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Transistor equivalent

Discussion in 'General Electronics Chat' started by Nikolai Petrenko, Oct 24, 2015.

  1. spec

    spec Well-Known Member Most Helpful Member

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    Last edited: Nov 3, 2015
  2. Willen

    Willen Well-Known Member

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    Hi,
    I already tested almost all radios of techlib.com. These are mindblowing. Circuit you mentioned at techlib.com needs just a meter of antenna and sounds really great! I will try your previous circuit and will say how efficiently it performs.
     
    Last edited: Nov 4, 2015
  3. spec

    spec Well-Known Member Most Helpful Member

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    Hi Willen,

    You are a fast operator. Glad to hear that the circuit works ok. Looking forward to hearing how the first circuit turns out. Keep up the good work!
     
  4. dave

    Dave New Member

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  5. Nikolai Petrenko

    Nikolai Petrenko Member

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    What a long answer!
     
  6. Nikolai Petrenko

    Nikolai Petrenko Member

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    C945, C1815, 2N3904, D965, S9014 are can use for audio. But I want know what is the best for audio of them? datasheet not talk much ( for sallen-key filter I asked )
     
  7. Willen

    Willen Well-Known Member

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    China widely uses S9014 as audio transistor. So use it as audio purpose. (however you can use all of them and perhaps you will get extremely tiny difference, almost like no difference if going to use in a general amplifier circuit)
     
  8. Nikolai Petrenko

    Nikolai Petrenko Member

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    I thing C945 lower noise than 9014, but less power
     
  9. Willen

    Willen Well-Known Member

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    Hi Spec,

    Few days ago I had tested your version of full wave rectifier radio on my very sensitive 32 ohms earpiece and I heard almost same level of audio from half rectifier and from full rectifier.

    I added a tuning circuit for AM (LC) same as crystal radio and took its output to your both circuits. Full wave rectifier played same level of audio, I don't know why. And I didn't get audio from transistor amplifier. Maybe I made mistake.
     

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  10. Willen

    Willen Well-Known Member

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    I found just a noticeable characteristic about noise in datasheet is 'Noise Figure (NF)'. According to datasheet, Maximum noise figures of BC549 and 2N2222 are 4dB. Maximum noise figures of S9014, C1815, BC547 are 10dB. Maximum noise figure of 2N3904 is 5dB. And maximum noise figure of C945 is 15dB.

    This shows that the BC549, 2N2222 and 2N3904 are nice. But I found no similarity between two datasheets of same transistor. However noise is only critical if you have very high impedance audio pre-amplifier.
     
    Last edited: Nov 13, 2015
  11. atferrari

    atferrari Well-Known Member

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    Not to derail this thread, Willen Raj K, but just abrief comment:

    Many years ago I also "discovered" :p that a 1N4148 could be used as a temperature sensor. Tested a big bunch of them between 0º and 100ºC. Still have the lab tubes in my drawers.

    Having heard about their linear response and how many mV they will change per ºC I found they showed a really non-linear response.

    For any practical purpose I think is better you test them to see how much they can go apart from a straight response. IIRC (done it more than 15 years ago) the highest separation from a straight line occurred somewhere around 55º or 60ºC.

    Those notes and my circuits from that time are lost. That moved me to never take again anything out of my home.

    Additional and curious note: I was truly amazed to see that the temperature of the water (furiously boiling) in the kettle, varied some degrees depending where the sensor was located. Hot experience that some of my fingers will never forget.
     
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  12. Nikolai Petrenko

    Nikolai Petrenko Member

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    Ok 2n3904 and c1815, i will test both
     
  13. Nikolai Petrenko

    Nikolai Petrenko Member

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    The preamp use to boost signal from subwoofer filter so 15db noise not a problem
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If you want good audio then you should use an opamp instead of a transistor. Transistors were used as preamps or audio filters 50 years ago before opamps were available and their distortion is fairly high. Opamps produce such low distortion that it is difficult to measure any. But do not use an LM324 or LM358 opamp because their low power design causes very bad crossover distortion and noise.

    The noise spec's for the transistors are all with different supply voltages, different frequencies, different currents and different source resistances so you cannot directly compare them. Many of the transistors have a HUGE range of noise so some are low noise and others are very noisy.
    The BC549 is a BC547 or BC548 SELECTED for its low noise. With the other transistor you are gambling that you might get a low noise one.
     
  15. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Did you test them using a constant current source or other method?
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    How about a synchronous receiver?
     
  17. atferrari

    atferrari Well-Known Member

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    A constant curren source maybe with LM334 IIRC but not sure.
     
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Oh ok that is probably good enough.
    If you want to read a good article on the diodes and temperature, check out Bob Pease's (RIP) articles. They are out on the web somewhere.
    I believe they are "somewhat" linear, but i guess not perfect.
     
  19. BobW

    BobW Active Member

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    I'm sorry, but this is completely false. Semiconductor diodes start conducting at zero volts, and the current to voltage relationship is a power law. The forward voltage Vf (also falsely referred to as turn-on voltage) is simply the voltage drop across the diode at a specified current (often 1 mA), which is often well higher than RF signal levels which can still be detected. So, relying on Vf values is misleading.

    Many weak radio signals are less than the typical Vf of a diode, but the diode will still detect the signal.

    While Vf does give a vague indication as to whether a diode may be suitable as a detector, there are other more important parameters that determine how well it will work for weak RF signals, (signal levels well below Vf).

    The correct theory of diode detection is given here by the late Ben Tongue:
    http://www.bentongue.com/xtalset/4opd_xfr/4opd_xfr.html

    I've built a number of crystal sets, and found that the 1N5711 is a very poor detector. There are much better schottky diodes made for detector use, but the best one will depend on the characteristics of the rest of the circuit. It's an impedance matching issue. To get the most audio out, the detector has to be impedance matched to both the RF tuned circuit and the audio output device. The 1N34A is still just about the best there is for the typical crystal set, and they can still be found with modest effort.
     
  20. spec

    spec Well-Known Member Most Helpful Member

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    Hi all,

    Just read all the recent posts- very intresting. Have been on other things including domestic duties. Also someone had this light controller and I spent a long time just trying to figure out the circuit diagram!

    Not too strange about the full wave detector sounding no louder than the the half wave, now that I have thought about it: like the eye, the ear has a logarithmic response, so doubling the power is just perceptable (3dB). To sound twice as loud you need 10x the power. I think all that is right. If your Xtal earpiece* is truely voltage driven, as opposed to power driven, the audio power would only double so you wouldn't hardly notice that. On the other hand, if it is power driven like a loudspeaker, the volume would go up 4x. You would notice that. If you use the 32 Ohm earphone you will swamp all effects out with the low impedance. Could I suggest staying with the xtal phone for the time being. We can do a hifi version, once the front end detector is sorted. (* appears to contradict physics but the low coupling efficiency, about 3%, is the cause)

    If I remember correctly, the full wave detector should have less noise though, for a given signal, that is because the wanted signal is coherent and the noise is just er... noise. I think the noise improvement is around √2. No, doubt the experts will will be able to advise on this if I have got it wrong.

    To develop the circuit you really net a meter that can handle up to about 500Hz or better still an oscilloscope.

    Great idea, were you thinking about useing RF FETS?

    I started looking at a switching demodulator using a compartator which is another approach, but not sure how noisy. If it works it should have a threshhold of about 1mv, compared to the about 150 mv for Ge and Schottkey diodes and probably about 100mv for the biased transistor detector. Now that we are allowed to use a power line for the xtal radio, all sorts of approaches are possible.

    A front end amp is probably the best way forward. I think the frequecies for AM go up to 30Mhz.

    You make me smile- I have done similar experiments way back. I was testing a circuit built with some expensive components (new fangled transistors) and read that the gain went up with temperature so I heated it all up with my mom's hair dryer. The gain sure went up, but so did the transistors. That cost me about 3 months pocket money.

    The voltage depends on the semiconuctor bandgap which should decrease fairly linearly with temperature. Have a look at this:

    http://ecee.colorado.edu/~bart/book/eband5.htm

    What you have probably seen is the nonlinear effects from the bulk resistance ( I think that is the term) , which cloud the issue, more so as the current through the diode is increased. The 1n4148 is a fast diode with low leakage but I think it has a relatively high resistance. The 1N4001 has a low resistance but a high leakage which would also cloud the issue. Try putting less current through the diode: 10uA would be good if you have a high impedence digital volt meter. The emitter base junction of a small signal transistor should be quite good for temperatue measuerements. Try both the base/emmiter junction and then the base/collector junction.

    Water can never get hotter than 100 degrees C, it's a pysical impossibility, unless it is pressurised that is (no doubt you know that). What you were measuring, I suspect, was small bubbles of steam.

    Nice reading your posts and talking to you all, but need a 48hr day :wideyed:


    junction_temperature_cct.png
     
    Last edited: Nov 13, 2015
  21. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    reminds me of a cube radio a friend had in Grade 9 It was 6 transistor superhet with battery and Xtal earbud. The cube was around 1.5cm cubed."
     

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