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Single Stage Common Emitter Amplifier part2

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The Diodes and R5 and C3 just turn AC power into DC power.

C1 blocks DC inputs so anly that AC part of the signal gets through (it makes a lowpass filter with R1 and R2 in parallel if you want to calculate the cutoff). I'm not sure your level of understanding of transistor amps so I'll start from the begininng.

R3 and TR1 provide the heart of the amplifier. I always like to think of the transistor as a variable current source (you can also think of them as a variable resistor but the current source is technicaly more correct). So the Current generated by the transistor creates a voltage across the resistor. Just use Ohms law to calculte it. R1 and R2 bias the transistor amp - by setting the DC voltage on the Base of the transistor. This sets the current of the transistor (which also sets the output voltage)when there is no input. C4 is a little confusing but because teh amplifier only amplifies the AC component of a signal C4 lets you ignore R4 as far as the AC output is concerned (In this case R4 just kills some of the transistors DC gain). C2 just kills the DC component of teh output.

When you pull out C4 things get a bit more complicated. The voltage across R4 depends on the current throught the transistor. At DC just use ohms law to get the voltage (C4 holds this voltage constant when there is an ac input therby removing the AC effect of R4). Without C4 as the Ac input causes the current in the transistor to increace the voltage across R4 increaces; this increaced voltage reduces Vbe (the voltage difference between The base and the emmiter). Since the variable current source is dependant on Vbe the current wil decreace. All that happens very fast so the output voltage will stay at the equilibreum point between the input voltage and the voltage across R4 (which since it depends on the current through the transistor is proportuonal to output voltage). This is called negative feedback.

Negative feedback takes a non-linear gain that is dependant on the transistors gain curve (which would screw up an AC signal) and throws away all the gain above a certain point to keep the gain constant over all inputs. This makes the amp much more predictable and easier to work with. It can also help to reduce the output impedance of an amplifier allowing you to drive more current.

Hope this helps.
Brent
 
thanks for that

just out of intrest what is Vbe?


other wise that is a great help and i understand it knwo

CHeers

adam
 
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