testing an unknown transistor

TESTING AN unknown TRANSISTOR

The first thing you may want to do is test an unknown transistor for COLLECTOR, BASE AND EMITTER. You also need to know if it is NPN or PNP.
You need a cheap multimeter called an ANALOGUE METER  - a multimeter with a scale and pointer (needle).
It will measure resistance values (normally used to test resistors) - (you can also test other components) and Voltage and Current. We use the resistance settings. It may have ranges such as "x10"  "x100"   "x1k"   "x10"
Look at the resistance scale on the meter. It will be the top scale.
The scale starts at zero on the right and the high values are on the left. This is opposite to all the other scales. .
When the two probes are touched together, the needle swings FULL SCALE and reads "ZERO." Adjust the pot on the side of the meter to make the pointer read exactly zero.

How to read:  "x10"  "x100"   "x1k"   "x10"
Up-scale from the zero mark is "1" 
When the needle swings to this position on the "x10" setting, the value is 10 ohms.
When the needle swings to "1" on the "x100" setting, the value is 100 ohms.
When the needle swings to "1" on the "x1k" setting, the value is 1,000 ohms = 1k.
When the needle swings to "1" on the "x10k" setting, the value is 10,000 ohms = 10k.
Use this to work out all the other values on the scale.
Resistance values get very close-together (and very inaccurate) at the high end of the scale. [This is just a point to note and does not affect testing a transistor.]
Step 1   - FINDING THE BASE  and determining NPN or PNP
Get an unknown transistor and test it with a multimeter set to "x10"
Try the 6 combinations and when you have the black probe on a pin and the red probe touches the other pins and the meter swings nearly full scale, you have an NPN transistor. The black probe is BASE
If the red probe touches a pin and the black probe produces a swing on the other two pins, you have a PNP transistor. The red probe is BASE
If the needle swings FULL SCALE or if it swings for more than 2 readings, the transistor is FAULTY. 

Step 2   - FINDING THE COLLECTOR and EMITTER
Set the meter to "x10k." 
For an NPN transistor, place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale.   

For a PNP transistor, set the meter to "x10k"  place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale.   

Little Giant -- The transistor

Part 1 of Little Giant, a 1959 U.S. government film about new industry applications for the transistor

Nobel Prize for Transistor

Newsreel from 1956, showing William Shockley, Walter Brattain and John Bardeen receiving the Nobel Prize for the transistor.

William Shockley Interview, 1969

In this video 1969, Jane Morgan interviews William Shockley, co-inventor of the transistor. The interview is part of a series done for the Palo Alto 75th anniversary celebrations. Thanks to the Palo Alto Historical Association.

4. Transistors

4. Transistors

Transistors are active components and are found everywhere in electronic circuits. They are used as amplifiers and switching devices. As amplifiers, they are used in high and low frequency stages, oscillators, modulators, detectors and in any circuit needing to perform a function. In digital circuits they are used as switches.


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. There are low, medium and high power transistors, for working with high and low frequencies, for working with very high current and/or high voltages. Several different transistors are shown on 4.1.

The most common type of transistor is called bipolar and these are divided into NPN and PNP types.
Their construction-material is most commonly silicon (their marking has the letter B) or germanium (their marking has the letter A). Original transistor were made from germanium, but they were very temperature-sensitive. Silicon transistors are much more temperature-tolerant and much cheaper to manufacture.

Fig. 4.1: Different transistors


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

The second letter in transistor’s marking describes its primary use:
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.
The number following the letter is of no importance to users.
American transistor manufacturers have different marks, with a 2N prefix followed by a number (2N3055, for example). This mark is similar to diode marks, which have a 1N prefix (e.g. 1N4004).
Japanese bipolar transistor 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.

Several different transistors are shown in photo 4.1, and symbols for schematics are on 4.2. Low power transistors are housed in 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 the metal housing. This lead is connected to the ground of the circuit, to protect the transistor from possible external electrical interference. Four leads emerge from some other types, such as two-gate FETs. High power transistors are different from low-to-medium power, both in size and in shape.

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

Fig. 4.3: Pinouts of some common packages

It might be useful to remember the pinout for TO-1, TO-5, TO-18 and TO-72 packages and compare them with the drawing 4.2 (a). These transistors are the ones you will come across frequently in everyday work.

The TO-3 package, which is used to house high-power transistors, has only two pins, one for base, and one for emitter. The collector is connected to the package, and this is connected to the rest of the circuit via one of the screws which fasten the transistor to the heat-sink.

Transistors used with very high frequencies (like BFR14) have pins shaped differently.
One of the breakthroughs in the field of electronic components was the invention of SMD (surface mount devices) circuits. This technology allowed manufacturers to achieve tiny components with the same properties as their larger counterparts, and therefore reduce the size and cost of the design. One of the SMD housings is the SOT23 package. There is, however, a trade-off to this, SMD components are difficult to solder to the PC board and they usually need special soldering equipment.

As we said, there are literally thousands of different transistors, many of them have similar characteristics, which makes it possible to replace a faulty transistor with a different one. The characteristics and similarities can be found in comparison charts. If you do not have one these charts, you can try some of the transistors you already have. If the circuit continues to operate correctly, everything is ok. You can only replace an NPN transistor with an NPN transistor. The same goes if the transistor is PNP or a FET. It is also necessary to make sure the pinout is correct, before you solder it in place and power up the project.
As a helpful guide, there is a chart in this chapter which shows a list of replacements for some frequently used transistors.

4.1 The working principle of a transistor

Transistors are used in analog circuits to amplify a signal. They are also used in power supplies as a regulator and you will also find them used as a switch in digital circuits.
The best way to explore the basics of transistors is by experimenting. A simple circuit is shown below. It uses a power transistor to illuminate a globe. You will also need a battery, a small light bulb (taken from a flashlight) with properties near 4.5V/0.3A, a linear potentiometer (5k) and a 470 ohm resistor. These components should be connected as shown in figure 4.4a.

Fig. 4.4: Working principle of a transistor: potentiometer moves toward its upper position - voltage on the base increases - current through the base increases - current through the collector increases - the brightness of the globe increases.

Resistor (R) isn't really necessary, but if you don't use it, you mustn't turn the potentiometer (pot) to its high position, because that would destroy the transistor - this is because the DC voltage UBE (voltage between the base and the emitter), should not be higher than 0.6V, for silicon transistors.

Turn the potentiometer to its lowest position. This brings the voltage on the base (or more correctly between the base and ground) to zero volts (UBE = 0). The bulb doesn't light, which means there is no current passing through the transistor.

As we already mentioned, the potentiometers lowest position means that UBE is equal to zero. When we turn the knob from its lowest position UBE gradually increases. When UBE reaches 0.6v, current starts to enter the transistor and the globe starts to glow. As the pot is turned further, the voltage on the base remains at 0.6v but the current increases and this increases the current through the collector-emitter circuit. If the pot is turned fully, the base voltage will increase slightly to about 0.75v but the current will increase significantly and the globe will glow brightly.


If we connected an ammeter between the collector and the bulb (to measure IC), another ammeter between the pot and the base (for measuring IB), and a voltmeter between the ground and the base and repeat the whole experiment, we will find some interesting data. When the pot is in its low position UBE is equal to 0V, as well as currents IC and IB. When the pot is turned, these values start to rise until the bulb starts to glow when they are: UBE = 0.6V, IB = 0.8mA and IB = 36 mA (if your values differ from these values, it is because the 2N3055 the writer used doesn't have the same specifications as the one you use, which is common 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.

Let's look at another experiment which will broaden our knowledge of the transistor. It requires a BC107 transistor (or any similar low power transistor), supply source (same as in previous experiment), 1M resistor, headphones and an electrolytic capacitor whose value may range between 10u to 100µF with any operating voltage.
A simple low frequency amplifier can be built from these components as shown in diagram 4.5.

Fig. 4.5: A simple 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 connected to headphones. The "turn-on" resistor - the resistor on the base, is 1M. When there is no resistor, there is no current flow IB, and no Ic current. When the resistor is connected to the circuit, base voltage is equal to 0.6V, and the base current IB = 4µA. The transistor has a gain of 250 and this means the collector current will be 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:

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

Put the headphones on and place a fingertip on point 1. You will hear a noise. You body picks up the 50Hz AC "mains" voltage. The noise heard from the headphones is that voltage, only amplified by the transistor. Let's explain this circuit a bit more. Ac voltage with frequency 50Hz is connected to transistor's base via the capacitor C. Voltage on the base is now equal to the sum of a DC voltage (0.6 approx.) via 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 slightly 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 low frequency signal source (CD player or a microphone).

There are literally thousands of different circuits using a transistor as an active, amplifying device. 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.

4.2 Basic characteristics of transistors

Selecting the correct transistor for a circuit is based on the following characteristics: maximum voltage rating between the collector and the emitter UCEmax, maximum collector current ICmax and the maximum power rating PCmax.
If you need to change a faulty transistor, or you feel comfortable enough to build a new circuit, pay attention to these three values. Your circuit must not exceed the maximum rating values of the transistor. If this is disregarded there are possibilities of permanent circuit damage. Beside the values we mentioned, it is sometimes important to know the current amplification, and maximum frequency of operation.
When there is a DC voltage UCE between the collector (C) and emitter (E) with a collector current, the transistor acts as a small electrical heater whose power is given with this equation:

Because of that, the transistor is heating itself and everything in its proximity. When UCE or ICE rise (or both of them), the transistor may overheat and become damaged. Maximum power rating for a transistor, is PCmax (found in a datasheet). What this means is that the product of UCE and IC should should not be higher than PCmax:

So, if the voltage across the transistor is increased, the current must be dropped.
For example, maximum ratings for a BC107 transistor are:
ICmax=100mA,
UCEmax = 45V and
PCmax = 300mW
If we need a Ic=60mA , the maximum voltage is:

 For UCE = 30V, the maximum current is:

Among its other characteristics, this transistor has current amplification coefficient in range between hFE= 100 to 450, and it can be used for frequencies under 300MHz. According to the recommended values given by the manufacturer, optimum results (stability, low distortion and noise, high gain, etc.) are with UCE=5V and IC=2mA.
There are occasions when the heat generated by a transistor cannot be overcome by adjusting voltages and current. In this case the transistors have a metal plate with hole, which is used to attach it to a heat-sink to allow the heat to be passed to a larger surface.

Current amplification is of importance when used in some circuits, where there is a need for equal amplification of two transistors. For example, 2N3055H transistors 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 means that in cases when two identical coefficients are needed, they should be measured. Some multimeters have the option for measuring this, but most don't. Because of this we have 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 1k resistors are used to protect the instrument if the transistor is "damaged". As we said, current gain 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 milliamps. The gain is equal to:

For example, if the multimeter shows 2.4mA, hFE = 2.4*100 = 240.

Fig. 4.6: Measuring the hFE

While measuring NPN transistors, the supply should be connected as shown in the diagram. For PNP transistors the battery is reversed. In that case, probes should be reversed as well if you're using analog instrument (one with a needle). If you are using a digital meter (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.

4.3 The safest way to test transistors

Another way to test transistor is to put it into a circuit and detect the operation. The following circuit is a multivibrator. The "test transistor" is T2. The supply voltage can be up to 12v. The LED will blink when a good transistor is fitted to the circuit.

Fig. 4.7: Oscillator to test transistors

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.

4.4 TUN and TUP

As we previously said, many electronic devices work perfectly even if the transistor is replaced with a similar device. Because of this, many magazines use the identification TUN and TUP in their schematics. These are general purpose transistors. TUN identifies a general purpose NPN transistor, and TUP is a general purpose PNP transistor.

TUN = Transistor Universal NPN and TUP = Transistor Universal PNP.

These transistors have following characteristics:

4.5 Practical example

The most common role of a transistor in an analog circuit is as an active (amplifying) component. Diagram 4.8 shows a simple radio receiver - commonly called a "Crystal Set with amplifier."

Variable capacitor C and coil L form a parallel oscillating circuit which is used to pick out the signal of a radio station out of many different signals of different frequencies. A diode, 100pF capacitor and a 470k resistor form a diode detector which is used to transform the low frequency voltage into information (music, speech). Information across the 470k resistor passes through a 1uF capacitor to the base of a transistor. The transistor and its associated components create a low frequency amplifier which amplifies the signal.
On figure 4.8 there are symbols for a common ground and grounding. Beginners usually assume these two are the same which is a mistake. On the circuit board the common ground is a copper track whose size is significantly wider than the other tracks. When this radio receiver is built 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 470k resistor are soldered. On the other hand, grounding is a metal rod stuck in a wet earth (connecting your circuits grounding point to the plumbing or heating system of your house is also a good way to ground your project).
Resistor R2 biases the transistor. This voltage should be around 0.7V, so that voltage on the collector is approximately equal to half the battery voltage.

Fig. 4.8: Detector receiver with a simple amplifier

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21_ Darlington Pair Tutorial

Darlington Pair Tutorial

The emitter current of Tr1 is the base current of Tr2.
A change in base current of Tr1 can give a change 100 times larger in its emitter current.
A change in the base current of Tr2 has a similar effect on its emitter current.
Therefore there is an overall amplification of 100 x 100 = 10000 times.
This circuit is sometimes called the Super Alpha Pair.
It is often used as a power output stage.
The two transistors can come in the same package.

20_ Directly Coupled Amplifier Tutorial

Directly Coupled Amplifier Tutorial

The PNP transistor is upside down with respect the NPN transistor and thus has the correct operating voltages.

If the circuit is designed so that the PNP collector voltage is a suitable value for the NPN transistor base, then there is no need for bias resistors for the NPN transistor.

The circuit will amplify dc signals as well as ac ones. This is because there is no coupling capacitor to block dc.
It will also amplify very low ac frequencies because there is no capacitive reactance to oppose them.

However, if temperature changes cause dc drift in the first transistor, then these will be amplified by the second transistor and change its normal operating voltages.
This is usually prevented by some form of dc stabilization.

19_ Complementary Push-Pull Amplifier Tutorial

Complementary Push-Pull Amplifier Tutorial

It is sometimes difficult to understand the behaviour of a circuit when the transistors are drawn upside down.

In diagram A,as the base voltage goes towards the + rail (goes more positive), the collector voltage goes away from the + rail (goes less positive).

In diagram B,as the base voltage goes towards the - rail (goes more negative), the collector voltage goes away from the - rail (goes less negative).

In diagram C,as the base voltage goes away from - rail (goes more positive), the collector voltage goes towards the - rail (goes less positive).

In diagram D,as the base voltage goes away from + rail (goes more negative), the collector voltage goes towards the + rail (goes less negative).

TR2 and TR3 are complementary. They have the same characteristics but one is NPN and the other PNP.

The NPN has + on its collector and less positive on its emitter.

The PNP Has + on its emitter and zero on its collector.

Therefore both have correct polarity voltages.

The voltage at the junction of R3 and R4 is half the supply voltage.

If R2 is replaced by a wire link, and R1 selected to give half the supply volts at TR1 and TR2 bases then they would both be biased in class B, and both would be non conducting.

Look at the waveforms on the bases of the output pair.

During the first half cycle the signal is going less positive.
This is reversing biasing the NPN transistor and increasing forward bias on the PNP transistor.
This means that the NPN remains cut off and the PNP conducts during this first half cycle.

In the next half cycle the signal on the bases is now increasing in a more positive direction.
During this half cycle the NPN conducts while the PNP is cutoff.

So the NPN is off and the PNP is on during the first half cycle, and current flows through the loudspeaker as C1 charges.

The NPN is on and the PNP off during the second half cycle, and current flows the other way through the loudspeaker, as C1 discharges.

R2 is added to give a small forward bias to both transistors thereby avoiding crossover distortion. (see the page on push-pull power amplifiers).

18_ Push Pull Amplifier Tutorial

Push Pull Amplifier Tutorial

TR2 and TR3 are biased in class B, which means that they are normally non conducting when there is no signal input.

TR1 and T1 form a phase splitter, opposite polarity signals appearing at the ends of T1 secondary.

When there is a signal in ,TR2 conducts on the positive half of the waveform only and TR3 conducts on the negative half.

Current flows down through the top half of T2 primary when TR2 conducts, and up through the bottom half when TR3 conducts.

Both halves are combined in the secondary of T2 to produce an amplified version of the input.

R1 and R2 apply a small forward bias to avoid crossover distortion, which is shown in the lower waveform.

This distortion is caused by the non-linear characteristics of the emitter/base junction. (for more details see the junction diode Vb/Ib curves).

The advantage of this circuit is that there is very little current taken if there is no signal input.

17_ Class a Power Amplifier Tutorial

Class a Power Amplifier Tutorial

The transistor is biased in class A which means that collector current flows all the time..

The collector current can increase or decrease.

The input signal increases and decreases the forward bias causing the collector current to change.

These changes in current in the primary of the transformer induce signal currents in the secondary.

The transformer matches the output impedance of the transistor to the loudspeaker impedance.

The disadvantage of this circuit is that the collector current is high even if there is no signal input.
Class B push-pull is more efficient.

16_ Phase Splitter Tutorial

Phase Splitter Tutorial

The phase splitter has one input and two outputs.
The two outputs are inverted with respect to each other. That is, as one increases in a positive direction the other increases in a negative direction.

In the first diagram the phase splitting is done by the transformer.

In the second diagram, the output from the collector is an inverted version of the input.
The output from the emitter follows the input.
There is no amplification from this circuit because the emitter is undecoupled.

The word PHASE indicates a shift in time. In actual fact there is no phase change here, only inversion of the signal.

15_ Common Collector Amplifier Tutorial

Common Collector Amplifier Tutorial

The positive power supply rail is joined to the zero volts rail by C3. As far as ac is concerned, both rails are joined together.

Therefore they, and the collector, are common to both input and output.

Since the emitter voltage follows the base voltage, it is also called the emitter follower.

Current gain is Ie/Ib which is quite high, typically 50.

Voltage gain is only 1 because of the undecoupled emitter.

The input impedance is high, typically 500k, requiring only low power to drive it.

The output impedance is low, typically 20 ohms.

The output signal follows the input. There is no inversion.

It is often used to match high impedances to low ones.

It can be used to drive several high impedance loads.

14 _ Common Base Amplifier Tutorial

Common Base Amplifier Tutorial

C3 connects the base to ground as far as ac is concerned.

Therefore both input and output are connected to the base. (common base amplifier).

Current gain is Ic/Ie which is less than 1.

The voltage gain is high since it is Rc/Re. (Approximately the same current flows through them). It is typically 250.

The input impedance is low, typically 20 ohms.

The output impedance is high, typically 1Megohm.

The output signal is not inverted with respect the input.

It is often used to match low impedance devices to high impedance ones.

It is commonly used at VHF.

13_ Common Emitter Amplifier Tutorial

Common Emitter Amplifier Tutorial

Sometimes called the grounded emitter, since the emitter capacitor connects the emitter to ground at ac frequencies.

Since, as far as ac is concerned, the emitter is joined to ground, both input and output are connected to the emitter.

Current gain is Ic/Ib and can be quite high, typically 50.

Voltage gain is high, typically 250

Input impedance is medium, say 5K.

Output impedance is medium, say 20k.

The output is inverted with respect to the input.

Its most common application is as a voltage amplifier.

12 _ Tuned Amplifier Tutorial

Tuned Amplifier Tutorial

Here the load resistor has been replaced by a tuned circuit, C4 and L1.
At resonance, the tuned circuit is high impedance.
At all other frequencies it is a low impedance.
Therefore only signals at the resonant frequency will be amplified

Since the tuned amplifier works at radio frequencies the capacitors can be much smaller in value than those used at audio frequencies

Amplifiers in Cascade Tutorial - 11

Amplifiers in Cascade Tutorial

If the gain of one stage of amplification is insufficient, then two or more stages can be connected in cascade, as shown.

If the gain of each stage is 50 times, then the overall gain is 50 x 50 =2,500.
If the input to Tr1 is 1 mV, then the output of Tr2 is 2,500 mV =2.5Volts.

If the gain is excessive the circuit may be unstable, or the output may be clipped.

9 _ Load Resistor

Load Resistor Tutorial

R1 and R2 bias the transistor in class A, so that a steady collector current flows.

R3 is the load resistor.

Look at the signal into C1.

During the first half cycle it is positive, and increases the forward bias on the base of the transistor.
This increases the base current and thereby the collector current through the load resistor.
Therefore the voltage across the load increases, and the collector voltage, with respect to the zero line, decreases.
The collector voltage falls as the base voltage rises.

During the second half cycle, the input signal goes negative, reducing the forward bias on the base.
This reduces both base and collector currents.
The voltage across the load resistor falls and the voltage on the collector rises.
Again, the collector voltage has done the opposite of the base voltage.

The transistor inverts the input signal as well as amplifying it.

8 _ Emitter Stabliser Resistor

Emitter Stabliser Resistor Tutorial

R1 and R2 bias the transistor in class A so that a steady dc collector flows.

R4 is the emitter stabilising resistor.

When collector current flows it causes the transistor to heat up.
This causes the base current to increase which in turn causes the collector current to rise.
This rise in collector current causes the temperature to increase even more, and the base and collector currents continue increasing.
This behaviour is called THERMAL RUNAWAY and will destroy the transistor.

If we consider the emitter/base junction as a diode as shown in the right hand diagram we can see that the base (anode) voltage is fixed by R1 and R2.

If the collector current tries to rise due to heating, then the voltage across R4 will try to rise, making the emitter (cathode) more positive.
This would reduce the voltage across the junction (diode) making it less forward biased and reducing the base current and hence the collector current, which was trying to rise.

Therefore the circuit has been stabilised against thermal runaway.

However, if an ac signal is applied to the base, the varying collector current will cause a varying voltage across the emitter resistor.
This voltage will follow the base voltage. This means that the base/emitter (anode/cathode) voltage will be constant instead of the base varying with respect to the emitter.
To fix this, the emitter is joined to earth, as far as the ac signal is concerned, by the emitter decoupling capacitor.

7 _ Biasing a Transistor

Biasing a Transistor Tutorial

Choose a general purpose transistor with a beta gain higher than 100.

Decide on the collector current.

The base bias voltage is be 1/3 of the supply voltage.

The current through the base bias potential divider is to be 1/10 of the collector current.

Calculate the two base resistor values, R1 and R2.

The emitter voltage is 0.6 volts lower than the base voltage.

The value of the emitter resistor R4 is the emitter voltage divided by the collector current.

The value of the collector resistor R3 is the supply voltage divided by three times the collector current.

The values of the capacitors depend upon the application. Study a few circuits.

For common collector and common base some of the capacitors are connected differently.