Geeky Power Amplifier Question

Factual Questions
1

I am trying to understand the basics of a power amplifier circuit and went to howstuffworks.com and found the page on amplifiers. The site has two seemingly conflicting diagrams and I am trying to understand which is correct. In diagram 1 the microphone is connected to the base electrode and what looks like the another contact for the base electrode (or a ground on the transistor). Diagram 2 shows the mic connected to the base electrode and the emitter. Which diagram is corect?

Basically if I wanted to build this and went to Radio shack and bought components which is the correct wiring diagram. (FTR I’m not building anything - I’m just trying to get this all in my head). I’ve tried googling Bipolar Junction Transistor but just got lots of formulas, no real wiring diagrams.

Any help is appreciated.

2

In diagram 1 the object in the bottom right is a depiction of an amplifier as a whole, not just the circuit but everything. The mic connection appears to infer a L/R stereo signal from a pre-amp and not a microphone itself.

In diagram 2 the theory behind how a transistor can amplify a smaller signal into a much larger one is explained. The object in the bottom right of this diagram depicts the transistor in an amplifier’s circuit. The mic connection here shows how the smaller signal is introduced to the circuit.

3

Neither diagram is a particularly accurate schematic, except in very general principles. Several important components are missing, such as biasing resistors (which determine the operating point of the transistor(s) being used) and input coupling to keep DC out of the input. A more useful general-purpose amplifier schematic can be found here. R1 and R2 form a voltage-divider bias circuit, while C1 is the input coupling (other types of coupling used include transformer coupling and diode coupling). Any general purpose silicon NPN transistor can be used here.

4

I see your problem, its due to the over-simplification of the circuit, or at least a lack of consistancy in the way they are represented.

What they should have done was show a simple circuit diagram in alongside the illustration with the equivalent points shown in each.

In the first diagram, we are looking at an amplifier as a whole, and this might be represented as a box within which there may be many components. We are not interested in what actually happens inside the box, just the inputs and outputs.

In the second diagram we have actually gone inside the box and are now examining one of the active components, a transistor.
There will be many such devices in a power amplifier.

Unfortunately they have chosen to use a battery to represent a large amount of output power capacity, this is not intended to represent the power supply and is misleading.

Although it is all correct, they have simplified it too much, and have not included the power supply or even referance to it.

In any amplifier, the input and the output will have one common referance point, often to the 0v or -Volt supply line. This may be done through resistors, but the path will be there.

A better representation at this kind of level would have been to draw a box with a pair of input wires at one end and a pair of output wires at the other end.

The power supply could be represented by a +Volts line joining the top of the box, and a 0 Volts line leaving the bottom of the box.

Once this has been done, it is an easier concept to join the lower pair of wires from the input output, to the 0 volts line.

5

Are you trying to build a useful device? Or are you just trying to teach yourself electronics theory? Don’t try to do both. A useful amplifier is too complicated for beginners to understand. A simple amplifer for beginners’ theory will have low gain, bad sound, and use up batteries like mad.

Here’s an excellent website for teaching yourself basic transistor circuitry:

NCSU electronics tutorials
http://courses.ncsu.edu:8020/ece480/common/htdocs/

6

Opengrave:

Those diagrams suck. Really.

On the other hand, looking at a schematic for a modern power amp would also suck, in that it would just look like a rat’s nest of resistors, transistors, and capacitors.

So is your goal to simply learn more about power amplifiers, and how they work? If so, I would start by reviewing how a simple single transistor, Class-A, NPN, common-emitter amplifier works. Once you have that mastered, move on to the totem pole/push pull (Class B) topology.

The above will show you how transistors amplify. But modern amplifiers contain a lot more than just amplification circuitry. Examples include:

Power supply circuitry
Protection circuits
Thermal compensation circuitry
Coupling circuits
Filtering circuits
Impedance matching circuits
Drivers
Self test / monitoring
Etc.

That’s why a modern amplifier schematic looks like a rat’s nest! So my advice is to take one step at a time, and first learn how transistors amplify.

7

My link shows a nice, simple example of this type of amplifier.

8

the simplicity of the concept can be lost in the explanations.
A transistor works by providing a path for electricity that can be turned off and on (or reduced).
picture a bridge with people walking across. in the middle is a gate that restricts the flow of pedestrians.
a gate master opens and closes the gate to allow 1, 2 3, 4, or 5 people through at a time depending on how wide it is open. How often it opens also is in his control.
Now on one side of the bridgeyou have a steady line of people coming up to the gate but on the otherside you will see a pattern of 1,2,3,4 or 5 people that corresponds to the opening and closing of the gate.
The gate is the middle section of the transistor, and the gatemaster in this case is a microphone.

9

Real amplifiers glow in the dark!

10

No, REAL amplifiers go to Eleven!!!
More than anything I was trying to understand the inconsistencies in the diagrams. I have no real desire to build anything. Thanks to all for the help.