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    • #6662

        Hi all,

        We’re back with an astonishing sensitive circuit : the darlington amplifier.

        A darlington pair is a circuit consisting of 2 transistors with their collectors connected to each other.

        The emitter of the 1st transistor is connected to the base of the 2nd as shown in the figure :

        Darlington pair

        What’s so special about this configuration is the fact that the amplification factor (Hfe) of the 2 transistors is multiplied ! This means that in our case with the 2N3904 transistors, the Hfe is 180 × 180 =32400 !

        But wait, what will happen when we connect more than 2 transistors in a Darlington configuration ?

        Indeed the total Hfe will be (180)4, which is a whopping 1 049 60 000 !

        Now, this sounds like fun, but there are a few drawbacks here. First of all, the voltage needed at the base of our first transistor need to be much higher than an conventional transistor (can you see why ?).

        The 2nd fact is that to obtain such a high Hfe, we need to carefully calculate the currents in our output circuit.

        But in our Lectron environment, we will use the normal parts to make some neat and magical experiments.

        As a starter, let’s setup the circuit as following, using the lamp without the meter :

        Darlington amplifier V1

        Now let’s do some magic !

        All you need is a woolen cloth and a plastic article, like a ruler or eye glasses with plastic frame.

        Put the woolen cloth at a table. Now take your plastic item at one end and rub it firmly on the cloth. Don’t touch the cloth with your hand !

        When you’re getting closer to L1, you will notice that the lamp glows ! Real magic, isn’t it ?

        Repeat this experiment, now with the meter assembly (as shown in the figure) connected instead of the lamp.

        Try some other items and note which one works best.

        Let’s do an experiment now which longs very very long …

        Darlington amplifier V2

        Try the circuit with the lamp first.

        It’s also best that you use the power adapter instead of the battery, since this circuit is a long term experiment.

        After pressing the switch, you will notice that the lamp lights up. Now, check on the watch and count the time till the lamp goes off.

        Repeat this experiment with the meter setup and compare.

        What are the results ?

        A tip, can you reconstruct the circuit so that the on-off times are shorter ?

        And now a last question, why are these experiments not possible with the legacy Germanium transistors ?


        To be continued …



        • This topic was modified 3 years, 11 months ago by Michael.
        • This topic was modified 3 years, 11 months ago by Michael.
        • This topic was modified 3 years, 11 months ago by Michael.
      • #6667

          Stunning revelations here (certainly for me!)

          Static electricity inductively coupled to then be amplified into useful current/voltage to light the lamp! 

          Professor Frank, you continue to astound!  You are a worthy successor to Mr. Wizard!

          • This reply was modified 3 years, 11 months ago by Michael.
          • This reply was modified 3 years, 11 months ago by Michael.
        • #6690

            Hello Michael,

            Well, let’s honor Mr. Wizard’s demonstrations and keep his spirit alive in this new era !

            I made experiments with a spring wire extending upwards (which I also use as FM antenna) and the needle moves from about 20 cm when approaching L1.

            Even rubbing the plastic makes the needle move !

            Concerning the question about the Germanium transistors, these are not usable here because they leak a very small amount of current, which is amplified and the output stays high.

            Today, we are going to build a Touch dimmer. We will combine two properties of the Darlington pairs.

            Let’s build the circuit first with our Lectron start and expansion set.

            Darlington dimmer

            You can see from the diagram that we use 2 Darlington pairs. The first pair (T1 & T2) form the touch amplifier.

            The 2nd pair (T3 & T4) form a sensitive amplifier with very high input resistance which forms (almost) no load on the previous stage.

            When you touch the 2 points indicated by the arrow (don’t touch the metal plate !), the lamp will gradually become brighter and the meter needle moves. When you release the touch points at a chosen time, the lamp retain for a very long time it’s brightness.

            So, how does the circuit works ?

            Darlington pair 1 amplifies the very weak current flowing through your fingers and feed it to C1 & C2 through R1.

            C1 & C2 slowly charge since R1 is relatively high.

            The charging current flows also through R2 to the 2nd Darlington pair. The lamp and meter connected to it’s output indicate this slowly increasing amplified current.

            Now, when you release the 2 touch points, the current charged into C1 & C2 can only flow through R2, since T1 & T2 block.

            Now, the magic happens. Since Darlington pair T3 & T4 has an Hfe  of about 32400 and it’s also connected as an emitter follower, it poses (almost) no load on the discharging current of C1 & C2.

            This results in a ultra long time till C1 & C2 discharge completely.

            Feel free to experiment with different values of C1 & C2.

            Also change the values of R1 and/or R2 into 10K and watch the results !




            • This reply was modified 3 years, 11 months ago by Michael.
            • This reply was modified 3 years, 11 months ago by Michael. Reason: Changed font and size styling for easier reading
          • #6695

              Wonderful Frank!

              Why do the Germanium transistors leak vs the silicon (I believe that is the material used) ones?

              Is this how the touch dimmers work commercially?

              At the rate you are going with new circuits and explanations, we are going to have to create a PDF for our Lectroneer friends to download and enjoy!  Well done!


            • #6697

                Hi Michael,

                Germanium transistors have a very small leakage current which grows when temperature rises. This is what makes them unique in vintage amplifiers. They tend to add a small amount of noise to the sound, causing a bit of “crunchy” effect.

                This is also a challenge to construct a Darlington circuit using these legacy transistors.

                Hmmm, let me think, there must a way to work around this.

                What a great idea to bundle all these project in a pdf book. Maybe in the same style as the original Lectron manuals.

                But I haven’t finished constructing circuits around the start and expansion set yet, new fresh ideas come to my mind !




              • #8185

                  Sorry for this insert, but I don’t want to open a new topic:

                  I could not cut a Darlington transistor into 2 pieces 😉 – so I used one Lectron block for it. It’s a common MPS-A13 (European equivalent: BC517) to test a circuit for voltages from 1.5 … 9 V for an AF amplifier stage. Low voltage amplification for e.g. the AM radio block #2454 (ZN414 / MK484 / TA7642 / LA1050 / … with Vcc= 1.3V max.) and a low-loss speaker connection (s. above) with a tapped coil transformer (medium impedance speaker and headphone socket) for best results and economic usage.



                  • This reply was modified 3 years, 8 months ago by Michael. Reason: Changed font and size styling for easier reading
                • #8200

                    Hi…these use virtually the same amplifier boards with electronically filtered power supplies so what you are left to compare is just the case, transformer type and possibly its slightly different supply voltage and hum issues.I have the 3rd example and find it performs well enough but thermally it’s a problem after an hour or so as it heats up and with that, also the volume control – That’s not good but I have a fix in mind for it.

                    • This reply was modified 3 years, 8 months ago by Michael. Reason: Changed font and size styling for easier reading
                  • #8203

                      Hi all,

                      A darlington amifier is not designed for AF amplification due to the temperature drift which shifts the bias. Sometimes the power stages of an amplifier use darlington transistors in a very controlled way by using strong DC feedback.

                      It’s preferable to use a feedback amplifier stage as the ones I proposed in this forum.

                      A second disadvantage of the darlington is the uncontrolled gain.

                      A better solution is to use a very high gain transistor such as the MPS-A18 or the BC549C both having a Hfe gain of 500 when a single transistor stage is preferred.


                      • This reply was modified 3 years, 8 months ago by LectronFan.
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