Come & get yer watts!!!

Discussion in 'General Electronics Discussion' started by Flurng, Aug 4, 2015.

1. Flurng

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Dec 29, 2012
I've been doinking around with some simple amplifier circuits, and I've discovered that increasing the load resistor value reduces the current through the transistor, thus increasing the voltage gain, while reducing current gain. And, of course, reducing the load resistor produces just the opposite effect. This got me thinking.... ( Stand back - this could get messy! ) What's the "best" voltage or current for a given circuit? I suppose it depends what type of device you're trying to drive, ( loudspeaker, motor, led, etc...) but if that's true, then who's hungry for voltage & who wants current? This thing's got me so befuddled, I'm not sure which way's down! Ideas, anyone?

2. duke37

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772
Jan 9, 2011
To transmit power you need voltage and current, this will be determined by the load. A small motor may need 12V and 1A, a big motor may need 400V and 200A, a light emitting diode may need a controlled current of 10mA at about 3V.

Audio amplifiers are normally made to amplify voltage and supply whatever current the load requires. If the load is too thirsty, then the voltage will drop.
The power supply will need to supply the required voltage and current.

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3. BobK

7,682
1,688
Jan 5, 2010
First of all, are you talking about a voltage amplifier with a collector resistor, or an output stage, which is typically push-pull and the speaker is the load, usually connected to the emitters? They operate quite differently. The output stage typically has no voltage gain.

Bob

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4. AnalogKid

2,515
718
Jun 10, 2015
Here's one from 1840:

https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem

ak

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5. Ratch

1,094
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Mar 10, 2013
The best voltage or current for doing what? Power transfer? Please specify.

Ratch

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6. Flurng

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Dec 29, 2012
Thanks, Duke37, for the valuable info! The power source in this case is a simple 9v "transistor" battery - I like to work with low voltages, so that when ( not "if" ) I screw something up, I generally don't fry my components. ( much less myself! ) I guess what I was hoping to figure out was a general rule of thumb for various types of devices. For instance, if I'm trying to drive an LED or a motor, I can just look at the spec sheets for those items to see what each requires, however for a speaker, it gets a bit more fuzzy - often small, cheap speakers are marked in terms of impedence, but they rarely show max power ratings. I know that it's important to match impedences when dealing with speakers, but if a particular speaker requires a low impedence ( say, 8 ohms ), how might that effect the voltage gain of the transistor? Thanks again, for the input - all info is appreciated!

7. Flurng

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Dec 29, 2012
Howdy, BobK! Thanks for the reply! The circuits I've worked with thus far are simple single-stage audio amps ( although, I suppose they'd probably qualify as pre-amps ), consisting of a single 2n2222 transistor & a handful of components. I guess, technically, they'd be called Class A, Common Emitter amplifier circuits and, yes, the "load" resistor I referred to is on the collector. I'm familiar with the push-pull configuration, but I'm more interested in a single-ended output stage, perhaps a darlington transistor? Basically, I am attempting to build a simple, reliable pre-amp stage for musical circuits, such as oscillators, active filters, guitar distortion pedals and such. So, I suppose line-level signals would be sufficient for coupling to further processing circuitry, though perhaps enough gain to drive a pair of headphones would be useful.

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Dec 29, 2012
9. Flurng

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Dec 29, 2012
Thanks, Ratch, for replying - as I mentioned in another post, I'm attempting to build a simple pre-amp stage for various musical circuitry; oscillators, active filters, distortion pedals and such. But, moreover, I'm just trying to get a handle on which types of devices (on average) require what type of power; for instance, are loudspeakers current-hungry, or voltage-hungry? Same for motors, LED's, transistors, etc..... I realize, of course, that every specific device has it's own requirements, but I'm just wondering if there are general "trends" for given device types.

10. Flurng

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Dec 29, 2012
If I haven't already said it, Thanks, all, for such prompt and helpful input! It's encouraging to see a forum so active and lively, and your interest in my conundrum is greatly appreciated!

11. Ratch

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Mar 10, 2013
Well, let's take them one at a time. I don't think "hungry" is the correct word. Speakers have a impedance and wattage, so the voltage and current are locked into those figures. A motor will draw more current when it is loaded. The windings will melt if the amount of heat caused by the higher current is exceeded. LED's and transistor are limited by heat and breakdown voltages. Heat is proportional to the product of the in phase voltage and current.

Ratch