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Double-Sziklai-Compound-Transistor (RFC)

Double-Sziklai-Compound-Transistor (RFC)

Double Sziklai Compound Transistor

Greetings, Everybody,   Now that I did some writing of my own, I kindly ask of those of you, who are “deep” in electronics, can you please comment on this picture as above, representing my contraption of what I call a  “Double Sziklai Compound Triplet”.   Probably it has been already invented by somebody else a long time ago, and it is “nothing new under the Sun”,  used in various applications, but if not, then just in case, I would like to hint that I drew this by myself today… {{   (C)  zjj_wwa, 2013-07-09  :)   }}.

If it turns out that this is something “new” – then as far as I am concerned, you are free to use this topology, for what it is worth, for NON-COMMERCIAL,    DIY purposes.

Those of you, who have built your own DIY amplifiers, are bound to have heard of a “Darlington Pair”, which consists of two n-p-n transistors, whereby the emitter of the driver transistor drives the base of the power transistor.
Their collectors are together, and such a configuration is known to have an amplification factor BETA(compound)  equal to the multiplication of the BETA’s of each of the two transistors building the Darlington PAIR.
But the Darlington Pair has a major drawback – two times the base-emitter voltage drop for such a compound transistor as a whole, which is not so good – especially if you are building a regulator that requires low dropout voltage.  In a voltage regulator application, such a serial pass element would result in a dropout voltage equal to two times the base-emiter junction voltage  (2 x 0,7 ~1,4V).

There was a Hungarian Guy, a Mr. Sziklai, who has invented some time ago a hybrid, two transistor pair, which consists of two heterogenous transistors, mainly an n-p-n driver transistor, and a p-n-p power transistor.
Sziklai - Traditional
As can be seen, a “Traditional Sziklai” .. sometimes called by people as a “Complementary Compound Darlington Pair” has an n-p-n driver transistor “pulling” the base of a “p-n-p” power transistor.  This configuration is very popular and much used in the LDO (Low Drop-Out) voltage regulators, because the total dropout of this combination consists of the emiter-base voltage drop of the power transistor (~0,7V) and the collector-emitter voltage drop of the conducting driver transistor.  This configuration achieves a total dropout that is less than as was the case with the Darlington Pair (0,3V+0,7V=~1.0V  <  1,4V),  but at the cost that the output impedance of such a pass device is high (the output is the collector of the power transistor) and this some times constitutes a problem.

{{ … another problem of the traditional, two transistor Sziklai pairing is that it is in essence a common collector output, meaning a fairly high output resistance. This, in turn, gives rise to various stability problems, be it in voltage regulators, or amplifiers, … issues that need to be dealt with by careful application of anti-parasitic capacitors … }}}

Now, my question was the following:  Can I have the best of BOTH worlds ?

Can I have:

  • a fairly low dropout voltage ?   …. and at the same time
  • a low output resistance ? … and at the same time
  • a very high compound value for the BETA  (amplification factor) ? … and, at the same time, 
  • a very low capacitance of the driver transistor’s base ?

Please have a critical look at my drawing at the top. It consists of THREE transistors, whereby the power transistor is an N-P-N, so there should not be an issue with the output impedance of such a serial compound pass device element (to be used in a regulator that I am building).  The base of the power transistor is “pushed” by the collector of a p-n-p driver transistor, which is sort of “Sziklai” connected, but not quite … .   A “reverse” or “complementary” Sziklai would probably be the better term.  Then again we have a third, pre-driver, n-p-n transistor, which “pulls” the base of the driver p-n-p transistor.

My big question is:

Will this setup of a  “Double Sziklai Compound Triplet” actually  WORK ???????? !!!!!!!!!!

My guess is that the compound beta of this contraption shall be in the order of:

400  (pre-driver)  x 300 (driver) x 50 (power transistor) = 6 Million.

Now, a beta of 6 million sounds like real trouble.  Will this thing be stable, or will it OSCILLATE ?

Where would you put capacitors here so as to stabilize this topology, if necessary ?

{{{ … or maybe a small amount of degenerative feedback, such as with an emitter resistor or something, to hammer down that big beta just a small bit ? … }

A final word regarding the resistors: R1 pre-loads the Q10 driver transistor with a small load current, so as to allow it to operate in a more favorable point on its transfer characteristics. Similarly, the R14 allows the Q8 pre-driver transistor to work with a slightly higher current, so as to allow it to operate in a more favorable point on its transfer characteristics. But at the same time, it also allows the Q10 transistor to “turn off” faster, if it is supposed to stop conducting.

Now, Dear All, please tell me: WHAT do you think ?  Will this setup work as anticipated ?

The input capacitance of such a contraption should be fairly small, a favourable attribute, when it comes to working up in the higher frequencies.  Any frequency related compound beta droop could be mitigated by some slick local degenerative feedback within this transistor triplet. 

Can somebody please throw this into a simulator, like the SPICE tool, and share with me what are the results ?

Do you agree that the power transistor, for voltage regulation purposes,  could be a D44H11 ?
The driving transistor – a BC560C, for the sake of it, and the pre-driver transistor – a BC550C … or maybe even a J-FET transistor, like a 2sk117 or something ?   (…. but this would be yet a totally different story … ).

Please kindly COMMENT.


P.S. As per suggestion of Mr. Sharma as in his comment, I present here below a simplified schematic of the voltage regulator, in which this Double Sziklai Compond Transistor configuration is intended to be used:
Regulator Simplified

Will this thing be stable or oscillate ?  I mentioned “simplified”, because there are plans to additionally insert a “shunt” topology on the output of this schematic, so the regulator would become a Serial-Shunt regulator.  But one step at a time. First I would like to be sure that this “basic” configuration shall be stable.   If it will NOT be stable, where should I put the capacitors, or other means, to stabilize this circuit ?

As a small commentary, of what I “wish to believe” that will be happening here (from right to left): From the right we enter the circuit with 10VDC. There is a FET transistor acting as a current source, that feeds the Zener Diode (or in the future – a more sophisticated circuit) acting as a stable voltage reference. The voltage reference is “silenced” by a capacitor, then it is “silenced” / “filtered out” some more by an RC network.  After the reference is cleaned up, it is directed to the base of one of the transistors in a Long Tailed Pair differential amplifier.   The differential amplifier is actually built out of “Darlington’ed” compound transistors, so that I can achieve more gain.   The lower “TAIL” of the amplifier is not a normal resistor, but a yet another FET based current source, that guarantees a stable current at all times. The differential amplifier has two routes feeding it from the “top”.  One route is via an ordinary resistor (the “load” of the differential amplifier).   The other route is through … yes, …. yet another FET based current source.

Now, the base of the transistor on the “other” side of the differential amplifier is fed from a voltage divider that monitors the level of the “stabilized” output voltage.   The Darlington’ed differential LTP amplifier compares this with my “clean” reference voltage.

Now, lets assume that the output voltage “suddenly becomes too high”.   In such case the divider produces a higher voltage. In consequence, the right hand side of the differential amplifier conducts more, hence the voltage on top of Q13 falls, and as a consequence the “Double Sziklai Compound Transistor” (DSCT) conducts less into the load of the regulator, hence correctively reducing the output voltage, which “suddenly became too high”.

Why does the DSCT conduct “less” ?  Because, IMHO, a falling voltage on the base of Q8 will cause it co conduct less; this in turn causes Q10 to conduct less;  This in turn causes the power transistor Q9 to conduct less.

Intuitively, it should work. But practically …. (?)    The gain of the DSCT is somewhere in the area of Beta= 6 million.

The gain of the differential amplifier is also very high.   Will this setup be stable ?

Even if it is stable, will the regulator “RING” when subjected to square wave transients on the input, or if it is subjected to “square wave” patterns of the output current drawn from the output of this regulator, as a result of it being loaded by a chip that is operating in a digital circuit ?

Let’s just assume that this regulator is supposed to provide the highest possibly stable power to a crucial Digital to Analog Converter chip …

Your Comments – Kindly Welcome.


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