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Old 18th May 2012, 06:17 PM   #21
Simon B is offline Simon B  United Kingdom
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Read your diagram! It shows 22k (upper) & 4k7 (lower) - note my use of the k as decimal point - helps avoid error.

This should give you a voltage of:

9 x [ 4k7 / (4k7 + 22k) ]

about 1.59 volts at the base of the transistor. This in turn should give about 0.9v at the emitter, and thus 9mA down the 100R emitter resistor.

Nearly all that emitter current flows through the collector (transistor has high gain) - so 9mA through the 1k0 collector resistor drops - oh dear, 9v, leaving nothing across the transistor......the design on your schematic is bad. This is why the base voltage falls so low - the transistor can't supply enough current to the emitter resistor to raise the emitter voltage.

Assuming you want linear amplification, not a simple fuzz box, you should aim to have about half the supply voltage dropped across the emitter and collector resistors in total -
4.5v, and half, 4.5v, across the transistor. That way your signal can go up and down roughly equal amounts.

If you're shooting for a voltage amplification of 10x, which is implied by the ratio of RC to RE, then you need to lower the bias voltage being produced by your bias network.

Unfortunately, you've accidentally raised it !

Sticking with
RC = 1k0 & RE = 100R for the moment, to drop 4.5v across 1100R, you need a current of 4.1mA.

Across the 100R
RE this will develop 410mV. Add 0.7v for the base-emitter drop of your transistor and you get 1.11v.

So you need to work out a viable bias chain to get close to that voltage. The resistors need to be in the ratio 1.11 to (9-1.11), ie 1.11 to 7.89,

ie about 1 : 7.11

If you use 22k at the top of the chain you need about 3k on the bottom. 3k is an E24 series value, less common than E12 series values, you might need to use 1k5 + 1k5.

This working is rough and sloppy. I'm making questionable approximations because I'm lazy and because I think the whole story might be a bit much for you just now.

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Old 18th May 2012, 06:18 PM   #22
shanx is offline shanx  Canada
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Hi Calx44, there are a few reasons why it alters the divider voltages. The Vbe junction and emitter resistor leg are parallel with the lower voltage divider. Now when I mentioned that it is important to look at the BJT transistors H parameters (Hfe, Gain) it is because as an active component the input impedance that you would see at the base of the transmitter is dependant on this parameter and also the Ic (DC collector current). It is not a simple resistor leg tacked on in parallel At 3 Volt at the base you would saturate the transistor on .
First you don't need the negative 9V supply voltage rail (were you meaning just the - side of the battery or supply? that should be 0V or signal ground) Just a single 9V positive supply will do.
Try these values with a single 9Volt positive supply, the signal ground tied in to the (-negative battery temirnal) ground.
500K for top of divider, 50K for bottom. You may need to add a potentiometer in the middle to fine adjust.
Increase emitter resistor 420 Ohm, and increase Collector resistor to 10Kohm. (You can boost the voltage gain somewhat by further adding a 25uF electrolytic capacitor across the 420 ohm, + side facing upwards toward the emitter)
You should have some ac signal voltage gain at the collector, at his point. Not much current so you can't load down output with an 8 ohm load, but you should be able to plug this into an amp input and have some boost. You would be better with a higher gain transistor like a 2N5088 or 2N5089 which would be about double the gain (Hfe) 300 to 400, instead of 100 to 150
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Old 18th May 2012, 06:29 PM   #23
Simon B is offline Simon B  United Kingdom
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In case it isn't apparent, shanx and I are saying similar things, at least in part.

shanx is suggesting a bias chain resistor ratio of 1 : 10, where I had 1 : 7 - note that they're both higher than the original design posted.

For the moment, I'd try to get the basic circuit working before adding the bypass cap on the emitter resistor that shanx suggests. The higher values that he suggests would give higher input impedance, which would be better for a guitar.
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Old 18th May 2012, 06:35 PM   #24
shanx is offline shanx  Canada
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Yeah, I loosely estimated.. a bit.
Friday before Victoria Day holiday weekend. Laziness is abounding here.
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Old 18th May 2012, 06:36 PM   #25
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Hi there Calx44, Im an A2 student myself, what course are you taking? (might be able to help in terms of requirements ) You can get a Power amp out of 2 transistors, say BD139/140 push pull configuration, put it in an op amp feedback loop to reduce distortion, but its not necessary, it may even add "harmonic character" if you just want 9v youll have to bias the power stage at about 4.5v. or i would suggest a look at the marshall MS-2 schematic. heres a link to a thread about it...

Marshall MS-2

I would say you want an input impedance of at least 100k, thats the lowest i've ever used with good results, any lower seems to reduce highs alot, final question, are you looking for squeaky clean tones? or is a bit of dirt ok?
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Old 18th May 2012, 07:05 PM   #26
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OK, Thanks guys for the help, Very much appreciated. As you have probably guessed, your not quite dealing with a novice but not far off.

With regards to the +-9V, that did just refer to the +- of a 9V battery.

Thanks Simon B and Shankx, I know appreciate the what the emitter/collector resistor combo does. I was looking at the problem from the wrong angle, trying to do divider network before the rest, but I think I am with you now

razorrick, I have just finished AS and now looking at what to do for A2 product design. planning to make the speaker housing and everything myself and wanted to make the amplifier so it was "my" product, even if I followed a schematic. I just didn't fancy ripping out an amp unit from something else, desperate times may lead to this though, I'm going to keep at this for now though. Don't mind a bit of dirt, might even be good...

Thanks again guys.
Callum
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Old 18th May 2012, 07:09 PM   #27
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Ok, product design probably doesn't require too much electronics,but it depends on how complex you want your circuit to be. Have you got a brief of any kind? I know my old Design teacher never stopped drilling on about a brief or specification for everything.
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Old 18th May 2012, 07:37 PM   #28
shanx is offline shanx  Canada
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No problem Cal, I think you will get this boost stage going. Once that gets done you could go for a 2nd stage power amp like a chip amp, or even a 741 op amp will drive a little 8 ohm speaker, or the push pull as Razorrick talked about. It takes some time and study, but go for it.
Another friendly advice is to study other practical proven circuits. Lots of them. What is working in the field, is not always conventional. Puzzling sometimes, but that is the fun part of discovery.
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Old 18th May 2012, 08:10 PM   #29
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Quote:
Originally Posted by shanx View Post
What is working in the field, is not always conventional. Puzzling sometimes, but that is the fun part of discovery.
I would agree with that completely

and my Teacher said if I can't figure out a simple circuit, as it only has to be a practice amp sort of thing, I can follow an existing schematic, which is looking like the way forward, or just rip from a scrap amp. No brief yet as such, only handed in AS coursework 2 weeks ago, on exam leave atm.
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Old 18th May 2012, 09:08 PM   #30
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i would say, unless your looking to go into electronics as a hobby or career, follow something already made and learn how it works, but if its the way you want to go, then design your own! Theres an endless amount of things to put into it depending on what you want (covered by a brief) any preference on controls you want?
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