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Electronic Parts [Explained] pls..

electronoobz

Jan 14, 2012
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According to http://www.williamson-labs.com/480_xtor.htm

Since the Transistor is a Current device, any signal Voltage must first be
Converted to a Current.

Voltage to Current Convertor

First, you must convert the input voltage to a current by
using a Voltage to Current Convertor--a resistor.

Next,

Since the Transistor is a Current in/Current out device, any Current Output is
Converted to a Voltage Drop by the Current flowing thru a Load Resistor.


Current to Voltage Convertor

Next, you convert the output current into a voltage by
using a Current to Voltage Convertor in the collector circuit--
you guessed it--a resistor


Abstractly, here are some Characteristics: 1. An equivalent circuit of a NPN transistor is two diodes tied anode to anode; one cathode being the emitter, the other the collector, and the junction of the anodes is the base. 2. When a NPN transistor is doing-its-thing, there is always a constant 0.6 volt drop between the base and emitter, i.e., the base is always ~ 0.6 volts more positive than the emitter--always! 3. There is no output at the collector, until the base has reached ~ 0.6 volts and the base is drawing current, i.e., any signal that appears at the base that is not up to ~ 0.6 volts (and not drawing base
current), is never seen at the collector. 4. The base requires a current, not a voltage to control the collector current. 5. The collector is a current source: it does not source a voltage. 6. The collector appears to output a voltage when a resistor is connected between it and power. 7. The collector is a high impedance when compared to the emitter. 8. The transistor can output an amplified signal either from the collector or the emitter (or both). 9. When operating with a collector resistor (RL): the output voltage from the collector is an amplified voltage. 10. When operating with only an emitter resistor (Re): the output voltage from the emitter is not an amplified voltage, because it is always ~ 0.6 volts, below the input (base) voltage--hence the name
voltage follower. But because the emitter can source large amounts of current to the "LOAD," it can be said, there was CURRENT amplification. 11. The collector--being high impedance--cannot drive a low impedance load. 12. The emitter--being a low impedance--can drive a low impedance load. 13. The voltage gain from the collector is greater than one (Gv > 1). 14. The voltage gain from the emitter is less than one (Gv < 1). 15. Both the collector and the emitter: output ~ the same power: E x I = P.
 
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electronoobz

Jan 14, 2012
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Back to load resistor..

How much volt do i need?? I thought current is the only thing i need to find out when it comes to output...

I'll just use the ohm's law for finding the resistance i need, but i need the voltage first... How much??
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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How much??

Once you know the maximum and minimum currents, you need to choose a resistor that will allow those currents to pass without exceeding the supply voltage (in fact it must be a couple of volts less than the supply voltage.

So if the output current varies from 10 to 50 mA and the supply voltage is 9V, then the maximum load resistor value is about 7/0.05 = 140 ohms.

Note that these calculations only apply to common emitter linear amplifiers, but that's what you're looking at. If there is an emitter resistor then that needs to be included in the calculations, but that will affect the current as well, so it's not something that can be explained with a single simple calculation.
 

electronoobz

Jan 14, 2012
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is this right?

i can use any voltage as long as it doesnt exceed the supply voltage??

why do i need to know the max and the min output current? if in the ohm law i would only use one current and one voltage?

and one more thing... what happens if let's say my supply voltage is 9v

and i use 8v to find the load resistor i need..

is there a difference if i used a larger voltage than what you give?

hmmm.. does it make my amp louder if i use larger voltage (not exceeding the supply voltage)
 

electronoobz

Jan 14, 2012
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i saw another vid here


it is the same as what i posted above

but there is something i confused at when he is in the part of "for better estimate"

help?
 

electronoobz

Jan 14, 2012
226
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is this right?

i can use any voltage as long as it doesnt exceed the supply voltage??

why do i need to know the max and the min output current? if in the ohm law i would only use one current and one voltage?

and one more thing... what happens if let's say my supply voltage is 9v

and i use 8v to find the load resistor i need..

is there a difference if i used a larger voltage than what you give?

hmmm.. does it make my amp louder if i use larger voltage (not exceeding the supply voltage)

hey steve.. help with this please??
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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is this right?

i can use any voltage as long as it doesnt exceed the supply voltage??

Typically you would actually chose a relatively small voltage swing in most cases so as to ensure that your amplifier remains linear (since a transistor is NOT linear -- it's gain is dependant on the collector current to some extent)

why do i need to know the max and the min output current?

Your input is a voltage that fluctuates. It is impressed on the bias voltage. This alters the base current. Thus you have a changing collector current.

If you know how much the base current can vary, you can calculate how much the collector current can vary.

From that, you can determine how much the output voltage varies with the input voltage. That is the voltage gain of the amplifier. Frequently it is important.

if in the ohm law i would only use one current and one voltage?

You can use the maximum current to determine the maximum value for the resistor.

Then using the resistor value you choose, using ohms law twice (once for each current) you can determine the voltage swing of the output.

and one more thing... what happens if let's say my supply voltage is 9v

and i use 8v to find the load resistor i need..

is there a difference if i used a larger voltage than what you give?

Clearly if your supply voltage is larger, your potential voltage swing is larger. And so the design will be slightly different.

hmmm.. does it make my amp louder if i use larger voltage (not exceeding the supply voltage)

Yes, but also more distorted.
 

electronoobz

Jan 14, 2012
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ow right yo!! hahahaha

now i can mod my pre-amp.. hahahha i'll use a larger voltage to slightly add a crunchy distortion to my guitar (just to add crunch not really distort it..)

what if i put 2 diodes to the output to clip the sound?? is this possible?
 
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electronoobz

Jan 14, 2012
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oh... i forgot to add this.. how could i measure the min output current?? it's an AC right? so that means it is not static.. OMG.. i think this would be hard isn't steve??

hmm.. i have an old multitester here but i don't know how to use it to measure an AC.. can i use it you think?

well can i just leave it? or just use the base current * gain (that's the collector current right?) for the max output current?

o yeah btw, i saw site on google.. it is titled a better volume control... is this true?

i should add let's say i have 100k pot..

i should add 15k res on the wiper to the ground?? hahahaha. just wondering..
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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I think we've got to (or gone past) the point at which your understanding cannor really progress until you've understood the fundamentals.

You need to understand how a diode junction works (because the base-emitter junction looks just like a diode).

You need to understand how voltage dividers work. You also need to understand what happens when you apply a load to them.

Then you need to understand some model of the BJT that fits the circuit you're talking about.

That's a minimum. People studying this stuff for real have a pretty extensive background before they start to deal with transistors as amplifying devices (treating them as switches is somewhat simpler).

There is quite a lot of work involved before it appears simple.

Unless you understand a transistor properly, you may get as far as (kinda) understanding a single simple common emitter amplifier, but you'll be flummoxed as you try to tackle something more complex.
 
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