Transistors operation

Discussion in 'General Electronics Chat' started by @hacker, Sep 2, 2015.

  1. @hacker

    Thread Starter New Member

    Nov 30, 2014
    While reading my textbook I came across this that all transistors which are used as switching elements are operated in saturation region. And reason for this was to maintain a low on-state voltage drop.But I didn't understood how it happens. So can any one here tell me how this happens and how the low on-state voltage drop is maintained?
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    If you consider a transistor with say a 1k collector resistor and a 10V supply.
    When the transistor is switched 'hard On' ie: in saturation almost all the voltage is dropped across the 1k resistor, so there is just under 10mA flowing thru the 1k and the transistor.
    The Collector voltage will be typically in the order of 0.1v thru 0.2v. [ the Vce sat is transistor dependent]

    When in saturation, the transistor gain falls to approx 10 to 20 , compared to say 200 when operating in the linear mode.
    So to ensure that the transistor remains in saturation the Base current must be at least 10mA/10 ie: 1mA.

    Is this what you are asking.?

    Clip from datasheet for a NPN 2N3904, showing Vce sat
    Last edited: Sep 2, 2015
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    By driving the base with a much higher base current than is implied by Ic/hfe.
  4. crutschow


    Mar 14, 2008
    Suppose you have a fixed collector load resistor and resistor voltage, and you slowly increase the current into the base of the BJT transistor.
    The collector current will then also slowly increase until the collector voltage becomes close to zero volts with all the current that can go through the collector resistor, at which point the base current is about equal to the collector current divided by this particular transistor's gain (Beta or hFE). But that value of base current does not give the minimum collector-emitter voltage.
    For that, the transistor's base current is typically increased to about 1/10 of the collector current (forced Beta of 10) to insure that the transistor is fully on (saturated) with the minimum collector-emitter voltage.
    If you look in the transistor data sheet, you will see that is how they test a transistor to measure its saturation voltage.

    A similar thing is done with MOSFETs (which are also transistors) to insure that they are fully on as a switch, but since they are voltage operated devices, its the gate-source voltage that's over-driven above its threshold voltage (Vth), typically to 10V for a standard MOSFET or 3-5V for a logic-level type.