Higher-gain version of 2N2222

[smoguzbenjamin]

Hi all!

I built the NPN beginner boost, and I'm real pleased about it. But the first time I built it, I screwed up somehow. I don't know what's wrong with the first build, but I've got the parts left. Now I was wondering -> this is basically a mini-amplifier. So enter the diodes, and here's a distortion... I'm still toying with the idea of making an overdrive out of it though, I've got a Boss MetalZone, which is all the distotion I need... But, I wanna have tons of gain  :twisted: . So instead of a 2N2222 which I used, does anyone know a similar tranny, but with heaps more gain?

cheers

[zachary vex]

well sure.  2n5088 or MPSA18 will probably do the trick.

[smoguzbenjamin]

I couldn't get the 2n5088, I wanted to start with that one too... I'll see if I can get the MPSA18

[Gringo]

Don't forget the allmighty BC549c, it saved my ass when i began DIY (2n5088 are everywhere but here, it seems )

[smoguzbenjamin]

2N5088's seem to be extremely difficult things to aquire... :?

[petemoore]

I just got a batches of 2n3904's in an old heath 2322 component tracer.
 I got many 5088's in an old DOD bass EQ, and have seen these in other
scavenge items.
 How close would a darlington from 2n3904's be...just curious...seems folks in some places have troubles getting high gain/low noise [even just high gain]  transistore sometimes, maybe a high gainer could be built from a smaller gainers into a Darlington???

[Brett Clark]

You are not going to get "heaps more gain" from a transistor substitution in the beginner boost, even using a Darlington. You might try going to a higher supply voltage (2 9V's in series) and adjusting the input resistors to correct the bias point. Better yet, add a gain stage.

What follows is a post that I made back in May about this subject. Paul Perry pointed out, correctly, that there are many circuits where higher beta will produce higher overall gain. But, this is because of impedence and loading effects, not because of higher unloaded voltage gain per stage. Also, the higher beta will change the biasing of many stages, changing the distortion characteristic considerably.

*******************************************************

I see a lot of statements about transistor "gain" on this forum that may be leading some folks down the wrong path.
The "hfe" or "beta" of a bipolar transistor does NOT determine the voltage gain of the stage that it is used in. It "helps" but it is not the only or even the most important factor. The beta is the ratio between input and output *currents*, but we are most interested (usually) in *voltage* gain. The beta (hfe) mostly determines the impedences (input and output) of the stage.


Most real use of bipolars in gain stages is as a voltage amplifier. A small ac voltage applied to the base produces a much larger output voltage at the collector, assuming biasing is OK. Right?
So what determines voltage gain? If there is an unbypassed emitter resistor (one without a cap across it), it produces degeneration, which is a form of negative feedback. It reduces the gain, but also makes it predictable, stable and linear. If it's large enough, the gain is (good approximation) = Rc/Re. So an EH LPB-1, which has Rc=10K and Re=390, should have a voltage gain of about 25, which it does. Putting a super-high gain Darlington in wouldn't increase the voltage gain much at all (but it would let you increase the bias resistors on the base and improve the input impedence).

So what if all of the resistance in the emitter path has caps across it? Or if there is no emitter resistor at all? The equation above say the gain would be infinite, which obviously it isn't. That's because a bipolar transistor has a "ghost resistor" in the emitter path, which is called re. That should be a small r with an e as a subscript. This is a characteristic of the transistor technology itself, not a defect or an actual resistor that the manufacturer puts there. The value of re is dependent on the current flowing into the collector: re = 25ohms/Ic, where Ic is the collector current in mA), at room temperature.
So the maximum gain = Rc/re = Ic*Rc/25. For a current of 1mA and Rc of 22K, max gain is 880. Again, this does NOT depend on hfe/beta, provided that you have ENOUGH hfe to drive the next stage and not load down the input.

An "interesting" side effect of re is the relatively high distortion of a Common-Emitter stage that doesn't have degeneration or feedback. Since the gain changes with the current, it actually varies over the cycle of the wave, producing distortion. The largest harmonic in this distortion curve is the 2nd, which shows that the audiophile cliche about transistors only producing odd harmonics is nonsense.

There is one more catch to the Rc/re max gain issue. You can't pick just any old values of Ic and Rc, because you have to keep the collector in the region where it can swing both positive and negative. This is dependent on the the available power supply voltage. If you want the maximum available collector swing and minimum distortion, the maximum gain is 20*Vcc (where Vcc is the power supply voltage) - so 180 for 9V applications. If you are willing to accept more distortion and smaller voltage swing (i.e. fuzzbox), you can certainly get more gain by biasing for more current.

[RDV]

Hi all!

I built the NPN beginner boost, and I'm real pleased about it. But the first time I built it, I screwed up somehow. I don't know what's wrong with the first build, but I've got the parts left. Now I was wondering -> this is basically a mini-amplifier. So enter the diodes, and here's a distortion... I'm still toying with the idea of making an overdrive out of it though, I've got a Boss MetalZone, which is all the distotion I need... But, I wanna have tons of gain  :twisted: . So instead of a 2N2222 which I used, does anyone know a similar tranny, but with heaps more gain?

cheers

An option for an easy build with a ton of distortion is my NPN Tonebender that's in Schematics 2 in the links above. You can use 2N2222 in it for Q1 & Q2. Just try to find something a little higher gain for Q3 like MPS18 or 2N5089, or BC549.

Regards

RDV

http://www.diystompboxes.com/pedals/schems/NPN_Si_TB_Mk.%20II.pdf

[Paul Marossy]

Try building Joe Davisson's "Easy Drive". Only one 2N3904 transistor and tons of gain, very easy to build. This thing probably has more gain than anything using two 2N2222 types.  

[smoguzbenjamin]

I wasn't really out to build a distortion, I was just thinking, since I have the PNP booster parts lying around, I could fiddle it around to a distortion. I have a metalzone, I don't need to build a distortion, nor was I planning on building a DS. I was just trying to put the old parts to good use...

I don't really have the resources to buy more parts etc... I was thinking that now I have the parts, that I could put m to good use

[Paul Marossy]

Got a breadboard? Break it out and start experimenting a little bit... you may come up with something different, weird, cool.

I would say a 2N3904 is in between the 2N2222 and higher gain types of transistors like a 2N5088 and a MPSA18. Maybe you could mess with some of those, too...

[Tim Escobedo]

As Brett states (and quoted), you technically won't get heaps more gain using any other transistor using a typical resistive load. But you can (subjectively) get much better ability to drive clipping diodes by using a higher hfe transistor. Which is where it really counts. Smallbear has several that would be suited for this. Hell, almost any transistor other than the 2N2222 would suffice.

[smoguzbenjamin]

I have a breadboard, but I allready put the circuit onto perf. But I was smart enough to socket the transistor....  :twisted:  Time to experiment, hehehe