BASIC 
ELECTRONICS COURSE 
Page 21
INDEX

Up to now we have shown how an NPN transistor can be connected to the power rails with the emitter near (or on) the negative rail (the 0v rail) and the collector going to the positive rail via a LOAD. This is called a COMMON EMITTER arrangement. 

The voltage on the base must be 0.7v higher than the emitter and the amount of current delivered to the base is multiplied by the GAIN OF THE TRANSISTOR and will be the maximum amount of current that can flow in the load. These are the basics of how a transistor works. Using these facts we can create a number of circuits, simply by connecting the transistor to the power rails in different ways. A single transistor and its associated parts is called  STAGE. Generally, a stage is separated by a capacitor at the front (called the input capacitor) and one at the output (called the output capacitor). This makes it easy to see the components associated with the stage as these parts play a part in turning the transistor on and setting the operating conditions for the stage. 

There are 3 ways to connect a transistor and the circuit produced for each connection has a special name, they are:
1. The Common EMITTER stage - the most OFTEN-USED arrangement
2. The Common COLLECTOR - also called EMITTER FOLLOWER stage 
3. The Common BASE stage.

It's easy to identify a COMMON EMITTER stage. The emitter is connected to the 0v rail. (It may be connected directly or via a low value resistor or via a resistor and capacitor - this will be discussed later). 
The COMMON COLLECTOR stage has the collector connected directly to the positive rail. 
The COMMON BASE stage has the base connected directly (or via a low-value resistor) to one of the rails. 

          


The potentiometer in the animations above and opposite is a VOLTAGE DIVIDER. When the wiper it is at 0v, (the centre of the pot is near the 0v rail) the output from the potentiometer is ZERO. As the potentiometer is rotated, the output rises. The COMMON EMITTER and COMMON COLLECTOR (EMITTER FOLLOWER) circuits do not turn on until the voltage on the base is 0.6v.  As the potentiometer is rotated further, the current delivered to the base of the Common Emitter stage increases and the transistor amplifies this current as shown in the animation. 
For the Common Collector stage, the voltage on the emitter is 0.6v less than the voltage on the base. As the voltage on the emitter rises, the current through the load increases. The voltage on the emitter is always 0.6v less than the base. That's why it is called an EMITTER FOLLOWER CIRCUIT. 



There are reasons why a particular stage is chosen for an application. The common-emitter stage is the most often used stage as it has medium input and output impedance (another name for impedance is "RESISTANCE") and has both voltage and current gain. 
If you want high input impedance, the EMITTER FOLLOWER stage is used. 
If you want very low input impedance, the COMMON BASE stage is used.
If you have a device (one that produces a signal) and it has a low output impedance, it should be matched up with an amplifying circuit that has a low input impedance. This will allow the maximum amount of energy to be passed from the signal source to the stage. 

A STAGE
A stage consists of a transistor and its biasing components. The diagram below shows the COMMON EMITTER stage with two base-bias resistors and emitter resistor. This is called the "BRIDGE BIASING" arrangement and a simpler arrangement is called the "SELF BIASING" stage. 

The self-biasing stage is shown below: It provide nearly the same features as the bridge arrangement but uses less components. 

The self-bias stage is the simplest and most often used. It has all the features necessary for a transistor to amplify a signal. It has:
1. A base-bias resistor to turn on the transistor
2. A load resistor.

HOW THE SELF-BIAS STAGE WORKS
The self-bias stage sets its own operating conditions due to the base-bias resistor between the base and collector. This is a very clever place to put it as it turns the transistor ON so that the collector sits at exactly half rail voltage. For a transistor to provide the maximum amplification, the collector must sit at half rail. This will allow the signal to be amplified equally in the positive direction as well as the negative direction. This needs a lot of explaining, so we will start at the beginning.

When power is first applied, the transistor is not turned on and current flows through the load resistor and base-bias resistor to turn it on very hard. This happens very quickly and when the transistors turns on, current flows in the load resistor and a voltage develops across it. This causes the collector voltage to drop and the voltage across the base-bias resistor is reduced. This causes less current to flow into the base and the rate at which the transistor turns on is reduced. The transistor keeps turning on but its rate of "turn-on" slows down as the collector voltage falls. 

As the collector voltage falls, the current through the load resistor increases and to provide this increased current, the current into the base must increase. But the base-bias resistor cannot provide this extra current and so the transistor turns off a slight amount. This creates a higher voltage across the base-bias resistor and allows more current to flow into the transistor to turn it on more
Eventually an equilibrium point is reached where the voltage across the base-bias resistor is exactly sufficient to allow current to flow into the base and turn the transistor on to a level called the "equilibrium point" or "bias point" or "set point." The value of the base bias resistor and load resistor creates this point. 
By careful selection of  the value for these two resistors the bias point can be set to HALF RAIL VOLTAGE. 

The transistor will sit with these voltages on the base and collector and the current through the load is called the QUIESCENT CURRENT. The quiescent current is also called the "idle current" or "set current" or "half current."  The voltage on the collector is shown graphically in the diagram below:

We can now add the input and output capacitors. These capacitors have no effect on the biasing of the stage. We have already learnt that capacitors do not pass DC and so the biasing voltages (the base and collector voltages) are not affected. The input capacitor passes the signal from a previous stage into the self-bias stage and the output capacitor passes the AMPLIFIED signal to the next stage. 

We can now apply a "rising and falling" voltage to the input line (this is called a WAVEFORM or AC voltage) and the transistor will amplify it. When the voltage is rising, (this is called the positive direction) the transistor is said to be amplifying the positive excursion and when the voltage is falling, the transistor is said to be amplifying in the negative direction. The voltage on the collector never rises above rail voltage (9v in this case) or below the negative rail (0v in this case). 

In the diagram below, you will notice the output waveform is "opposite" (INVERTED) to the input waveform. This is due to the transistor. The stage is called an INVERTING STAGE. When the input voltage is rising, the output is falling and vice versa. The size of the waveforms do not indicate the gain of the stage. The stage-gain may be 20, 100 or even more depending on the gain of the transistor, the value of the resistors and the voltage of the supply. At the beginning of the input waveform, the voltage is RISING. This will turn the transistor on MORE and the voltage on the collector will FALL. This is shown by the output waveform as a falling or NEGATIVE-GOING waveform. Only one cycle of the waveform is needed to show how the output responds.  

 

Question 85: Name the three leads of a transistor:

Ans: collector,   base,    emitter

Question 86: What is another name for the negative rail?

Ans: 0v rail, 

Question 87: What is a STAGE?

Ans: A stage consists of a transistor and its biasing components. 

Question 88: Name the three ways a transistor can be connected the power rails. 

Ans: common emitter,   common collector (emitter follower),  common base. 

Question 89: For a COMMON EMITTER stage; when the input is rising, is the output:
(a) rising   (b) falling

Ans: falling

Question 90: For a common emitter stage, name the resistor between the collector and positive rail. 

Ans: LOAD

Question 91: For a common emitter stage, name the resistor between the base and collector.

Ans: base-bias resistor

Question 92: How many resistors are needed to bias a transistor in  a BRIDGE arrangement?

Ans: Four resistors

Question 93: How many resistors are needed to bias a common emitter transistor in self-bias arrangement?

Ans: two resistors

Question 94: Is the common emitter stage called a NON-INVERTING or INVERTING stage?

Ans: Inverting

Question 95: For a self-biasing common emitter stage, what is the quiescent voltage on the collector?

Ans: Half-rail voltage

Question 96: For a self-biasing transistor in a common-emitter stage, name the resistor that turns the transistor on.

Ans: Base-bias resistor

Question 97: If the base-bias resistor is removed, what voltage will appear on the collector?

Ans: The collector voltage will rise to RAIL VOLTAGE. This is because the transistor will turn off and you can consider it to be removed from the circuit. The only component will be the LOAD resistor and if no current is flowing through this resistor, rail voltage will appear on BOTH ENDS. 

Question 98: Name the two capacitors (we have described them above) that are associated with a common-emitter transistor STAGE:

Ans: input capacitor,  output capacitor

Question 99: When a transistor "turns on," will the collector voltage be: (a) rail voltage (b) lower than rail voltage.

Ans: lower than rail voltage. The exact value will depend on the type of transistor and the value of the biasing resistors. 

Question 100: To turn a transistor "ON," the voltage on the base must be (a) 0v,   (b) 0.3v  (c) 0.7v  (d) 1.7v

Ans: 0.7v  The other value  0.3v  is not sufficient to turn the transistor ON,  and the voltage can NEVER go above 0.7v, so if the voltage is 1.7v, the transistor is DAMAGED! This voltage is measured between the base and emitter and we are assuming the emitter is connected to the 0v rail for these measurements.

Question 101: When a transistor is FULLY TURNED ON, the voltage on the collector will be:   
(a) rail voltage,    (b) half-rail voltage     (c) 0.7v     (d) 0.35v     (e)  0v 

Ans: 0.35v  This is the accepted value for a fully turned on transistor however it may be higher than this due to a number of reasons. It can NEVER go as low as 0v and 0.7v is reserved for identifying the voltage on the base to turn a transistor ON. 

We are now going to describe a product that turns a transistor ON and OFF at exactly the right instant to energise a number of coils. These coils are placed inside a circular magnet and the magnet is allowed to rotate. The result is a BRUSHLESS MOTOR. This is a very interesting product and comes in the form of a BRUSHLESS FAN and has been incorporated into almost every computer, to keep the active components (mainly the microprocessors) from overheating. Miniature versions are also available and are placed over the individual microprocessors with its own cooling fin. 
It a very simple circuit but the way the motor works needs a full page of explaining. 
The next page covers the: BRUSHLESS MOTOR.


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