Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Transistor Resistance

  1. Mar 4, 2016 #1
    Do transistors give a certain resistance?
    If so how do you find this resistance?
    Say Hypothetically for the following example. Refer to (fig.1).
    The resistor's value is 100Ω, source is 3V.
    Say if the transistor wasn't there, the amp would be 0.03A or 30mA.
    But when I run this in a simulation, the amp across the ciruit is 0.02252A or 22.52mA.
    How is the transistor affecting the current and is there a formula?
     

    Attached Files:

  2. jcsd
  3. Mar 5, 2016 #2
    The base-emitter junction inside the transistor is actually a diode. This diode needs at least 0.7V across it before it will conduct appreciable current. This leaves 2.3V or less across the resistor. The 22.53mA shows that the actual voltage drop across the base-emitter diode is 0.748V, which is reasonable for a small transistor.
     
  4. Mar 5, 2016 #3
    Thankyou.
    That helped alot.
     
  5. Mar 11, 2016 #4

    LvW

    User Avatar

    Looking at the current-voltage characteristics of the B-E path (as for any pn junction) we see a typical exponential function with a current even for voltages as low as 0.1 volt. What is an "appreciable" current? I think, it is necessary again to mention that the pn junction does not "suddenly" open at a voltage of 0.7 V (as some newcomers might think).
     
  6. Mar 11, 2016 #5
    @LvW

    I agree. The 0.7V is just a 'typical' rule-of-thumb used during a quick-look circuit evaluation. Especially for design, a look at the characterisric curves is essential. (as the OP 0.748V example points out!)
     
  7. Mar 12, 2016 #6

    LvW

    User Avatar

    Each transistor (BJT and FET) as well as each semiconductor device (diode) has a strong non-linear voltage-to-current characteristics.
    Therefore, speaking about "resistances" it is very important strongly to discriminate between STATIC (R=V/I) and DYNAMIC (r=dV/dI=v/i) resistances.
    So - what is your problem? A transistor is a three-terminal device and we can define two resistances (static, dynamic) at the input as well as at the output port.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted