Any use for PNP darlington transistors

[mozz]

Any use for PNP darlington transistors? Just picked up a 1000 cheap to92 transistors, date coded 1983, house number marked, turned out they are pnp darlington with a gain of about 25k and up. Maybe a hybrid fuzz face? Neg ground version of a booster or bazz fuss of sorts?

[Steben]

Because of the high gain, they can feature high input impedance while still having some punch. The slightest emitter resistor (without decoupling cap) boosts input impedance while taking away not that much gain and it even gives some mellowing of the non linearity. An alternative for jFET inputs. RE of 100 ohms gives +3 Mohms at the base at normal currents!
They are indeed very useful for boosting and bazz fussing etc...

[pacealot]

They'd be great for a positive ground Scrambler — which is something I'd like to try myself, come to think of it.... 

[mozz]

Any neg ground circuits use them?  Think I will try some in PNP fuzz just to see if they are usable.  My pcbs have a open q1 slot for a emitter resistor & cap anyway which I usually jump.  Think I have some scrambler pcbs here, somewhere.

Edit: really tempted to make my pedalboard half positive ground and half neg gnd.  Isolated handmade power supply just waiting. 

[antonis]

An alternative for jFET inputs. RE of 100 ohms gives +3 Mohms at the base at normal currents!

But they will need a bias configuration, no...??

[Steben]

An alternative for jFET inputs. RE of 100 ohms gives +3 Mohms at the base at normal currents!

But they will need a bias configuration, no...??

of course, but only with one resistor more (or even not). And then you have a bias that usually works with any transistor of the same type. jFET circuits depend mostly on the characteristics which are unpredictable.

[antonis]

 

Of course but I was talking about equivalent input impedance..
A 3M or so Base "reflected" resistance makes no difference on a 30k bias configuration..

[Steben]

Of course but I was talking about equivalent input impedance..
A 3M or so Base "reflected" resistance makes no difference on a 30k bias configuration..

Exactly! A single PNP would!

[antonis]

 
(sorry Steven, but..)

I'm trying to tell you that bias configuration impedance is the major dominator of stage input impedance, in the case of ultra high gain transistor..

[Rob Strand]

I'm trying to tell you that bias configuration impedance is the major dominator of stage input impedance, in the case of ultra high gain transistor..

FWIW the gain of a Darlington can drop off significantly at low currents.    The hFE might be 25k to 40k at 5mA to 10mA (using MPSA13 data) but it will drop a hFE of say a bit over 1k at 10uA and 4k at 100uA.    The hFE on power Darlingtons can drop off a lot more because the low value internal BE resistors effectively bypass the second transistor in the Darlington - and so it doesn't contribute any gain.

Even at hFE = 1000 the input impedance is going to be high and the voltage drop across the base-bias resistor is still quite low.

[antonis]

FWIW the gain of a Darlington can drop off significantly at low currents.

Totally agree, Rob..

That's the main reason (other than linearity) for R4 be placed between Q1 & Q2 Emitter..
(Q1 Collector current must be much larger than Q2 Base one, for the Darlington configuration reasonably high beta so R4 is set there..
No problem from signal point of view 'cause it's lower end is bootstrapped from Q2 Emitter..)

[Rob Strand]

Yes, that's the resistor.   (I don't know what the MPSA13 Darlington has inside.   No resistor shown on the internal schematic.   I might have something in my archives.)

Interesting buffering of only the bootstrap with Q4.

IIRC, Doug Self's book seems to show the MPSA42's as being quite linear.   The BC capacitance is low and also the collector resistance (r0) is high.

Actually maybe that circuit is from Doug Self?

[antonis]

Actually maybe that circuit is from Doug Self?

Definatelly yes..!! 
(my bad for not mention it..)

>Interesting buffering of only the bootstrap with Q4.<
There is another one, slightly more complicated, with current source for Q4 and also bootstrapping Q1 Collector resistance (rc = Early voltage / Collector current) and Base-Collector capacitance (Cbc) via R6 & C3..
(but, IMHO, it's an overkill for casual effects bulding.. - Douglas Self uses moderate beta BJTs for their large V_CE spec in contrast to lower bipolar op-amps supply margin..)

[Steben]

Actually maybe that circuit is from Doug Self?

Definatelly yes..!! 
(my bad for not mention it..)

>Interesting buffering of only the bootstrap with Q4.<
There is another one, slightly more complicated, with current source for Q4 and also bootstrapping Q1 Collector resistance (rc = Early voltage / Collector current) and Base-Collector capacitance (Cbc) via R6 & C3..
(but, IMHO, it's an overkill for casual effects bulding.. - Douglas Self uses moderate beta BJTs for their large V_CE spec in contrast to lower bipolar op-amps supply margin..)

Well if you need a REALLY sturdy input buffer 

[Rob Strand]

>Interesting buffering of only the bootstrap with Q4.<
There is another one, slightly more complicated, with current source for Q4 and also bootstrapping Q1 Collector resistance (rc = Early voltage / Collector current) and Base-Collector capacitance (Cbc) via R6 & C3..
(but, IMHO, it's an overkill for casual effects bulding.. - Douglas Self uses moderate beta BJTs for their large VCE spec in contrast to lower bipolar op-amps supply margin..)

Doug Self has some cool designs that push the envelope.    Some of those are used to show how far you can stretch things and what limits the performance (he's really done a great job on this over years).   If you were building test equipment where you need something better than all other circuits then I can see these types of circuit have their place.

The extra bootstrap on the collector of Q1 is to reduce the input capacitance, which is pretty small already but  perhaps letting it compete with a JFET.   
[Actually, going back to some 60's Wireless World articles, the motivation boot-strapped collector is quite obscure, it reduces the effect of the *base-collector* resistance on the input impedance.]