BASIC 
ELECTRONICS COURSE 
Page 34 INDEX

THE VOLTAGES ON A STAGE
Discussing the voltages on a stage is exactly the same as talking about the voltages on a transistor in a simple circuit as there are so few components in the circuit (stage) that the reference points coincide with the leads of the transistor. 
The demonstration circuit below shows a COMMON EMITTER stage directly connected to a COMMON COLLECTOR stage. These two stages are DC connected (no capacitor between the two stages) and thus they will amplify DC voltages. 
The purpose of this demonstration is to show how the voltage at each part of the circuit rises (or falls) and the magnitude of the change - compared with the size of the input voltage. 
When designing a circuit, there are two factors you must take into account. The voltage at the output and the current at the output. 
At the moment we are only covering the voltage at the output. The reason for this is quite simple. You may have a 12v motor or siren or globe and wish to drive it with full voltage. You need to know if the circuit is capable of  supplying full (or nearly full) rail voltage. Once you know the circuit is capable of supplying the required voltage, you can design for current requirements. 
One point to note: The circuit below is not designed for high output current as the current through the base resistor of the emitter follower will be about 1/20th of the output current and this current is called "wasted current."  There are other circuits more suited to driving high current loads. This is purely a demonstration circuit to show how the voltages change. 

One of the points we mentioned above needs to be answered. It's the current capability of the output of a circuit. In the 2-stage circuit above, the emitter follower transistor is designed to deliver a "high current" to a load.  Suppose we require a current of 1 amp. If the gain of the transistor is 20, the current entering the base of the emitter follower transistor will be 50mA. This current must flow through the base resistor and when the first transistor turns on, it turns off the output transistor BUT current still flows through the resistor. This is wasted current and the circuit can be re-designed as shown below. 
In the re-designed circuit below, maximum current flows through the base resistor when the output transistor is fully turned on, but when it is turned off, no current flows through the base resistor. 

 

GOING FURTHER
You now have enough theory to go to our projects section and/or our PIC programming section and continue with your learning. All our projects and programming articles explain the operation of a project IN COMPLETE DETAIL and you will be able to continue learning with a "hands-on" approach. 
The whole purpose of this course is to get you into construction as soon as possible because you learn ten-times FASTER when you are actually building a project.  We don't mind where you get the parts, but the PC boards can only be purchased from Talking Electronics. The prices for the boards is less than making them yourself and each board has an overlay showing exactly where the parts are placed and the underside is solder-masked so that each connection becomes a very neat round solder-joint. 
If you don't intend doing things absolutely perfectly from the beginning - don't start!  Peg-board, perf-board, strip-board, matrix-board and all these terrible layout arrangements are an insult to electronics.  Don't get into bad design-habits by building on a board that dictates a layout. Start with projects we have produced on PC boards and you will see photos of how things should be laid out and presented. 
The cost of our projects is exactly the same as you would pay for the individual parts over the counter. That's how we work out the price. But every kit contains one or two parts that you can't get over the counter or have to buy in a pack of 3 or 10! Some of the parts we make ourselves (the coils) and some we import especially for a kit. All our components are miniature and MODERN. We get so many kits sent in for repair containing ugly, bulky, old-fashioned components or parts with the wrong value! Don't take the risk. Buy a kit and you are guaranteed success. 
Every time you build a kit, you add to your knowledge-base.  Each project is designed to add a  building-block to your library. All our kits have been designed for a purpose and after building a few projects you will see things starting to come together. 

It's not too early to go to our PIC programming section as most of the circuits surrounding the microprocessor are very simple. Any components around a micro are needed to interface (connect) the outside world to the micro or the micro the outside world. All the clever features of the project are done with a program inside the micro and since this doesn't involve electronics, as such, now is the time to start.


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