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.