Author Topic: Transistors in Parallel  (Read 4838 times)

seedlings

Transistors in Parallel
« on: October 02, 2012, 12:23:20 PM »
Please forgive my inexperience in SS.

Triodes can be paralleled for a moderate increase in gain with reduced noise (because the resistor values are halved).  Does this principle also apply to transistors?  Application: I built a rangemaster-type boost and it has a bit too much background white noise.  Would a paralleled transistor and half value resistors to reduce noise?

Thanks for your guidance,
CHAD

R.G.

Re: Transistors in Parallel
« Reply #1 on: October 02, 2012, 01:25:02 PM »
In theory, theory is the same as practice. In practice, it isn't.
(Yogi Berra, I believe).  :)

Yes, kind of. That's the theory.

However, you'll probably do better by selecting a single low noise germanium instead of messing with paralleling them to run the impedance down. That's because the noise mechanisms in most germaniums are not the simple resistive-derived mechanisms, nor the flicker/shot noise of low current quantization. A lot of germanium noise is from surface leakages and contamination that isn't a strictly resistive mechanism.

Paralleling germaniums for lower noise will be as experimental as selecting for a low noise one.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

seedlings

Re: Transistors in Parallel
« Reply #2 on: October 02, 2012, 01:53:29 PM »
Understood, and thanks RG  ;)

The ge I selected sounded nice for tone and drive.  It was later when I noticed the background noise, and attributed that to the carbon film resistors (odd values required).  I'll sift through some other specimens.

CHAD

LucifersTrip

Re: Transistors in Parallel
« Reply #3 on: October 02, 2012, 02:04:50 PM »
every time I've paralleled transistors, one transistor always "took over"...so, my guess would be, if you put one in parallel and got less noise, it would simply be that the 2nd one was less leaky....and I can tell you for a fact, as RG noted, that leakage causes noise in Rangemasters...which is why those insanely low leak Russians are perfect
always think outside the box

seedlings

Re: Transistors in Parallel
« Reply #4 on: October 02, 2012, 02:12:53 PM »
which is why those insanely low leak Russians are perfect

Ha!  I'm using a GT108V.  I have dozens more, and they used to be organized by gain, but my 2 year old took care to disorganize them for me.  I also have some GT313A.

CHAD

R.G.

Re: Transistors in Parallel
« Reply #5 on: October 02, 2012, 02:46:37 PM »
Carbon film - as opposed to carbon comp - resistors are not really all that bad for causing thermal noise. Metal film is a little better, but not hugely so. In many cases it's the active devices and/or high gain that's the real root culprit. 
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

Jazznoise

Re: Transistors in Parallel
« Reply #6 on: October 02, 2012, 02:49:34 PM »
Have you tried it on a battery? Alot of pedal noise simply comes from ripple on the voltage rails.
Expressway To Yr Null

amptramp

Re: Transistors in Parallel
« Reply #7 on: October 02, 2012, 08:37:22 PM »
When you put bipolar transistors in parallel, you have to find some way to force the current to be shared more or less equally.  In a number of cases, the transistors use separate emitter resistors so if one transistor tries to hog the current, the extra voltage drop across the emitter resistor will reduce the bias until there is a more equitable sharing.  I have a 13.8 volt 8 amp linear supply that uses four series pass 2N3055's in parallel and has a separate resistor in series with each emitter for exactly this reason.  If one of your transistors turns on with a Vbe of 0.20 volts and the other with 0.19 volts (typical for germanium), then the one that turns on at 0.19 volts will carry all the current and the base-emitter diode at 0.19 volts will clamp the drive to that level and the transistor with a 0.20 volt turn-on will remain off.  Once you add emitter resistors, the current through the more sensitive one will reduce the bias until the other one can carry a small amount of current.

PRR

Re: Transistors in Parallel
« Reply #8 on: October 03, 2012, 01:00:56 AM »
LM394 is _100_ matched transistors wired-up as "two transistors" 50 each.

Noise (hiss) is somewhat lower than many other simple transistors.

The actual factors are Current Density and Parasitic Base Resistance.

Noise _voltage_ goes down as Square-Root of number of devices. Two is better than one. If not good enough, skip 3 and go to 4. I have seen 8-parallel for a specific application (super low impedance).

But that whole theory is verifiable only in very-clean Silicon process, where a dumb little 2N5089 gets awful close to Theoretical Noise at guitar impedance.

Also NOTE: guitar amp gains were developed so that hiss was not a big deal. A booster like the Rangemaster has gain around 10, a LOT. You boost the guitar; you also boost the guitar's (thermal) hisss.

And that's a way to check. Build a Rangemaster with neg-ground and a 2N5089 (or pos-ground with a good PNP Si device), making the 68K bias resistor 100K to keep current similar. That's a good low-hiss for guitar application. But there's always hiss, and if you boost you will hear it. If the Si booster is not-much-quieter than the Ge booster, then you are near as good as it gets. If there's a big difference, you may have bad Ge. (Or bad joints, bad caps, crap from cell-phone or PC.... low-noise work is frustrating).

Working in older Germanium, I bet paralleling devices is more like mixing bad beers. Or two good players and one bad player. You don't have odd flavors averaging-out, instead the worst of the worst comes through.

Agree that selecting, within a batch or among batches of different vintages, is your first step. Find and lose the hissers.
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brett

Re: Transistors in Parallel
« Reply #9 on: October 03, 2012, 08:07:54 AM »
Hi
maybe you are just amplifying the cosmological white noise picked up in your guitar-like antenna. You know - the 'snow' between radio stations. Any and every booster makes the snow louder.
(The reason I say this is because in my experience GT308s are very quiet. 99% or more of GT308Bs are)
cheers
Brett Robinson
Let a hundred flowers bloom, let a hundred schools of thought contend. (Mao Zedong)

seedlings

Re: Transistors in Parallel
« Reply #10 on: October 03, 2012, 11:43:59 AM »
Hi
maybe you are just amplifying the cosmological white noise picked up in your guitar-like antenna. You know - the 'snow' between radio stations. Any and every booster makes the snow louder.
(The reason I say this is because in my experience GT308s are very quiet. 99% or more of GT308Bs are)
cheers

Could be, but the noise is louder than my Fuzz built with two of them.  I added 10pf at a time across Rc to reduce hiss when it was on the bench (using a 1-spot for power).   There is a 220R, 1000uF cap RC filter on the power coming in.  I didn't notice the noise floor until it was on the pedal board with other pedals (and the same 1-spot), but that doesn't mean the noise wasn't there to begin with.  I'll try out some different Ges pretty soon - that's probably it.

It doesn't help that I use this in front of my Vulcan and Fuzz to boost for lead stuff  :o  It sounds very nice, and while playing it's, of course, not a big deal.  Just have to punch it out when not in use otherwise the sound guy will turn me down and forget about me.  On it's own there is a gentle woosh in the background, but in front of those beastly boys it's more of a nuisance.

CHAD
« Last Edit: October 03, 2012, 11:51:38 AM by seedlings »