why npn tr not suitable as switch

Discussion in 'General Electronics Chat' started by ajay2539, Nov 7, 2014.

  1. ajay2539

    Thread Starter New Member

    Nov 7, 2014
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    why npn tr not suitable as switch for analog signals
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Why do you think it would be?

    Is this a homework question?
     
  3. ajay2539

    Thread Starter New Member

    Nov 7, 2014
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    i read that pnp tr . are suitable for switch so, i want to know why ?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    both are suitable, just differnt polarity.
     
  5. ajay2539

    Thread Starter New Member

    Nov 7, 2014
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    yes both can be used but which one will be best to use ...
     
  6. studiot

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    What are the characteristics of an ideal switch?
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    low on resistance, high off resistance.
     
  8. alfacliff

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    the one you design the switch for. as voltage rating, current rating, speed rating, polarity and such. also whether you are using high side or low side switching.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    A FET would be a better choice for an analog switch.
     
  10. crutschow

    Expert

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    A BJT has a small offset as well as base current that goes into the channel when used as a switch. A FET has no offset or gate current, and just looks like a resistor when used as a switch. That's why FETs are usually preferred for switching analog signals.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    A FET is usually preferred, but I wouldn't say an NPN BJT is "not suitable". It can perform just fine as a switch. If you have just 1V to put on the base or gate, it's the best game in town.
     
  12. crutschow

    Expert

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    A BJT can work well as a digital or power switch. But you should note that the offsets of a BJT can be problematic in low level analog signal switching.
     
  13. kubeek

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    Sep 20, 2005
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    Unless OP is talking about an analog switch like CD4066, then I have to say I havent seen one made with bipolar transistors. Do such even exist?
     
  14. crutschow

    Expert

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    Not in integrated form, but you could conceivably make one from discretes.
    Many years ago I worked on a FLIR system that used a 175 channel 10.5 MHz analog multiplexer consisting of discrete BJT switches. It used an unusual configuration where the output consisted of a series of emitter followers sharing a common emitter resistor. Each base was connected to an IR preamp detector output and the analog switching was performed by sequentially turning on the collectors of each emitter follower. The emitter follower transistors were matched to minimize Vbe offset difference between each transistor. Worked quite well.
     
  15. studiot

    AAC Fanatic!

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    The more you put into your question the better folks will be able to help and the more you will get out.

    We have no idea from this what you want to do or why, or even if it is just a passing question over a beer that you have already forgotten.

    It is you question and your problem, so you need to help people help you.
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here you go:
    NPNasAnalogueSwitch.gif
    There are slight glitches at the start and end of the switching period, and some attenuation, but otherwise the AC signal is gated with little distortion.
     
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