Biasing?

[Luke]

Hi everyone,
I dont know ANYTHING about biasing transistors for a fuzz circuit. I just do what the schematic says, and hope for the best. I was reading before about biasing for the Tychobrahe- can anyone tell me where I can findout how to bias transistors in a fuzz circut- I mean, what it is, what to look(or listen)for, and how to actually do it?
I have made a few pedals in my time- but I have never even thought about 'biasing'
Thanks very much all,
Take care,
Cheers,
Luke

[Luke]

anyone?

[Peter Snowberg]

The basic concept is that (talking NPN here) the collector must be higher than the base which must be higher than emitter.

There are a lot of variables depending on the design of the circuit so sometimes you bias by adjusting the base voltage, sometimes the collector, and sometimes the emitter. I think that's why no quick answers.... too general a question for an easy answer. You get in to the "dark side" of electronics where you're dealing with current flow which is not as easy as dealing with saying adjust to X.X volts.

Sorry, that's not much of an answer, but without the circuit it's kind difficult.

If you can point to the circuit in question, then the answers get MUCH easier because now people can wrap themselves around the question.

Take care,
-Peter

[Luke]

thanks Peter,
I had no idea that it was so involved . I was basically planning to 'bias' (if nessessary) the Fulltone 69 pedal (I havent made it- the germanium transistors have been ordered though) - the "PNP- Positive ground" schematic (and also the tychobrahe).
Would the '69 benifit from this? Could someone tell me what to do? Id appreciate it!
Take care,
Cheers,
Luke

[petemoore]

The 5k above Q2's collector set bias there.
 I'ts setup I think to be used as a panel knob, so that biasing [or slight misbiasing] adjustments can be used as an efkt.
 This 5k brought My Q2's [for the most part er the ones I substituted] just up to around 4.5v [probly the most common bias point for 9V fx]
I usually use a 10k on Q2's C in a FF, but with the way the 70's setup, other resistors, this smalller amount of fine tune [the 5k] is enough to precisely tune bias]...but not go 'way off' so dummy won't shut the transistor off and this knob is suitable for panel mount...though mine's a board trimpot in this case so far.
 I always have a bias trimpot [10k?][instead or with a fixed resistor] on Q2's collector in a FF type ckt. Though sometimes I set the 10k range to be between 4k7 and 14k7 by adding a 4k7 resistor between trimpot and Pos rail [in an NPN ckt]. Just right for finding bias [ or close enough] usually the trimpot is somewhere in the middle of it's throw...you could use a 47k pot, but fine tuning [like really fine touch] is harder/touhier to find.

[Luke]

Thanks Pete,
But I'm a bit confused- are you talking about the Fulltone '69 of the Fulltone '70? The '69 is PNP germanium, whilst the '70 is NPN Silicon, so I am a bit confused, sorry  :oops:  
Is this how to biase the '69 pedal as well?
Thanks,
Luke

[bwanasonic]

In a Fuzz Face (either PNP  or NPN) , a trim in series with a *minimum* resistor with the collector works as a "bias" adjustment. I do the actual *biasing* by ear, but then read the voltage. I find I generally prefer the sound with the voltage highr than the *normal* 4.5v (6.66 actually!  :twisted:). It's other circuits like the Tyc. Octavia and Green Ringer, and maybe some three trasistor fuzzes that I would like more info where to look for *optimal* adjustment. I made room on my Tyc. Octavia layout for a trim at the emitter of Q1 to grnd, as per another post here, but I'd like to have a better idea of what I'm actually doing also.

Kerry M

[Jay Doyle]

Biasing BJTs is tricky and confusing for a lot of people so don't get discouraged. The nuts and bolts of it can be found on many basic electronics web sties so I will just cover the basics.

First off a transistor is a current controlled device not voltage as with JFETs and Tubes. The easiest way to learn this is to think of a transistor as a current sink with the path of the current flow being from the collector to the emitter. For the transistor to be "on" at all the base needs to be at least .6V higher than the emitter. Think of the base-emitter junction as a diode, which it is, and this diode needs to be biased on for the transistor to be on.

Now once the transistor is on, the collector passes a much larger current that is proportional to the amount of current flowing from the base to emitter. To increase the current through the collector, increase the current through the base-emitter, decrease the current through the b-e junction to decrease the current the current through the collector.

Now imagine that the current flowing through the b-e junction is your guitar signal (for ease of thought think of it as a sine wave). As your signal goes up it passes a larger current into the b-e junction and the current at the collector goes up as well, as the signal falls the current falls at the collector. SO the transistor is acting as a current controlled current source, with your guitar signal controlling the amount of current through the collector.

Now how do we get a signal amplified? Well we attach a resistor to the collector, the other side to the V+ and take the signal at the junction of the resistor and collector. Now as the b-e current goes up the collector lets more current through, which drops a larger voltage across the resistor bringing the voltage at the collector DOWN, as the b-e current goes down, the current through the collector goes down, dropping less of a voltage across that resistor and bringing the voltage at the collector UP. This is why the signal is inverted, and because the collector is passing a larger current proportional to the b-e current the signal is amplified.

Well that is how a transistor works in theory but in reality you have to set certain conditions so that it can work properly. First you have to guarentee that the base is .6V above the emitter, not that hard really, a voltage at the base via a resistor will do this, especially if the emitter is connected to ground. BUT you also have to set the b-e current to a point so that when there is no AC signal present (your signal is AC btw) the collector is "resting" at a point where it will allow the largest signal swing before distortion (if that is your aim, you can set it anywhere you want really, higher or lower so if you get it to clip it will clip on one side first for assym distortion).  This b-e current is a DC signal, DC in that it is static, it doesn't change. So, you "DC Bias" the transistor. You do this by setting the current through the b-e junction (via V+ through that resistor on the base, via Ohm's Law) to the point where the corresponding current through the collector sets the voltage drop across the collector's resistor to be 1/2V+ (or any other point you are looking for) at the collector. Then when your signal IS present it's flucuations in the b-e current will cause the collector to flucuate (though inverted) proportionally around that set DC Bias point, amplifing your signal.

There are other issues that are important as well but that is the gist of it, it would be well worth your time to read up on this on other basic electronics websites.

Hope this helped somewhat.

Jay Doyle

[Gringo]

I don't know if you helped Luke, but sure as hell you helped me!!

Worth a couple more reads to digest it completely, though..

Thanks a lot, man!

[smoguzbenjamin]

this explains a lot... Now to implement this into real life. Help  :?  :wink:

[Luke]

thanks- thats going to be a huge help for certain!
Luke