Connect transistors together?

[airguitar]

Can i connect transistors together for higher gain in a pedal ?
If yes how ?

[GibsonGM]

You sure can, Air!    Each transistor can be set up in what is known as a Gain Stage, which amplifies the input signal applied to it.  If you connect them together, or CASCADE them, and they multiply the amount of gain.   The venerable Fuzz Face is one example of this:   http://fuzzcentral.ssguitar.com/fuzzface.php  Tho it is a special case in how it actually operates.

This is way above our pay grade, but 6.2 in the following PDF shows, and discusses the basic gain stage, which is a bit different than the fuzz - mainly just found this to show you what one looks like.  There are a lot of tutorials out there about "BJT gain stage" design, how to just start using them...   http://ee.sharif.edu/~faez/Chapter_6.pdf   Look up "Big Muff Pi" to see how a bunch of stages have been cascaded in a pedal.

This also applies to FETs, Opamps, tubes...they cascade in a very similar way, just a few details will differ.   They will all have, essentially, an input and output cap, a source of power, and some form of biasing.     These are the components we 'tweak' to get the stage to do what we want.

[samhay]

Another option is to use a pair of transistors configured to act as a single amplifier stage.
Look for e.g. a 'Darlington transistor' or 'Sziklai pair'.

[GibsonGM]

Again, don't get too bogged down in the heavy details, but feel free to check this out:  http://www.electronics-tutorials.ws/transistor/tran_1.html   

Be aware that common emitter configuration is the most-used one you'll see in pedals.  Look around for simpler/better tutorials, esp. on Youtube, where you can see them in operation.    Once you get the gain stage understood, you move to cascading them...or working with Darlingtons, as Samhay pointed out. 

[EBK]

Another option is to use a pair of transistors configured to act as a single amplifier stage.
Look for e.g. a 'Darlington transistor' or 'Sziklai pair'.

I just happy that we live in a society where a transistor couple can be either Sziklai or Darlington and have their orientation accepted and respected.

[Aph]

I'm glad you feel that way, Eric, because some times I feel PNP and other times I feel NPN.

[airguitar]

So how i can do that on a breadboard ? I somehow connect them ?

[EBK]

So how i can do that on a breadboard ? I somehow connect them ?

Depends on what kind of transistors you have....

Do you have NPN, PNP, a mixture of both?

You also have to know the pinouts of your transistors....

[airguitar]

So how i can do that on a breadboard ? I somehow connect them ?

Depends on what kind of transistors you have....

Do you have NPN, PNP, a mixture of both?

You also have to know the pinouts of your transistors....

Is this right ? https://www.youtube.com/watch?v=GWVq4UiIfKs
I have 2 transistors 2N3904.

[GibsonGM]

"Right"?  Yes, it is, for parallel operation. That allows the transistors to pass more POWER than one alone can handle - that's ALMOST like a Darlington, but Darlington transistors come in 1 package (you CAN make them with 2 transistors)....a typical power rating for the ones we use, like 2N3904, is 500mW, or 1/2 Watt (same thing).    Paralleling them allows you to pass about 1 W through them.   This application doesn't have that much to do with our audio stuff, though.   That is a DC application, at least in the video you linked.  Some good info on there - yes, the BJT is a current-controlled device, rather than voltage controlled as a FET or opamp would be....that can make working with them intimidating for some people.

CASCADING gain stages, what I thought you wanted to do, will get us more Voltage gain....AC amplification, since our guitar signals are AC.   Look at the Big Muff circuit to see (many) gain stages cascaded.  1st stage takes your tiny guitar signal, and outputs a measly low voltage signal...that goes to stage 2, which takes the maybe 1V signal, amplifies it as much as it can (within the limits of the power supply - can't put out 20V if you run on a 9V battery, ha ha)...so we get some clipping (the diodes help too!).....and so on.    It is how you get big distortion.    Other things come into play too, though, like tone-shaping using caps and resistors. That comes after you get started...

To do so with the 3904's, you could simply build the first 3 stages of the Muff - leave out the 1N914 diodes if you like, or put them in for more clipping.   That will get you going experimenting!     Then you can look at how to design stages of your own!

[Steben]

If the current flow is doubled, doesn't it behave as if the Vbe (which is log) goes down? In other words almost in Ge-land?

[antonis]

Doubling current flow by doubling series pass transistor keeps every transistor's current same as previously..
(as long as you place some Emitter balast resistors, you're fine with equal current sharing..)

P.S.
V_BE is actually ln (not log)..