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                                           email Colin Mitchell:   talking@tpg.com.au




A lot of time is spent on the web showing and explaining how to design a transistor amplifier.
Most of the discussions are vague, inaccurate and fail to explain anything.
I have produced a revolutionary article on what a transistor REALLY IS.  It is really a variable resistor and you can see the discussion HERE.
And you can see the other article on understanding how to design transistor stages HERE.
You must not spend a lot of time, worrying about how to design a transistor stage such as a common emitter amplifier as the gain of the transistor and the waveforms and output load are not know and cannot be determined and you cannot get any sort of accuracy.  The only way to approach this is to build the circuit and experiment with the values.  That's why you don't need to spend any study-time on this topic and certainly don't need any mathematics. 
It is much more important to spend time learning and understanding how a transistor can be interfaced and connected to devices (and chips) to solve all sorts of difficult problems.
That's where the skill of the electronics engineer comes in.
I have designed hundreds of circuits and modules and sold them on the internet and not one module has been copied or resold as a "clone." That's because very few people understand how circuits work and if one of the modules does not work, they are incapable of fixing it.
That's also the reason why not a single University or School or College has bought a single kit from me in the past 50 years as the instructors are incapable of diagnosing and fixing anything if the student messes up the construction.
I have evidence that the instructors are petrified if a student brings a module for repair.
That's because the electronics they have learned and teach is completely WORTHLESS. It does not give you any ability to see how things "work."
This brings me to the point that none of the courses are worth the money you pay, and you come out with absolutely no understanding.  It's just a whole lot of  mesmerizing mathematics, to hide the real fact that they have taught you NOTHING. 
If you can't see how a transistor REALLY WORKS, you will not be able to design anything out of the ordinary. That's why nothing like I have designed, has been produced or presented on the web.
That's why it is important to read the article:  What a 
transistor REALLY IS: HERE.


We will start with some very simple circuits and show what can be done:

These are what we call the "common 10% range." These are suitable for 90% of all projects and you only need a different value for some special circuits where an accurate voltage is required. You can always get any value you want by combining two resistors in parallel or series, so you don't have to buy 1% resistors.


The simplest circuit is a transistor connected to a microcontroller.
A microcontroller produces a 5v output with a current capability of 20mA.
The base of a transistor never rises above 0.7v because the base is connected to a diode inside the transistor this is one of the features we cannot change.
We have to "match" 5v to 0.7v and this is called INTERFACING.  The simplest may to connect these two voltages is with a resistor. The resistor will have a voltage across it of 4.3v and depending on the value of the resistor, it will allow a current to flow.  This current will flow into the base of the transistor and turn it ON.  The transistor will have the capability of allowing 100 times this current to flow through the collector-emitter terminals but the actual current is not determined by the transistor but by the LOAD in the collector.
The LOAD in the collector is actually a resistor connected between the collector terminal and the positive rail.
When this resistor has a low value of resistance, a HIGH current will flow. When the value of resistance is high, a small current will flow.  This flow is nothing to do with the transistor. The transistor is capable of delivering a high current and the load can take any current up to this value.
I am capable of giving you one million dollars a day, but you are only capable of spending $100,000.   If I increased your capability, like reducing the value of resistance of the LOAD, you may be able to spend $200,000 a day.

The maximum current from a microcontroller is 20mA.  A 270R resistor connected the output of the micro to a transistor will allow about 20mA to enter the base.
A 1k resistor will allow about 5mA and 10k will allow 0.5mA. 
You cannot connect the output of the micro directly to the base because this will not allow the output to rise higher than 0.7v and the FET transistor on the output will not be turned on correctly and FULLY and it will get hot.  Don't forget the FET is microscopic and may get damaged.

This is called a DIRECT CONNECTION or DC connection because there is no capacitor involved.
When the output of the microcontroller goes HIGH, the transistor turns ON and the LED illuminates. You have "activated" the transistor and it has turned into a low-value resistor and the current through the transistor and the current-limiting resistor is sufficient to illuminate the LED. Reducing the value of the current-limiting resistor will make the LED glow brighter. But you cannot remove the current-limiting resistor. The LED will glow too bright and be destroyed.
A LED requires from 1mA to 20mA for 3mm or 5mm LEDs.  Larger LEDs need 250mA to 350mA and they illuminate a vary big area. You have to know how much current the LED needs before you start but if you do not know, start with 1k resistor. The CURRENT-LIMITING resistor determines the current.
Every LED and every different colour LED will produce a voltage across it called the CHARACTERISTIC VOLTAGE.  Do not worry about this voltage. Just put a 1k resistor in series with the LED and see how bright it is.  You can vary the brightness by changing the value of the resistor. If you make the resistance too low, the LED will go brightly and go "pop." This will be your first lesson.
Do not worry about any of the formulas for LED resistor values or any of the voltages across a LED.  These values will change slightly from different manufacturers and we will cover this topic later.


Fig:1 shows a microcontroller driving a transistor and illuminating a LED. This is just a simple example as a microcontroller can drive a LED directly from an output.
But if you use a transistor, the microcontroller can drive a lot of LEDs and the micro will not get hot.
If the transistor has a gain of 100, and the LED requires 10mA, you only need 0.1mA into the base and a 50k base resistor can be used. But if we use say 10k the current will be 0.5mA and the micro can drive other things.
The supply voltage has been shown because a micro does not like supply voltages above about 5.5v.   When you see 470R and LED in series, it does not matter if the resistor is above the LED or below. Components in series can be in any order as the current through them will be the same and it just depends on the convenience on the printed circuit board.

Fig:2 (we fixed the base resistor)

You don't deliver more current than necessary to the base of a transistor because the project may be battery operated and you want the module to take the lease current. Base-current is effectively "wasted current" and need to keep it a small as possible. You can get LEDs that take 1 to 5mA and are so bright they blind you, so you may not need the transistor. This is just a point to remember as the microcontroller will deliver up to 20mA.

If we replace the LED with a motor, it will not work.


The circuit above is designed to deliver about 50mA maximum and this may not be sufficient to activate a motor when it is under load.
In addition, the BC547 transistor is only capable of delivering about 100mA so a 1amp transistor such as BC338 will be needed.
But the main component to be changed is the base resistor.
But before we do this, we have to know that a motor takes about 5 times more current when starting and so a 50mA motor can take 250mA when starting and although we don't have to be aware of this, when you are talking a digital circuit,  it has to be designed correctly.  So we will design for 250mA collector current to get the following circuit:


We have changed the transistor to one capable of delivering up to 1 amp. We have changed the base resistor to 1k.  We have changed the gain of the transistor to 50 to be sure it will deliver the current.  The 5mA x 50 = 250mA.  This is what the circuit will deliver IF the motor asks for up to 250mA.
What happens if the motor asks for 350mA??   The transistor, in this arrangement, will only deliver 250mA and when 250mA is being delivered, the transistor is fully turned ON. This is called fully saturated and the voltage across the collector-emitter leads of transistor will be less than 0.5v and it will not get very hot.
If the motor asks for more than 250mA, the transistor will try to provide the extra current and it will not remain fully saturated.  The voltage across the collector-emitter terminals will rise and the voltage to the motor will be less.
But one of the unfortunate features of this situation is the voltage and current through the transistor will increase and the transistor will start to get very hot and that's why it is very important to keep the transistor fully saturated. You will be able to feel the huge rise in temperature with your fingers and watch the transistor "blow-up."   This is due to taking the transistor OUT OF SATURATION.

Connecting LEDs to a circuit is very simple and easy.
You don't have to worry about voltages or current or any special connection.   All you have to do is connect a LED to a 6v battery with a 1k resistor in series and look at the brightness. You need a 6v battery because some LEDs will not work with a 3v battery and none with work with a single 1.5v cell.
However all LEDs will work with a 3.7v lithium cell.

We will start with the simples circuit. It is a 5mm red LED, 1k resistor and 6v battery. The LED must be connected around the correct way. If is connected around the wrong way it will not illuminate. (It may not get damaged). There are two ways to identify the leads. The short lead is called the cathode (k) and some LEDs have a flat side to indicate this lead.  The cathode lead goes to the negative lead of the battery and we also call the negative of the battery the 0v rail.
You can try red, yellow, green, blue and white LEDs. They will all have different brightness and you can increase the resistor to 10k and down to 220R. Some high bright LEDs need just 1mA to produce good brightness and some can be delivered 20mA (MAX), via a resistor, to produce high-brightness for a key-hole torch.  


It is unnecessary to use any formulas or mathematics to work out the value of a resistor as you don't know the current required by the LED.  The only way is to experiment with different values and pick a value that uses the least current for the brightness you accept, especially if the project is powered by batteries.
Here are 4 ways to connect a LED and only one way will work:

The two leads of a LED are called ANODE and CATHODE. The short lead is the cathode and is identified on a circuit with the letter "k."  All LEDs have a characteristic voltage that develops across the leads according to the colour of the LED. This is a fairly constant voltage for each colour, but will change slightly for each LED (in a batch) and from different manufacturers and according to the brightness.  But it will not change very much for each colour. However you must remember each colour is different. The only two colours that have almost the same voltage across them is blue and white.

LEDs can be connected in parallel and series. There are laws and limitations to doing this, so we will simplify the complications by outlining just a few requirements.

A single red LED can be connected across a 6v supply as a red LED only needs about 1.7v

A single blue LED can be connected across a 6v supply as a blue LED only needs about 3.2v

Two red LEDs can be connected across a 6v supply as they only needs about
1.7v +
 1.7v  = 3.4v    We have 2.4v across the resistor.


Two blue LED cannot be connected across a 6v supply as the need about 3.2v  + 3.2v = 6.4v  and we only have 6v.  The LEDs will not illuminate. The voltage is insufficient.
This applies to all LEDs connected in series. The voltage of the supply must be greater than the total characteristic voltage across the LEDs.


You can connect any colour LEDs provided the total combined voltage is not higher then the supply voltage.
The value of the resistor is done experimentally.  Select a 1k resistor and reduce the value until you have the brightness you want.

Connecting LEDs in parallel is a problem. You can only connect LEDs of the same colour in PARALLEL.

Two red LEDs connected in parallel

Two blue LEDs connected in parallel


When a red LED and blue LED are connected in parallel (or any colour is connected in parallel with a red LED) the red LED will have a characteristic voltage across it of 1.7v and this voltage WILL NOT RISE.  This voltage is not enough for any other colour to illuminate and only the red LED will illuminate.   

Now you know the limitations of connecting LEDs in Parallel and Series, you can create LED displays THAT WILL WORK.


Here's the first trick that no-one has ever mentioned. For low-power output from a speaker, you can use 8R, 16R, 32R or even 50R and a quality speaker will deliver the same volume.
A speaker is just like a motor. It has an electromagnet winding called a voice-coil and a magnet. When current flows through the voice-coil moves due to the interaction with the magnet. Exactly the same as motor revolving.
Both of these items work on a principle called AMP TURNS. It's the multiplication of current and the number of turns that produces the "magnetism." And the stronger the magnetism (magnetic flux) the stronger the motor and the the louder the sound.
When you multiply the current through an 8 ohm speaker and the number of turns on the voice-coil, you will find it is the same as a 16R speaker and even a 50 ohm speaker. The advantage of a high-impedance speaker is the low current and the circuit likes a low current.

5  50R voice coil  

When you see a transistor with current of 1mA entering the base, you can assume the collector current can be a maximum of 100mA. This is the called the "amplification factor" of the transistor. A BC547 transistor can only handle 100mA, so this applies to all the "small-signal" transistors. By "small-signal," the data sheets really mean small current.
If a transistor is rated at 1 amp, only allow 500mA, etc.

Fig:16  Current gain of 100

The circuit may not produce a gain of 100 due to the surrounding components, but this is the way you "see" a transistor "in action."  Every time you see a transistor in a circuit you assume it has the capability of increasing the current by 100 times. Voltage may also be increased but there is no "accepted trick" for dealing with voltages.
When the transistor is passing )allowing to flow) a current of 50% or more, the gain of the transistor drops considerably.  For instance a 1 amp "small signal" transistor can only be guaranteed for a gain of 20 to 50. All these figures are very VARIABLE and that's why it is pointless doing any mathematical calculations. The result could be 200% or 50%.

Too much time is spent on YouTube, about the 0.7v base voltage.   It is so small that we are not going to take it into account. 

Fig:17 The Base voltage

However, you need to know the voltage on the base must be higher than 0.7v for the transistor to be operating.
This voltage must be very accurate and only the transistor can produce this voltage.
We deliver a voltage that is higher than 0.7v and the transistor produces the 0.7v via the base resistor.
This voltage will increase very slightly as the current delivered by the collector-emitter circuit increases, but we have no control over the actual base voltage.
All we do is two things: we deliver a voltage that is higher than 0.7v, via the base resistor and at the same time we deliver a current that the transistor uses, so it able to deliver current through the collector-emitter leads.
It is the load resistor that asks for the current and if the transistor is able to supply the current, it is able to go to a FULLY SATURATED state, in which the voltage across the collector-emitter terminals is as low as 0.3v. 

All we are covering at the moment is two things:
1. Deliver a voltage above 0.7v to the base resistor
2. Deliver a current to the base so the transistor SATURATES (if required).

The only time when you need to know about the 0.7v base voltage is when you want to turn the transistor OFF.
Some input signals only go to a LOW of 0.7v and the transistor will be turned ON slightly.  You need to go as low as 0.55v to be sure the transistor is OFF.

When you see a transisto0%.  

When you see a transisto0%.  

When you see a transisto0%.  

When you see a transisto0%.  

When you see a transisto0%.  

A transistor can be connected to the output of a microcontroller with a resistor and a simple way to remember the base current is to remember 1k base resistor delivers 5mA.  470R delivers 10mA. 220R delivers 20mA ad this is the maximum for a microcontroller.
The transistor will deliver 100 times this current (in theory) but you should never go for the maximum. As the current through a transistor increases, its gain decreases. Normally a small-signal transistor has a gain of 250, but we normally allow a gain of 100 and we can reduce this to 30 or 50 if the current is high.  This make sure the circuit will always work.