Saturated NPN base to emitter voltage drop?

Discussion in 'General Electronics Chat' started by morphy_richards, Jan 3, 2013.

  1. morphy_richards

    morphy_richards Thread Starter New Member

    Jan 3, 2013
    I'm checking for possible problem with a simple H bridge motor control circuit I'm designing using only NPN transistors. The motor supply is 4.5 V and the base will be 'switched' using a 5 V supply.

    I understand that as long as the transistor is in saturation there is no appreciable voltage drop from the collector to the emitter but what about from the base to the emitter?

    Something I have read elsewhere suggests there is a 0.7 v drop here which would put the base voltage at 4.3 V and below the value of the (4.5 v) emitter voltage which means the transistor wont work as expected? Thanks for any help and advice.
  2. MKCheruvu

    MKCheruvu Member

    Nov 20, 2012
    Please provide Schematic diagram to understand H Bridge and its Drive configuration.
  3. GopherT

    GopherT Well-Known Member

    Nov 23, 2012
    Western PA (USA) (GMT -5)
    In general, you are right. Emitter will be 0.5 to 0.7 volts below base voltage (it can be more). If you have a darlington NPN, it will be double. Check your DATASHEET.
  4. StayatHomeElectronics

    StayatHomeElectronics Well-Known Member

    Sep 25, 2008
    NE Ohio, USA (GMT - 5)
    A schematic would definitely be helpful.

    If you tie the emitter of your drive transistor, npn, to 4.5 volts, the base of the transistor would have to reach approximately 5.2 volts in order to turn the transistor ON. The 5.2 volts is above both your drive and logic supplies. It requires additional "high side drive" circuitry to use only npn transistors.
  5. tinamishra

    tinamishra New Member

    Dec 1, 2012
    A bipolar junction transistor could be a design of transistor that relies on the get in touch with of 2 types of semiconductor for its function. BJTs are often used as built-in amps already built in, changes, or in oscillators. BJTs are often discovered either as personal individual areas, or in large figures as components of incorporated tour.

    Bipolar transistors place system thus known as due to their function includes each electrons and gaps. These 2 types of cost providers place system attribute of the 2 types of doped semiconductor content. In difference, unipolar transistors like the field-effect transistors have just one quite cost service provider.
  6. Audioguru

    Audioguru New Member

    Dec 20, 2007
    Ontario, Canada
    Please "get in touch" with translation software that works properly.
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