as follows:

There are several main modes of operation of the transistor switch: normal active mode, saturation mode, cut-off mode and active inverse mode. Although a transistor switch circuit is in principle a common-emitter transistor amplifier circuit, its functions and modes differ from a typical amplifier stage.

In key applications, the transistor serves as a high-speed switch, and the main static states are two: the transistor is off and the transistor is on. The locked state is an open state when the transistor is in cutoff mode. Closed state - the state of transistor saturation, or a state close to saturation, in this state the transistor is open. When a transistor switches from one state to another, this is an active mode in which the processes in the cascade proceed nonlinearly.



Static states are described in accordance with the static characteristics of the transistor. There are two characteristics: the output family - the dependence of the collector current on the collector-emitter voltage and the input family - the dependence of the base current on the base-emitter voltage.

The cutoff mode is characterized by a bias of both p-n junctions of the transistor in the opposite direction, and there is a deep cutoff and a shallow cutoff. Deep cutoff is when the voltage applied to the transitions is 3-5 times higher than the threshold and has the opposite polarity to the working one. In this state, the transistor is open, and the currents of its electrodes are extremely small.

With a shallow cutoff, the voltage applied to one of the electrodes is lower, and the electrode currents are greater than with a deep cutoff; as a result, the currents already depend on the applied voltage in accordance with the lower curve of the family of output characteristics, this curve is called the “cutoff characteristic” .

As an example, we will carry out a simplified calculation for the key mode of a transistor that will operate on a resistive load. The transistor will remain for a long time in only one of two main states: completely open (saturation) or completely closed (cutoff).



Let the load of the transistor be the winding of the SRD-12VDC-SL-C relay, the coil resistance of which at a nominal 12 V will be 400 Ohms. Let's neglect the inductive nature of the relay winding, let the developers provide a snubber to protect against surges in transient mode, but we will carry out the calculation based on the fact that the relay will be turned on once and for a very long time. We find the collector current using the formula:

Iк = (Upit-Ukenas) / Rн.

Where: Iк - D.C. collector; Upit - supply voltage (12 volts); Ukanas - saturation voltage bipolar transistor(0.5 volts); Rн - load resistance (400 Ohm).

We get Ik = (12-0.5) / 400 = 0.02875 A = 28.7 mA.

To be sure, let’s take a transistor with a margin for the maximum current and maximum voltage. The BD139 in the SOT-32 package is suitable. This transistor has parameters Ikmax = 1.5 A, Ukemax = 80 V. There will be a good margin.

To provide 28.7 mA of collector current, a corresponding base current must be provided. The base current is determined by the formula: Ib = Ik / h21e, where h21e is the static current transfer coefficient.

Modern multimeters allow you to measure this parameter, and in our case it was 50. This means Ib = 0.0287 / 50 = 574 µA. If the value of the coefficient h21e is unknown, for reliability you can take the minimum from the documentation for a given transistor.

To determine the required value of the base resistor. The base-emitter saturation voltage is 1 volt. This means that if control is carried out by a signal from the output of a logic chip, the voltage of which is 5 V, then to provide the required base current of 574 μA, with a drop of 1 V at the transition, we obtain:

R1 = (Uin-Ubenas) / Ib = (5-1) / 0.000574 = 6968 Ohm

Let's choose a 6.8 kOhm resistor from the standard series to the smaller side (so that there is definitely enough current).

BUT, in order for the transistor to switch faster and for the operation to be reliable, we will use an additional resistor R2 between the base and emitter, and some power will drop across it, which means it is necessary to lower the resistance of resistor R1. Let's take R2 = 6.8 kOhm and adjust the value of R1:

R1 = (Uin-Ubenas) / (Ib+I (through resistor R2) = (Uin-Ubenas) / (Ib+Ubenas/R2)

R1 = (5-1) / (0.000574+1/6800) = 5547 Ohms.

Let R1 = 5.1 kOhm and R2 = 6.8 kOhm.

Let's calculate the losses on the switch: P = Ik * Ukenas = 0.0287 * 0.5 = 0.014 W. The transistor does not require a heatsink.

The transistor switch is the main element of digital electronics devices and many power electronics devices. The parameters and characteristics of the transistor switch to a very large extent determine the properties of the corresponding circuits.

Switches on bipolar transistors . The simplest switch on a bipolar transistor, connected in a circuit with a common emitter, and the corresponding timing diagram of the input voltage are presented in Fig. 14.5.

Rice. 14.5. Bipolar transistor switch

Let's consider the operation of a transistor switch in steady states. Until a moment in time t 1 The emitter junction of the transistor is locked and the transistor is in cutoff mode. In this mode i To =– i b =I co (I co– reverse collector current), i uh≈ 0. Moreover u R b ≈u R To ≈ 0;u bae ≈ –U 2 ;u ke ≈–E To .

In the interim t 1 …t 2 transistor is open. In order for the voltage across the transistor u ke was minimal, the tension U 1 usually chosen so that the transistor is either in saturation mode or in a borderline mode very close to saturation mode.

Field-effect transistor switches characterized by low residual stress. They can switch weak signals (a few microvolts or less). This is a consequence of the fact that the output characteristics of field-effect transistors pass through the origin.

For example, let's depict the output characteristics of a transistor with a control transition and a channel p-type in the region adjacent to the origin of coordinates (Fig. 14.6).

Rice. 14.6. Field effect transistor with p-type channel

Please note that the characteristics in the third quadrant correspond to the specified voltages between the gate and drain.

In a static state, the field-effect transistor switch consumes very little control current. However, this current increases as the switching frequency increases. The very high input resistance of the switches on field-effect transistors actually provides galvanic isolation of the input and output circuits. This allows you to do without transformers in control circuits.

In Fig. Figure 14.7 shows a diagram of a digital switch based on an MOS transistor with an induced channel n-type and resistive load and corresponding timing diagrams.


Rice. 14.7. Digital key on a field-effect transistor

The diagram shows the load capacity WITH n, which models the capacitance of devices connected to a transistor switch. Obviously, when the input signal is zero, the transistor is switched off and u si =E With. If the voltageuin is greater than the threshold voltage U winter threshold transistor, then it opens and the voltage u si decreases.

Logic elements

A logical element (logic gate) is an electronic circuit that performs some simple logical operation. In Fig. 14.8 shows examples of conventional graphic symbols of some logical elements.

Rice. 14.8. Logic elements

The logic element can be implemented as a separate integrated circuit. Often an integrated circuit contains several logic elements.

Logic gates are used in digital electronics devices (logic devices) to perform simple conversion of logic signals.

Classification of logical elements. The following classes of logical elements (so-called logics) are distinguished:

    resistor-transistor logic (TRL);

    diode-transistor logic (DTL);

    transistor-transistor logic (TTL);

    emitter-transistor logic (ETL);

    transistor-transistor logic with Schottky diodes (TTLS);

    R(R- MDP);

    logic based on MOS transistors with channels like n(n- MDP);

    logic based on complementary switches on MOS transistors (CMOS, CMOS);

    integrated injection logic I 2 L;

    logic based on gallium arsenide semiconductor GaAs.

Currently, the most widely used logics are: TTL, TTLSh, CMOS, ESL. Logic elements and other digital electronic devices are produced as part of the following microcircuit series: TTL – K155, KM155, K133, KM133; TTLSH – 530, KR531, KM531, KR1531, 533, K555, KM555, 1533, KR1533; ESL – 100, K500, K1500; CMOS – 564, K561, 1564, KR1554; GaAs–K6500.

The most important parameters of logical elements:

    Performance is characterized by the signal propagation delay time t sp and maximum operating frequency F Max. The delay time is usually determined by level differences of 0.5 U input and 0.5Δ U out. Maximum operating frequency F Max– this is the frequency at which the circuit remains operational.

    The load capacity is characterized by the input integration coefficient TO about (sometimes the term “output pooling coefficient” is used). Magnitude TO about is the number of logical inputs, the value TO once– the maximum number of similar logical elements that can be connected to the output of a given logical element. Their typical meanings are: TO about =2…8,TO once=4...10. For elements with increased load capacity TO once =20…30.

    Noise immunity in static mode is characterized by voltage U pst, which is called static noise immunity. This is the maximum permissible static noise voltage at the input, at which the output levels of the logic element do not yet change.

    The power consumed by the microcircuit from the power supply. If this power is different for two logic states, then the average power consumption for these states is often reported.

    Supply voltage.

    Input high and low threshold voltages U input1threshold And U input0threshold, corresponding to a change in the state of the logical element.

    Output voltages high and low levels U output1 And U output0 .

Other parameters are also used.

Features of logical elements of various logics. A specific series of microcircuits is characterized by the use of a standard electronic unit - a basic logical element. This element is the basis for building a wide variety of digital electronic devices.

    Basic TTL element contains a multi-emitter transistor that performs a logical AND operation and a complex inverter (Fig. 14.9).


Rice. 14.9. Basic TTL element

If a low voltage level is simultaneously applied to one or both inputs, then the multi-emitter transistor is in a saturation state and transistor T 2 is closed, and therefore transistor T 4 is also closed, i.e. the output will have a high voltage level. If a high voltage level is simultaneously applied to both inputs, then the transistor T 2 opens and enters saturation mode, which leads to the opening and saturation of the transistor T 4 and the turning off of the transistor T 3, i.e. The AND-NOT function is implemented. To increase the speed of TTL elements, transistors with diodes or Schottky transistors are used.

    Basic logical element TTLSH (using the example of the K555 series). The element used as the basic element of the K555 series of microcircuits

AND-NOT (Fig. 14.10, A), and in Fig. 14.10, b a graphical representation of a Schottky transistor is shown.


Rice. 14.10. Logic element TTLSH

Transistor VT4 is a regular bipolar transistor. If both input voltages u input1 And u vx2 are at a high level, then diodes VD3 and VD4 are closed, transistors VT1, VT5 are open and there is a low level voltage at the output. If at least one input has a low level voltage, then transistors VT1 and VT5 are closed, and transistors VT3 and VT4 are open, and there is a low level voltage at the input. TTLSh microcircuits of the K555 series are characterized by the following parameters:

    supply voltage +5 IN;

    low level output voltage no more than 0.4 IN;

    high level output voltage not less than 2.5 IN;

    noise immunity – not less than 0.3 V;

    average signal propagation delay time 20 ns;

    maximum operating frequency 25 MHz.

Features of other logics. The basis of the basic logical element of the ESL is a current switch, the circuit of which is similar to that of a differential amplifier. The ESL microcircuit is powered by negative voltage (–4 IN for K1500 series). The transistors of this microcircuit do not enter saturation mode, which is one of the reasons for the high performance of ESL elements.

In microcircuits n-MOS and p-MOS switches are used respectively on MOS transistors with n-channels and dynamic load and on MOS transistors with p-channel. To eliminate power consumption by a logic element in a static state, complementary MIS logic elements (CMDP or CMOS logic) are used.

Logic based on gallium arsenide semiconductor GaAs is characterized by the highest performance, which is a consequence of high electron mobility (3...6 times more compared to silicon). GaAs-based microcircuits can operate at frequencies of the order of 10 GHz.

When working with complex circuits, it is useful to use various technical tricks that allow you to achieve your goal with little effort. One of them is the creation of transistor switches. What are they? Why should they be created? Why are they also called “electronic keys”? What features of this process are there and what should you pay attention to?

What are transistor switches made of?

They are performed using field or The first ones are further divided into MIS and switches that have a control p-n junction. Among bipolar ones, non/saturated ones are distinguished. A 12 Volt transistor switch will be able to satisfy the basic needs of a radio amateur.

Static operating mode


It analyzes the closed and open state of the key. In the first, the input contains a low voltage level, which indicates a logical zero signal. In this mode, both transitions are in the opposite direction (a cutoff is obtained). But the collector current can only be affected by thermal current. In the open state, the key input has a high voltage level corresponding to the logical one signal. It is possible to work in two modes simultaneously. Such operation can be in the saturation region or the linear region of the output characteristic. We will dwell on them in more detail.

Key saturation

In such cases, the transistor junctions are forward biased. Therefore, if the base current changes, the value on the collector will not change. In silicon transistors, approximately 0.8 V is required to obtain a bias, while for germanium transistors the voltage fluctuates between 0.2-0.4 V. How is switch saturation achieved in general? To do this, the base current increases. But everything has its limits, as does increasing saturation. So, when a certain current value is reached, it stops increasing. Why do you need to saturate the key? There is a special coefficient that reflects the state of affairs. As it increases, the load capacity of transistor switches increases, destabilizing factors begin to influence with less force, but performance deteriorates. Therefore, the value of the saturation coefficient is chosen from compromise considerations, focusing on the task that will need to be performed.

Disadvantages of an unsaturated key

What happens if the optimal value has not been achieved? Then the following disadvantages will appear:

  1. The open key voltage will drop to approximately 0.5 V.
  2. Noise immunity will deteriorate. This is explained by the increased input resistance that is observed in the switches when they are in the open state. Therefore, interference such as voltage surges will also lead to changes in the parameters of the transistors.
  3. Saturated key has significant temperature stability.

As you can see, it is still better to carry out this process in order to ultimately obtain a more advanced device.

Performance

Interaction with other keys

For this purpose, communication elements are used. So, if the first switch has a high voltage level at the output, then the second one opens at the input and operates in the specified mode. And vice versa. Such a communication circuit significantly affects the transient processes that occur during switching and the speed of the keys. This is how a transistor switch works. The most common are circuits in which interaction occurs only between two transistors. But this does not mean at all that it cannot be made into a device in which three, four or even more elements will be used. But in practice it is difficult to find an application for this, so the operation of a transistor switch of this type is not used.

What to choose

What is better to work with? Let's imagine that we have a simple transistor switch whose supply voltage is 0.5 V. Then using an oscilloscope it will be possible to record all the changes. If the collector current is set to 0.5 mA, the voltage will drop by 40 mV (at the base it will be approximately 0.8 V). By the standards of the problem, we can say that this is a rather significant deviation, which imposes a limitation on the use in a whole range of circuits, for example, in switches. Therefore, they use special ones where there is a control p-n junction. Their advantages over their bipolar counterparts are:

  1. Insignificant value of residual voltage on the key in the wiring condition.
  2. High resistance and, as a result, low current that flows through the closed element.
  3. Low power consumption means no significant control voltage source is needed.
  4. It is possible to switch low-level electrical signals that amount to units of microvolts.

A transistor relay switch is an ideal application for field applications. Of course, this message is posted here solely to give readers an idea of ​​their application. With a little knowledge and ingenuity, a great many possibilities for implementations that include transistor switches will be invented.

Example of work

Let's take a closer look at how a simple transistor switch functions. The switched signal is transmitted from one input and removed from the other output. To lock the key, a voltage is applied to the transistor gate that exceeds the source and drain values ​​by an amount greater than 2-3 V. But care should be taken not to go beyond the permissible range. When the key is closed, its resistance is relatively high - more than 10 ohms. This value is obtained due to the fact that the reverse bias current also influences p-n junction. In the same state, the capacitance between the switched signal circuit and the control electrode fluctuates in the range of 3-30 pF. Now let's open the transistor switch. The diagram and practice will show that then the voltage of the control electrode will approach zero, and strongly depends on the load resistance and the switched voltage characteristic. This is due to a whole system of interactions between the gate, drain and source of the transistor. This creates certain problems for operating in chopper mode.

As a solution to this problem, various circuits have been developed that provide stabilization of the voltage that flows between the channel and the gate. Moreover, due to its physical properties, even a diode can be used in this capacity. To do this, it should be included in the forward direction of the blocking voltage. If the necessary situation is created, the diode will close and the pn junction will open. So that when the switching voltage changes, it remains open and the resistance of its channel does not change, a high-resistance resistor can be connected between the source and input of the switch. And the presence of a capacitor will significantly speed up the process of recharging the containers.

Transistor switch calculation


For understanding, here is an example of a calculation, you can substitute your data:

1) Collector-emitter - 45 V. Total power dissipation - 500 mw. Collector-emitter - 0.2 V. Cutoff frequency - 100 MHz. Base-emitter - 0.9 V. Collector current - 100 mA. Statistical current transfer coefficient - 200.

2) Resistor for 60 mA current: 5-1.35-0.2 = 3.45.

3) Collector resistance rating: 3.45\0.06=57.5 Ohm.

4) For convenience, we take the nominal value of 62 Ohms: 3.45\62=0.0556 mA.

5) We count the base current: 56\200=0.28 mA (0.00028 A).

6) How much will be on the base resistor: 5 - 0.9 = 4.1V.

7) Determine the resistance of the base resistor: 4.1\0.00028 = 14.642.9 Ohms.

Conclusion

And finally, about the name “electronic keys”. The fact is that the state changes under the influence of current. What is he like? That's right, a collection of electronic charges. This is where the second name comes from. That's all. As you can see, the operating principle and design of transistor switches is not something complicated, so understanding this is a feasible task. It should be noted that even the author of this article needed to use a little reference literature to refresh his own memory. Therefore, if you have questions about terminology, I suggest you remember the availability of technical dictionaries and search for new information about transistor switches there.