Understanding Electronic Components

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4. Transistors

 

Transistors are active components which are often found in many different electronic circuits. Their play their roles in circuits usually as amplifiers or switch components. As amplifying components they are used in different LF (low frequency) and HF (high frequency) amplifiers, oscillators, modulators, detectors, etc, or in any circuit which needs amplification of voltage, current or power stage. Transistors are used as switches mainly in digital circuits, although they are common in analog ones as well. This means that if the conditions are met, they could switch on or off a part of a circuit, or even in some cases the whole circuit.

There is a large number of manufacturers around the world who produce semiconductors (transistors are members of this family of components), so there are literally thousands of different types of transistors. There are low, medium and high power transistors, for working with low and high frequencies, for working with very high current flows, etc. Several different transistors are shown on 4.1.

Depending on their principle of work they could be divided into: FET-s and single connection transistors. Most commonly used transistors are bipolar which are divided into NPN and PNP transistors.
Their base material is most commonly silicon (then their marking has a letter B) or germanium (in that case their marking has a letter A). There are transistors made of other materials, depending on that their first letter could be C, D or R.


Fig. 4.1: Different transistors


Fig. 4.2: Transistor symbols: a - bipolar, b - connection FET, c -
MOSFET, d - dual gate MOSFET, e - inductive channel MOSFET,
f - single connection transistor

Second letter in transistor’s marking is describing the type of that transistor and it’s primary usage:
C - low and medium power LF transistor,
D - high power LF transistor,
F - low power HF transistor,
G - other transistors,
L - high power HF transistors,
P - photo transistor,
S - switch transistor,
U - high voltage transistor.

Here are few examples:
AC540 - germanium core, LF, low power,
AF125 - germanium core, HF, low power,
BC107 - silicon, LF, low power (0.3W),
BD675 - silicon, LF, high power (40W),
BF199 - silicon, HF (to 550 MHz),
BU208 - silicon (for voltages up to 700V),
BSY54 - silicon, switching transistor.
There is a possibility of a third letter (R and Q - microwave transistors, or X - switch transistor), but these letters vary from manufacturer to manufacturer, and because of that they are not reliable.
Number following the letter marking is of no importance to users.
American transistor manufacturers have yet different marks, with a 2N prefix followed by a number (2N3055, for example). This mark standard is similar to diode marks, which have a 1N prefix (e.g. 1N4003).
Japanese bipolar transistor marks are prefixed with a: 2SA, 2SB, 2SC or 2SD, and FET-s with 3S:
2SA - PNP, HF transistors,
2SB - PNP, LF transistors,
2SC - NPN, HF transistors,
2SD - NPN, HF transistors.

A picture of several different transistors is given on picture 4.1, and symbols in which they are represented in schematics are on 4.2. Low power transistors are housed inside of a small plastic or metallic cases of various shapes. Bipolar transistors have three leads: for base (B), emitter (E), and for collector (C). Sometimes, HF transistors have another lead which is connected to metal case of the housing. This lead is connected to the ground of the circuit, to protect the transistor from possible external electrical interferences. Four leads exist with some other types of transistors as well, two-gate FET-s, for example. High power transistor packages are different from the ones for low-to-medium power, both in size and in shape.

It is important to have the manufacturer’s catalog or a datasheet of the exact component to know which lead is connected to what part of the transistor. These documents hold all the valuable information about the component's proper usage (maximum current rating, power, amplification, etc.) as well as a diagram of that component with it’s exact pinout. Placement of leads and different housing types for some commonly used transistors are on picture 4.3.


Fig. 4.3: Pin placements of some common packages

It might end up useful to remember the lead placement in TO-1, TO-5, TO-18 and TO-72 packages and compare them with the drawing 4.2 (a). As you can see, placement remains the same in each case, and it is easy to remember because it is analogous to transistor representation symbol. These transistors are the ones you will come across frequently in everyday work.

TO-3 package, which is used to house very powerful transistors, has only two pins, one for base, and one for emitter. Collector is connected to the packaging, and the wire which is used to connect it to the rest of the circuit is mounted on one of the screws which fasten the transistor and the heat-sink.

Transistors used with very high frequencies (like the presented BFR14) have pins shaped differently. One of the breakthroughs in the field of electronic components was the invention of SMD (surface mounted devices) circuits. This technology allowed manufacturers to achieve tiny form factor electronic components with the same properties as their larger counterparts, and therefore reduce the size and cost of the complete user's design. One of the SMD housings is the SOT23 package. There is, however, a trade-off to this, SMD components come as somewhat difficult to solder to the circuitboard for an unexperienced person and they usually need special soldering equipment which might be a bit expensive (there are some workarounds to this, but as a beginner it is advisable to stick to the regular, or as commonly referred to through-hole components).

As we said, there are literally thousands of different transistors, many of them have similar characteristics, which makes it possible to replace faulty transistor with different one. These characteristics and similarities could be found in comparison charts. If, for some reason, and there should not be that reason if you are planning to become serious in this field, you do not have those charts, there is no other than to try some of the transistors you already have. If the circuit continues to operate in the right manner, everything is ok (this should be applied only as the very last option, your whole circuit could be rendered completely useless if the new transistor isn't supposed to do what you try with it). Some other important things you should take care of, if you absolutely have no other but doing it this way, is to for one replace a NPN transistor only with a NPN transistor. Same goes if the transistor in question is PNP or a FET. It is also good to say that pin placement must be taken into account before you solder it all together and power it up.
As a "first aid" there is a chart in Chapter 4.4 which shows a list of replacements for some frequently used transistors. You can find these transistors in many magazines, all kinds of schematics for different electronic devices, and they will be common in future isssues of "Practical ELECTRONICS".

4.1 Working principles of transistors

Often use for a transistor in analog electronic circuits is to amplify the electric signal, but it is commonly used in many devices as a regulator for different purposes. Their digital usage is most often as a switch of some kind. Best way to explore the basics of transistors is by experimenting. For a simple one, you will need one medium to higher power transistor, whose maximum current rating is around 1A. Some of these are BD135 and 2N3055. Other things you would need are a battery (or transformer which is capable of delivering 4.5V), a small light bulb (taken from the flashlight) with properties near 4.5V/0.3A. Linear potentiometer (5K or higher) and a several hundred ohms regular resistor. These components should be connected in a manner shown on 4.4a. There is a picture (4.4b) showing the end result using the 2N3055 transistor. Isolated copper wire is used as a conducting material to connect the components. Components are soldered to the wire. If you're not comfortable with soldering, you should go study Section 2 of this book, called "Practical realization of electronic devices". Connecting battery leads and bulb's body could be done solderless, by simply wrapping and fastening them with wire. This could also be done with the collector of the transistor, but it is easier to use a 3mm screw. Last component to be connected is the battery (it's negative side) to close the circuit.


Fig. 4.4: Working principle of transistors: potentiometer moves toward it's upper position - voltage over the base is rising - current flowing through the base is rising - current flowing through the collector is rising - light bulb's brightness rises

Resistor (R) isn't really necessary, but if you don't use it, you should watch not to turn the potentiometer (or as more commonly referred to a "pot") in it's high position, because that would destroy the transistor. So it is safer to actually use the resistor with this circuit. This is because the DC voltage UBE (voltage between the base and the emitter), which is called pre-voltage, should not be higher than 0.6V, for silicon transistors, or 0.2V, for germanium ones. When there is no resistor R, slider in high position means pre-voltage is 4.5V, and that means almost certain death of your transistor.

Line (4.4a) which connects lower end of the potentiometer, emitter and the negative side of the battery, which is symbolically representing the wire which connects these three on 4.4b. In case this circuit is to be enclosed in a metal housing, this wire should be connected to the box. So when there is a need to measure something (like a DC voltage) inside of the circuit, negative probe of the multimeter goes to the ground. If there was written, for example UC = 3V or just 3V, it means that DC voltage between the collector and ground is equal to 3V.

Turn the knob of the potentiometer to it's leftmost position. This brings the voltage on the base (or more correctly between the base and the ground) to zero volts (UBE = 0). Bulb doesn't light, which means that there is no electricity passing through the transistor.

As we already mentioned, potentiometer's lowest position means that UBE is equal to zero, and it's highest position (if we forget the resistor R) produces 4.5V on the base of the transistor. When we turn the knob from it's leftmost position toward the other end, voltage UBE gradually increases, only to become equal to power supply's maximum in the rightmost position. This means that UBE could be equal to any voltage level between the 0 and 4.5V. With having resistor R in the circuit, situation is a bit different. It lowers the UBE voltage range to some acceptable level. In the highest pot position, voltage on the output is equal to 0.6V which makes the bulb red, and that means that the collector current (IC) is passing through the serially connected battery, bulb and transistor. Now you should try to spin the knob in both directions randomly. You will see that light from the bulb is increasing and decreasing it's brightness, accordingly to your actions.

If we connected an amperemeter between the collector and the bulb's body (to measure IC), another amperemeter between the pot and the base (for measuring IB), and a voltmeter between the ground and the base and repeated the whole experiment, we could find some interesting data. When the pot is in it's low position UBE is equal to 0V, as well as currents IC and IB. While the knob is turning these values start to rise until the bulb starts to flicker when they are: UBE = 0.6V, IB = 0.8mA and IB = 36 mA (if your values differ from the values acquired here, it is because the 2N3055 author used doesn't have the exact same specifications as the one you use, which is a common thing when working with transistors).
The end result we get from this experiment is that when the current on the base is changed, current on the collector is changed as well: higher the voltage on the base, higher the current on the collector, and other way round: lower the voltage, lower the current. Beside the collector current there is also a base current IB. When UBE is equal to zero, it is equal to zero as well. (Just to be on the safe side, there is also a I current, which flows through the circuit constantly independently on the state of the potentiometer, but it is of no importance to us at this moment).

Let's look at another experiment which will broaden our knowledge of the transistors. It requires the BC107 transistor (or any similar low power transistor), supply source (same as in previous experiment), 1 MO resistor, 1 kO, or higher value, speaker phones and an electrolytic capacitor whose value may range between 10 to 100 µF with any operating voltage. Simple low frequency amplifier could be built from these components if they were connected in a way shown on schematic 4.5.


Fig. 4.5: Transistor amplifier

It should be noted that the schematic 4.5a is similar to the one on 4.4a. The main difference is that the collector is here connected to the speakerphones and on 4.5 it was connected to the light bulb. Beside that, needed prevoltage is acquired in much simpler fashion - using only one resistor (R). When there is no resistor, there is no current flow IB, so it means that IC is zero as well. When the resistor is connected to the circuit, base voltage is equal to 0.6V, and through the transistor flows the base current IB = 4µA and the collector current IC = 1 mA. Since both of these currents enter the transistor, it is obvious that the emitter current is equal to IE = IC + IB. And since the base current is in most cases insignificant compared to the collector current, it is considered that:

Relation between the current flowing through the collector, and the one flowing through the base is called transistor's current amplification coefficient, and is marked as hFE. In our example, this coefficient is equal to:

Put your speakerphones on, and place a fingertip on point 1. You will hear noise. In every human body there is a 50Hz AC voltage which is inducted under the magnetic field of the 220V network voltage. Noise heard over speakerphones is that voltage, only amplified using transistor. Let's explain this circuit a bit more. Ac voltage with frequency 50Hz is connected to transistor's base over the capacitor C. Voltage on the base is now equal to the sum of a DC voltage (0.6 approx.) brought over resistor R, and AC voltage "from" the finger. This means that this base voltage is higher than 0.6V fifty times per second, and fifty times somewhat lower than that. Because of this, current on the collector is higher than 1mA fifty times per second, and fifty times lower. This variable current is used to shift the membrane of the speakerphones forward fifty times per second and fifty times backwards, meaning that we can hear the 50Hz tone on the output.
Listening to a 50Hz noise is not very interesting, so you could connect to points 1 and 2 some LF signal source (CD player's or gramophone's output, a microphone) and possibly connect some smaller speaker instead of speakerphones.
(There is a possibility that your amplifier based on this schematic operates faulty, or even ceases it's functioning, in that case try varying the values of the resistor R)

There are literally thousands of different schematics using a transistor as an active, amplifying component. And all these transistors operate in a manner shown in our experiments, which means that by building this example, you're actually building a basic building block of electronics so it is good to know it's operation inside-out.

Variable voltage on the base is creating variable current on the base and on the collector. Variable current of the collector flows through some output (speakerphones in our case, but that could be a resistor, coil, speaker, etc. as well) and creates a variable voltage over it. This voltage is shaped in the same way as the one over the base, but with significantly higher value than it. Amplitude relation between the output voltage, and the input voltage connected between the points 1 and 2 is called amplifier's voltage amplification.

4.2 Basic characteristics of transistors

Selection of transistors for some practical usage is commonly based on their main electrical characteristics, which are: maximum voltage rating between the collector and the emitter UCEmax, maximum collector current ICmax and maximum power rating PCmax. If you need to switch a faulty transistor, or you feel comfortable enough to build a brand new circuit design pay attention to these three values, your circuit must not exceed the maximum rating values of the transistor which is to be used at any time, if this is discarded as unimportant there are possibilities of permanent circuit damage . All information there is in existence for a certain component could be found in a spreadsheet published by the manufacturer. Beside values we mentioned, sometimes it is important to know the current amplification coefficient, and in some cases border frequencies.
When there is a DC voltage UCE between the collector (C) and emitter (E) with collector current flowing through them, transistor acts as a small electrical heater whose power is given with this equation:

Because of that, transistor is heating itself and everything in it's proximity. When UCE or ICE are risen (or both of them), transistor overheats, and when it reaches some certain point material from which it was made melts, and that renders transistor useless. Maximum power rating for a transistor, within which it functions properly is PCmax (found in a spreadsheet). What this means is that a product of UCE and IC should should not be higher than PCmax:

So, if the voltage flowing through the transistor should have been higher, current must be dropped, and contrary.
For example, maximum ratings for a BC107 transistor are:
ICmax=100mA,
UCEmax = 45V and
PCmax = 300mW
If we need a Ic=60mA current flowing through the component, voltage over it should be less than:

We could calculate maximum current rating for UCE = 30V in similar fashion:

Among it's other characteristics, this transistor has current amplification coefficient in range between hFE= 100 to 450, and it could be used for frequencies under 300MHz. According to the recommended values given by the manufacturer, optimal results (stability, low distortion and noise, high gain, etc.) are with UCE=5V and IC=2mA.
There are occasions when a transistor generated heat could not be overcame by adjusting voltages and amperages. Because of that, transistors which are most likely to overheat have a metal plate with screw hole, which is used to attach a heat-sink to the component's body, and lower the overall temperature by spreading it over large surface. This will be discussed in more detail in one of the later chapters.

Current amplification coefficient is of importance when used in so called coupled transistor circuits, where there is a need for near equal amplifications. This is a bit complicated since there is a large possibility that even two exact transistors (same model, same manufacturer) have different hFE. For example, 2N3055H transistor is said to have hFE within range between 20 and 70, which means that there is a possibility that one of them has 20 and other 70. This is, as you may guess, not good. It means that in cases when two identical coefficients are needed, they should be measured. Some better multimeters have option for measuring this, but most of them don't. Because of this we provided a simple circuit (4.6) for testing transistors. All you need is an option on your multimeter for measuring DC current up to 5mA. Both diodes (1N4001, or similar general purpose silicon diodes) and 100Ohm resistors are used to protect the instrument if the transistor is "broken". As we said, current amplification coefficient is equal to hFE = IC / IB. In the circuit, when the switch S is pressed, current flows through the base and is approximately equal to IB=10uA, so if the collector current is displayed in milliamperes coefficient is equal to:

For example, if the multimeter shows 2.4mA, coefficient is equal to hFE=2.4*100= 240.


Fig. 4.6: Measuring the hFE

While measuring NPN transistors, battery should be connected in a manner displayed on the picture. For PNP transistors polarity of the battery is reversed: plus is connected to Y, and minus to X. In that case, probes should be reversed as well if you're using analog instrument (one with a needle), if you're using digital (highly recommended) it doesn't matter which probe goes where, but if you do it the same way as you did with NPN there would be a minus in front of the read value, which means that current flows in the opposite direction.

All components should be mounted in a housing, a plastic, wooden or some other non-conducting material box would do perfectly. On the top side of the box you should drill holes for four connectors: two (X and Y) for power supply, and two (A and B) as contacts for your multimeter's probes. Beside them, place a switch S, three connectors (or even ordinary screws) for C, B and E in a way shown on picture. Transistor's pins are placed on the screws, switch is flipped and current is read out from the multimeter. One practical usage of this circuit will be discussed in the next "Practical ELECTRONICS".

4.3 The safest way to test transistors

Author of these lines was often in situation to talk to repairmen of different electronic devices who claimed that when they tested a transistor using the multimeter it seemed to operate properly, but placed back into it's circuit it acted faulty. What this means is that using only multimeter to check the transistor might not be sufficient and 100% fullproof. Repairmen suggested simulating a circuit using some small oscillator, like the one on 4.7. This circuit is called a multi vibrator. If both transistors inside it are operable LED blinks several times per second. If the transistor in question is functional, replacing the T2 with it would have kept the LED blinking. Supply voltage ranges between 12V and 4.5V. Brightness of the LED could be improved by lowering value of the resistor R. Frequency of the whole circuit could be modified as well by changing the values of the resistors R2 and R3, or capacitors C1 and C2. Whole device should be placed in a small box, much like the previous circuit. There should be holes for a LED and three screws (E, B and C).


Fig. 4.7: Tranistor testing oscillator

To test PNP transistors, same would go, only the transistor which would need to be replaced is the T1, and the battery, LED, C1 and C2 should be reversed (which makes building two circuits specifically for NPN and PNP transistors possibly a better option for an unexperienced electrician).

4.4 TUN and TUP

As we previously said, many electronic devices work normally even if the transistors with a certain mark is replaced with another, similar transistor. This said, you would find many magazines and Internet sources using marks TUN and TUP in their schematics. These are general purpose transistors. TUN marks a general purpose NPN transistor, and TUP is a general purpose PNP transistor.
These transistors have following characteristics:

 

4.5 Practical examples

The most common role of a transistor in an analog circuit is as an active (amplifying) component of different amplifiers (1.5a and 2.6a), oscillators (1.5b) and other similar circuits. As another example of transistor usage is a 4.8a. This is a schematic for a simple detecting radio receiver.

Variable capacitor C and the coil L are forming a parallel oscillating circuit which is used to single out a signal of a single radio station out of many different voltages. AA121 diode, a 100pF capacitor, and a 500kiloohm resistor are forming a diode detector which is used to transform the low frequency voltage into an information (music, speech). Information acquired on the 500kiloohm resistor is forwarded further through a 5uF capacitor to the base of a transistor. Transistor, resistor, speakerphones and the battery are forming a low frequency amplifier which amplifies and reproduces the signal.
On 4.8 there are symbols for a common ground and grounding. Beginners are usually asuming these two as same which is a mistake. On the circuit board common ground is a copper line whose size is significantly bigger than other lines. When this radio receiver is placed on a circuit board, common ground is a copper strip connecting holes where the lower end of the capacitor C, coil L 100pF capacitor and 500 kiloohm resistor are soldered. On the other hand, grounding is a metal rod stuck in a wet earth (connecting your circuit's grounding point to the plumbing or heating system of your house is also a good way to ground your device).
Some home appliances have grounding for security reasons (boiler, fridge, heater), some don't but all have common ground. Grounding is always connected to the common ground, which is possibly the source of the discussed often misconception that common ground and grounding are the same thing.
Resistor R2 is used to bring the needed DC voltage to the base of the transistor. This voltage should be around 0.7V, so that voltage over collector is approximately equal to one half of the battery voltage. If this is not the case, resistor should be replaced with the appropriate one as discussed previously. This could be done manually resorting to your "ear" instrument, by changing the value of the resistor until the best signal quality is achieved.


Fig. 4.8: Detector receiver with a simple amplifier

 

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