sine to TTL with a SMD transistor

Discussion in 'General Electronics Chat' started by rompelstilchen, Jul 22, 2014.

  1. rompelstilchen

    Thread Starter New Member

    Jul 22, 2014
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    hello ,

    i try to convert a sine wave (0.25v peak to peak) to a ttl signal with a transistor

    some guy could do it with a bc547 here : https://www.youtube.com/watch?v=eKHKGpOcllA

    so i tried a simulation

    [​IMG]

    it seems ok, but in practice i can barely have a tension follower (gain=1)

    i use a BC848 that has a gain of 90 to 180
    not much but it should be able to at least double the input and i cant make it work

    any advice?

    thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want to turn a sinewave into a real squarewave, a schmitt trigger would be the tool to use.
    On this page of the wiki, there is a two transistor schmitt trigger:
    http://en.wikipedia.org/wiki/Schmitt_trigger

    Bertus
     
  3. rompelstilchen

    Thread Starter New Member

    Jul 22, 2014
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    sorry but i have a cd4093(smd) and it never output anything as i give the mentioned signal
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You are right that the 4093 will not put anything out, as the hysteresys is about 1.5 - 2 volts at 5 V powersupply.
    The 0.25 v signal you have available will be to low.

    Bertus
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    google comparators. That would be the way to go with such a small signal.
     
  6. rompelstilchen

    Thread Starter New Member

    Jul 22, 2014
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    yep, hence my attemps to amplify with (small) discreet components
     
  7. rompelstilchen

    Thread Starter New Member

    Jul 22, 2014
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    but comparators seem to need a lot of peripheral componants

    [​IMG]
     
  8. rompelstilchen

    Thread Starter New Member

    Jul 22, 2014
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    0
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Nowadays you can get single gate packages for various logic families - fair bet there's an integrated Schmitt trigger on the market somewhere, for 1 or 2 input gates; the package is barely any bigger than a SS SMD transistor.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello Ian,

    Do you mean the 1G14?

    [​IMG]

    [​IMG]

    Bertus
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Though perhaps not as many as an amplifyer. Never more.

    Don't use a gate for this, your signal is too small. Adding an amp so you can use the gate is silly when just a comparator will do.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What load impedance is the collector of the BC848 driving (not the 1K pull-up)?

    Are you sure you had the EBC connections correct?
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That circuit should work perfectly fine with the right value resistors and a sine of 0.25v p/p.

    You need to change the base voltage divider to have less impedance, and better adjustability.

    Try a 100k instead of the 330 ohm resistor, and keep your 25k trimpot.

    Now you will be able to adjust it with a lot more accuracy, and it won't be loading your sine amplitude down as much.

    You can change R1 collector resistor to 10k too, it does not need to be as low as 1k.

    To adjust the trimpot to suit TTL output remove the sine signal, and adjust the trimpot until the collector (output) voltage is 2.0v. That is about halfway between the TTL digital threshold voltages.
     
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Here is a 1-transistor approach. It needs a low source impedance for the input signal. The average transition level is around 1.8V for TTL and 2.5V for CMOS. Adjust R2 and R3 for the right combination of duty cycle at the transition level and keeping the positive output peaks at 4V or more.

    ak
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    A comparator needs a voltage reference (including pull up resistor) and a high resistance for positive feedback if you want hysteresis like a Schmitt.

    An op-amp needs all those things - and a much higher price tag if you want it to switch as fast and clean as a comparator.
     
  16. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That circuit performs far worse than the original with the parts changes I suggested.

    It's better to have a high impedance voltage divider for the base, and bias it with the transistor half on. Then the output vaveform will be more evenly distributed (closer to 50:50 duty) and more square.
     
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