How transistors work in a circuit

how transistors work in a circuit


Sep 09, Transistors work as either amplifiers or switches. When working as an amplifier, a transistor takes a small input current and amplifies it to produce a larger output current. On the other hand, while working like a switch, a low input current at the input terminal switches on and drives a larger current at the output terminal. In this article. By Cathleen Shamieh. Bipolar junction transistors (BJTs) and field-effect transistors (FETs) work basically the same way. The voltage you apply to the input (base, for a BJT, or gate, for a FET) determines whether or not current flows through the transistor (from collector to emitter for a BJT, and from source to drain for a FET). To get an idea of how a transistor works (specifically, a .

Home Transistor Basics. What good is a what is a comprehensive background check Sure, integrated circuits ICs are full of transistors, thousands of them. Before the IC and microprocessor revolutions, there was a transistor revolutionwhere televisions, radios and computers were built using the new solid-state devices.

The transistor was the father of the IC. What use does a lowly transistor have in a world where the current Intel microprocessors have over a billion transistors each? It would not be possible to build a modern microprocessor with discrete transistorsthe lead lengths alone would make the speeds impossible. But the reverse is also true. A discrete transistor can be a simple way to solve some problems. Transistors, for example, typically have much higher operating voltage and russian garlic how to plant limits in simple circuits than those of comparable ICs.

Electronics manufacturers and distributors still make and sell individual transistors because the parts still have some uses.

In this article, I want to go over some very basic things about transistors, how they are used and how you can include them in your applications. Both are usually silicon devices. The silicon is modified doped with impurities to produce N-type or P-type material. The N-P-N structure is just representative. In an actual transistor, the collector region is normally larger than the emitter region, and none of them is square as shown in the diagram. The diode representation of the transistor indicates how current flows, not how the actual part is constructed.

Operation of an NPN transistor is conceptually easy to understand. Referring to the diode model, if you connect the collector to a positive voltagesay 5 Vand the emitter to ground, you end up with two diodes back-to-back with their anodes connected together. The junction of the two anodes represents the base of a transistor. If you apply a positive voltage greater than 0.

The collector diode will be reverse-biased, and no current will flow through that diode. If the base-emitter voltage is below 0. The collector-emitter current flow is inherent in the construction of the transistor.

The magic in a transistor is determining how to get the amount of current you want flowing through the collector. Generally, if the transistor is operated within its current, power and voltage ratings, the current in the emitter will be the current flowing into the base plus the current flowing from the collector to the emitter. A very small base current controls a much larger collector current, so the collector current is approximately equal to the emitter current.

If there is enough current flowing that the collector-emitter voltage is as low as it can go generally around 0. In this state, changes to base current no longer affect collector current. Figure 2 shows a simple circuit. The forward voltage of the 2N is 0. Conventionally, 0. So, the voltage at the emitter VE will be 1 V 0. The collector current is the same. Therefore, by controlling the base voltage, we control the emitter current and thereby the collector current. Figure 3 shows how we can make an amplifier with this circuit.

Since the current in the emitter what is standard international shipping on amazon fixed at 1. This current flows through R2, producing a voltage across R2 of 1.

So, the voltage at the collector, VC, is the 5 V supply minus the voltage across R2, or 2. Now, what happens if the voltage at the base is raised to 1. When that happens, the voltage at the emitter is now 0. The collector current is also 1. So, a 0. The collector voltage dropped by 6. If you work through the math, this makes sense, because the collector current is the same as the emitter current.

But since the collector resistor R2 is 6. If you did the same calculation after lowering the base voltage from 1 V to 0. This circuit is an inverting amplifier with a gain of A positive voltage change at the input produces a negative voltage change at the output and vice-versa.

This circuit has some limitations. If you put 1. The limitation of this specific circuit, therefore, is a maximum input voltage of about 1. At the other end, anything less than 0. So, the useful input voltage range of this circuit is 0. Still, that would be adequate for boosting a low-level audio signal to something that can be further amplified. Speaking of audio, how would you connect audio signals into the circuit? Audio signals typically swing between negative and positive voltages.

If you put that into the base, the transistor will be in cutoff most of the timeall the time if the positive signal peaks never reach 0. This brings us to biasing. Figure 4 is a modification of Figure 3 with some biasing resistors added to the base. Resistors R3 and R4 make a voltage divider that brings the base to about 1 V.

This is halfway between the 0. Now say that we apply a signal to the input that swings between Because of the DC blocking capacitor C1, this will become 0. There are other ways to bias a transistor base. A voltage reference diode, as shown in Figure 5fixes the base at a known voltage. In this circuit, the emitter voltage, VE, will be about 1.

The point is not to show all the possible ways to bias a transistor, just that there are other ways to do it. Transistors have other characteristics. For example, the 2N used in these examples has a maximum collector-emitter voltage of 40 V. Any more than that, and the transistor fries. The base-emitter reverse voltagewhere the base is taken negative with respect to the emitterhas a maximum value of 6 V.

Beyond that, the emitter-base junction breaks down. The collector can handle a maximum continuous current of mA. The device has a maximum power dissipation of about mW. So even though the collector-emitter can withstand 40 V and the collector current can be as high as mA, if you try to put mA through it at 40 V, it will fail. The point of all this is that, like any semiconductor device, your design has to stay within all the maximum ratings: Power, collector-emitter voltage, collector current, emitter-base reverse breakdown voltage and so on.

One of the key characteristics of the transistor is the current gain. This number describes how much the emitter current changes for a given change in the base current. The current gain varies with what does john hagee say about obama amount of current flowing in the collector.

For the 2N, the minimum current gain at 0. At 10 mA, the minimum gain is The maximum gain per the datasheet is Just before writing this paragraph, I measured a handful of 2Ns All of them had gain exceeding The practical implication of the gain is to affect how the emitter interacts with the base. But if you have to what channel oprah come on lower value resistors in your biasing circuit, this in turn presents more load to whatever is driving it.

In the case how to wash the engine of your car the amplifier, it reduces the overall end-to-end gain. Where you get into difficulty is when you need a very low value of emitter resistance. If you do, the emitter will pull down the voltage.

One common addition to an audio amplifier is to bypass the emitter resistor with an electrolytic capacitor. The capacitor has a very high impedance nearly infinite at DC, but the impedance decreases as frequency increases.

This allows the DC biasing to work, but it raises the gain for audio signals by making the emitter impedance the resistance in parallel with the impedance of the capacitor a very low value at audio frequencies. This makes the ratio of the collector resistance to emitter resistance much higher at audio than at DC, which raises the gain.

Remember: The gain is the collector resistor divided by the emitter impedance. However, this also has the effect of significantly lowering the input impedance of the circuit at those audio frequencies.

In Figure 6I have modified Figure 5 by making the reference voltage 2. Because the voltage at the base is fixed at 2. Obviously, there are upper limits to this, and at some point, the voltage or power dissipation limit of the 2N will be exceeded and it will go up in a cloud of smoke.

But if you wanted a constant current through an LED regardless of supply voltage within reasonable limitsthis circuit will do it.

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Inverters (NOT gates) are available on logic ICs but if you only require one inverter it may be better to use this simple transistor circuit. The output signal (voltage) is the inverse of the input signal: When the input is high (+Vs) the output is low (0V). When the input is low (0V) the output is high (+Vs). The transistor can handle higher voltages than most logic-level translator circuits. A transistor could translate between a V circuit and a 12 V circuit, for example. I hope my explanation of how transistors work has helped you understand them better, and that the examples are enough to let you experiment with transistors in your. HOW TRANSISTORS WORK? (BEGINNERS EDITION) Transistors are indeed one of the revolutionary inventions that man has ever made. The function of the transistor is to switch and amplify electrical currents. The number of transistors in dense integrated circuit doubles about every two years.

This page explains the operation of transistors in simple circuits, mainly their use as switches. Practical matters such as testing, precautions when soldering and identifying leads are covered by the transistors page. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. This page is mostly about NPN transistors and beginners should initially focus on this type.

The leads are labelled base B , collector C and emitter E. These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels. A Darlington pair is two transistors connected together to give a very high current gain. In addition to standard bipolar junction transistors, there are field-effect transistors which are usually referred to as FET s.

They have different circuit symbols and properties and they are not covered by this page. Rapid Electronics: transistors. When the switch is closed a small current flows into the base B of the transistor. It is just enough to make LED B glow dimly.

The transistor amplifies this small current to allow a larger current to flow through from its collector C to its emitter E. This collector current is large enough to make LED C light brightly.

When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off. It is a good way to test a transistor and confirm it is working. A transistor amplifies current and can be used as a switch , as explained on this page.

With suitable resistors and capacitors for AC a transistor can amplify voltage such as an audio signal but this is not yet covered by this website. This arrangement where the emitter E is in the controlling circuit base current and in the controlled circuit collector current is called common emitter mode.

It is the most widely used arrangement for transistors so it is the one to learn first. The operation of a transistor is difficult to explain and understand in terms of its internal structure.

It is more helpful to use this functional model. It must never be partly on with significant resistance between C and E because in this state the transistor is liable to overheat and be destroyed. In the fully ON state the voltage V CE across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current Ic. When choosing a transistor to use as a switch you need to consider its maximum collector current Ic max and its minimum current gain h FE min.

Transistor voltage ratings may be ignored for supply voltages less than 15V. Most suppliers provide data for transistors they sell, for example Rapid Electronics. Power developed in a transistor appears as heat and the transistor will be destroyed if it becomes too hot.

This should not be a problem for a transistor being used as a switch if it has been chosen and set up correctly because the power developed inside it will be very small.

Relays are suitable for all these situations but note that a low power transistor may still be needed to switch the current for the relay's coil. For more information, including the advantages and disadvantages, please see the relays page. If the transistor is switching a load with a coil such, as a motor or relay , a diode must be connected across the load to protect the transistor from the brief high voltage produced when the load is switched off.

The diagram shows how a protection diode is connected 'backwards' across the load, in this case a relay coil. Current flowing through a coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil and diode so the magnetic field dies away quickly rather than instantly.

This prevents the induced voltage becoming high enough to cause damage to transistors and ICs. Rapid Electronics: relays.

Most ICs cannot supply large output currents so it may be necessary to use a transistor to switch the larger current required for devices such as lamps, motors and relays. The timer IC is unusual in being able to supply a relatively large current of up to mA, sufficient for many relays and other loads without needing a transistor.

A base resistor limits the current flowing into the base of the transistor to prevent it being damaged but it must also allow sufficient base current to flow to ensure that the transistor is fully saturated when switched on. The next section explains how to choose a transistor and base resistor to ensure full saturation. A transistor can be used to enable an IC connected to a low voltage supply such as 5V to switch the current for a load with a separate DC supply such as 12V.

The two power supplies must be linked. Transistor data is available from most suppliers, for example see Rapid Electronics. Do you want the load to switch on when the IC output is high? Or when it is or low?

NPN and PNP transistors are connected differently as shown in the diagrams below but the calculations and properties required are the same for both types of transistor. NPN transistor switch load switched on when IC output is high. PNP transistor switch load switched on when IC output is low.

The base resistor R B must allow sufficient current to flow to ensure the transistor is fully saturated when switched on and it is good to make the base current I B about five times the value which just saturates the transistor.

Use the formula below to find a suitable resistance for R B and choose the nearest standard value. If the load being switched on and off is a motor, relay or solenoid or any other device with a coil a diode must be connected across the load to protect the transistor from the brief high voltage produced when the load is switched off.

Note that the diode is connected 'backwards' as shown in the diagrams above. The supply voltage is 6V for both the IC and load. The IC can supply a maximum current of 5mA. The diagrams below show how to connect an LDR light sensor to a transistor to make a light-sensitive circuit switch on an LED.

There are two versions, one switches on in darkness, the other in bright light. The variable resistor adjusts the sensitivity. Any general purpose low power transistor can be used to switch an LED. If the transistor is switching a load with a coil such as a motor or relay instead of an LED you must include a protection diode across the load.

Note that the switching action of these simple circuits is not particularly good because there will be an intermediate brightness when the transistor will be partly on not saturated.

In this state the transistor is in danger of overheating unless it is switching a small current. There is no problem with the small LED current but the larger current for a lamp, motor or relay is likely to cause overheating. Other sensors, such as a thermistor , can be used with these circuits, but they may require a different variable resistor. You can work out an approximate value for the variable resistor Rv by using a multimeter to find the minimum and maximum values of the sensor's resistance Rmin and Rmax and then using this formula:.

You can make a much better switching circuit with sensors connected to a suitable IC chip. The switching action will be much sharper with no partly on state. Inverters NOT gates are available on logic ICs but if you only require one inverter it may be better to use this simple transistor circuit. The output signal voltage is the inverse of the input signal:. Any general purpose low power NPN transistor can be used. With these values the inverter output can be connected to a device with an input impedance resistance of at least 10k such as a logic IC or a timer trigger and reset inputs.

Darlington pair A Darlington pair is two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The pair behaves like a single transistor with a very high current gain so that only a tiny base current is required to make the pair switch on.

The Darlington pair current gain h FE is equal to the two individual gains h FE1 and h FE2 multiplied together - this gives the pair a very high current gain, such as Note that to turn on a Darlington pair there must be 0. Rapid Electronics: Darlington transistors. Darlington pairs are available as a 'darlington transistor' package with three leads B , C and E equivalent to those of a standard transistor. You can also make your own Darlington pair from two ordinary transistors.

TR1 can be a low power type but TR2 may need to be high power. The maximum collector current Ic max for the pair is the same as Ic max for TR2.

A Darlington pair is sufficiently sensitive to respond to the small current passed by your skin and it can be used to make a touch-switch as shown in the diagram. For this circuit which just lights an LED the two transistors can be any general purpose low power transistors. The k resistor protects the transistors if the contacts are linked with a piece of wire. Rapid Electronics have kindly allowed me to use their images on this website and I am very grateful for their support.

They stock a wide range of components, tools and materials for electronics and I am happy to recommend them as a supplier. Next Page: Capacitance Study. Transistor circuit symbols.

The diagram shows the two current paths through a transistor. The small base current controls the larger collector current. Common Emitter Mode This arrangement where the emitter E is in the controlling circuit base current and in the controlled circuit collector current is called common emitter mode.

The base-emitter junction behaves like a diode. The device switched by the transistor is called the load. Transistor technical data Most suppliers provide data for transistors they sell, for example Rapid Electronics. Would a relay be better than a transistor switch?

A signal diode such as the 1N is suitable for this. Why is a protection diode needed?

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