AC with DC Lesson

[The_Bloody_Omen]

So I am new to guitar pedal circuits but have some classes under my belt in ac and dc circuitry but do not know how they work together. Do we just "look" at the DC part for the biasing of transistors and then put it to a side and then look at the ac for the signal part? Also has anyone found any good sources for learning how transistors function under this config?

Thanks,
Mike

[MaxPower]

Electronic Principles and Transistor Circuit Approximations both by Malvino. You should be able to get them cheaply. EP covers various topics (transistors, op amps, capacitors, etc.). TCA focuses mostly on transistors.

And yes, you look at the dc and ac separately.

[antonis]

Do we just "look" at the DC part for the biasing of transistors and then put it to a side and then look at the ac for the signal part?

Well established as "Superposition Theorem"...
http://electronicslab.eu/en/analog-electronics/349-dc-and-ac-equivalent-circuits.html

But for guitar pedal circuits you don't have to mess with imaginary and/or complex parameters and phasors paperwork..
(let some simulator do the dirty job..)

[Brisance]

why not look at everything just as voltages, some of which vary over time?

[GibsonGM]

why not look at everything just as voltages, some of which vary over time?

This is how I approach it.

1) there are bias voltages, DC...you measure these at 'quiescence', with no signal.  No AC to worry about

2) if making a measurement of gain, etc., with an input, just measure the AC component...i.e,  500mV in,  2V out, no worries about the DC.  You would do well to do this after the output cap if you really want them separated.

[tubegeek]

You can think of capacitors as the main tool for separating AC from DC.

If a capacitor is in series with a DC voltage that has an AC voltage superimposed on it, it will block the DC levels on either side of it from influencing each other - typically what you'll see when running a signal into the gate of a transistor or either input of an opamp. DC can be thought of as 0 frequency, and the capacitive reactance (opposition to AC current) is infinitely high at 0 Hz.

At high enough frequencies of AC, the cap will have a rather small reactance - it will behave for AC purposes as though it's not there.

At in-between frequencies the cap will have an in-between reactance that will interact with the other impedances around it to set the amount of signal transmission.

The value of "in between" is set by the value of the capacitor itself: "in between" is lower for large capacitance and higher for small capacitance. Also what value of "in between" we are interested in depends on the impedances in the rest of the circuit.

The reason some circuit blocks can get away with no capacitor between them is because they are already at appropriate DC bias levels to connect to the next section: very typical with opamps, once the input is biased the output will usually be at nearly the same DC voltage so it can go right into another opamp.

The formula for capacitive reactance is

X_c=1/(2•∏•ƒ•C)

Notice at ƒ= 0 Hz, this formula is undefined. But we can see that X_c would approach infinity as the frequency gets lower and lower. And in fact that is what we will see with DC voltages on the cap.

The larger ƒ gets, the smaller the reactance for a given capacitor. The larger the capacitor gets, the smaller the reactance for a given frequency. The larger the ∏ gets, the more there is for me.

Also: many thanks to everyone who has recommended Malvino on this forum. Never heard of him and his Transistor book is excellent. Looking for the other one now. Nothing like a nice, clearly written book!

[Transmogrifox]

why not look at everything just as voltages, some of which vary over time?

This is how I prefer to see it.  It's all the same, we just identify it as something different when the level is changing over time.

One example is measuring tide.  The "DC" value of the ocean is Mean Sea Level (MSL).  If the tides could stop (launch the moon off to the sun or something) and the ocean settled, it would end up at the MSL.  The AC part is the tides.  If you look at tide charts, you see a sinusoidal variation in water level superimposed about the mean sea level.  One is a 24-hour cycle dependent upon the rotation of the earth, and a second AC component hiding in there is due to the phases of the moon (much slower cycle).

Another analogy:
Consider a transistor like the valve on sink where the water comes out.

Voltage is like the pressure behind the water held back by the valve.

Current is the amount of water that flows when you open the valve (like gallons per minute).

DC current is like the water flow when the valve is left steady in one position (assuming mains pressure remains constant).

DC voltage is like the steady part of the pressure (like the constant pressure in your water mains), and the constant pressure measured just upstream of the valve.

Bias is like turning on the faucet.  A constant unchanging stream of water comes out.

Now wiggle the handle back and forth -- the stream is increasing and decreasing.

The thing you don't "see" is what the pressure directly on the other side of the valve does.  Because of the resistance in the pipes, you probably don't have a well-regulated pressure at the faucet, so when you turn it on all the way, the pressure drops at the valve.  When you turn it off pressure increases at the valve.

Wiggling the handle makes the pressure go up and down.

With a transistor bias is this:
Turn the handle on about half way.

AC signal is like this:
Wiggle the valve.

A capacitor then behaves somewhat like a membrane between two bodies of water.  The two bodies of water can be at completely different pressures, but one pressure doesn't influence the other as long as all is held constant.

Then if you change the pressure in one side of the membrane, the membrane moves and causes a pressure fluctuation to propagate into the other side of the membrane.

If you had pressure-controlled valves, you could make the equivalent of a transistor amplifier with pipes and valves (probably would be more like a vacuum tube).  Then your pressures and flows could be created either with air (air compressor as your power source) or water (water mains as power source).

I have always thought it would be interesting if some day I become independently wealthy to build a pneumatic audio amplifier.  The principle is sound, but implementing air valves with good dynamic response would be a challenge.

[WhiskeyMadeMeDoIt]

why not look at everything just as voltages, some of which vary over time?

I have always thought it would be interesting if some day I become independently wealthy to build a pneumatic audio amplifier.  The principle is sound, but implementing air valves with good dynamic response would be a challenge.

This is a little unrelated but ...
I have always wanted to build a an amp around the flame triode. Using a flame triode amp a flame rectifier and a flame speaker you can rock out with flames! I don't think it could get more metal than that.

triode and rectifier description:
http://www.sparkbangbuzz.com/

flame speaker:
http://swtpc.com/mholley/PopularElectronics/May1968/Flame_Amplification.htm

[The_Bloody_Omen]

MaxPower: I will check out those books next time I can get to a library thanks

antonis:I dont mind doing the phasors and magnitudes I have done those in some classes and plan to be an ME anyho. I have limited time on a computer but am chugging through the link you sent me.

Brisance: thats what I am trying to do and work my way through I and P eventually.

tubegeek: I get ya man. You are more talking about caps as filters but I understand the coupling caps to "remove" DC from AC.

Transmogrifox: pneumatic. that sounds bad ###. I wanted to make an instrument for dubstep that would work a diapram back and forth. The instrument would be like one of those scientific instruments used to demonstrate friction. You put a little piece of paper down a cylinder closed at one end and then ram a piston down it and it starts a little fire for a bit.

WhiskeyMadeMe:that is badass haha so badass man