Discussion in 'Homework Help' started by Permia, Oct 17, 2010.

  1. Permia

    Thread Starter New Member

    May 6, 2010
    Figured this would be best posted here, eventhough its not quite homework it will help me understand my lab results a little better...

    Okay, built a common emitter amplifier (below) with 15V supply. Recorded Vb, Vc and Ve. Then changed the transistor for another, identical component (both BC109) and recorded Vb, Vc and Ve again.

    The results are below
    .....T1 T2
    Vb 2.5 2.6
    Vc 2.0 1.9
    Ve 8 8

    Now, the base and collector voltages being slightly different with each transistor isn't a problem as far as my experiment goes, I'm just interested as to why these values are different.

    Any explanation or HINT at explanation would be fantastic! Thanks
  2. Georacer


    Nov 25, 2009
    Can you post a larger image? This one is hardly readable. We could help you better if we knew the values of the resistors.

    The usual suspect is either the beta value or the temperature.

    Beta is never exactly dialed in. Two transistors of the same model have never the same exact beta. Read a datasheet. It has a minimum and a maximum value for the beta - for every base current level!

    Another factor of variability is temperature, that affects Vbe. If you had the first transistor working for a long time before you took the mesurements, take it out, replace it and take the new measurements immediately after, then you propably took two measurements in different temperatures.
  3. Permia

    Thread Starter New Member

    May 6, 2010
    Sorry about the picture...I didnt actualy think resistor values had much to do with it so didn't bother posting them. The thumbnail was just to give a basic idea of the layout. Here's a larger pic.

    R1 = 22k
    R2 = 4.7k
    R3 = 1.2k
    R4 = 330

    Thanks for your advice.
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Hello Permia,

    You appear to have mixed up your reported Vc and Ve values. The collector voltage is always higher than the emitter voltage for the NPN configuration shown in the schematic.
  5. hobbyist

    Distinguished Member

    Aug 10, 2008
    Therews always going to be variations, as mentioned by the posters, but transistor parameters fluctuate even with the same model number.

    Here is a good experiment to understand parameter changes in transistors.

    Build your entire circuit but leave out the transistor temporarily.

    1). take a voltage reading from ground to the junction of the divider R1 and R2.
    2). now keep your voltmeter at that junction, and hook up ONLY the transistor EMITTER AND BASE, do not hook up the collector.
    3). see how much your voltage drops.
    the reason the voltage drops is because there is a series circuit with the base emitter diode, and R1 wich means all this current will flow through the base from the emitter.

    4). Now keeping everything hooked up connect the colector of the transistor to R3, and watch how the voltage increases at the junction of R1 and R2.

    What that shows is with a open collector all current flows through the emitter into the base, when the collector is connected then majority of the current that flows into the emitter will flow out the collector, while a small amount of the emitter current flows through the base.

    This is the function of the Beta of the transistor,
    Now beta's change due to temprature, as well as manufacturing tolerances, whioch are the parameters built into the transistor device.

    So if Beta of one transistor is low, than more current will flow out the base and drop the base voltage, as well as the collector voltage will go higher, and if a beta of another transistor is higher , than less current will flow out the base and the base voltage will be higher, causing the collector voltage to go lower, (common emitter config).

    So that's the reason designs are done with external components, such as the resistors, in the emitter and the ground resistor at the base to try to compensate as much as possible for the extreme changes in transistor parameters, be it manufacturing tolerances or temperatiure, etc...
  6. Georacer


    Nov 25, 2009
    This particular transistor configuration the stabilization coefficients of beta and temperature are as follows:

    According to my caclulations...:p

    Temperature relation:
    \frac{\Delta I_{CQ}}{\Delta T}=\frac{K \cdot \Delta T}{R_E} approximately, where K=2mV/^oC.
    For a fluctuation of 50 degrees the collector current will change by 0.3mA which will change V_E by 0.1V. Pretty significant huh?

    Beta relation:
    S_B=\frac{I_{CQ1}\cdot (R_B + R_E)}{\beta _1 \cdot (R_B+(\beta _2 +1)\cdot R_E)}
    That means that for an initial measurement of your first circuit with V_E=2V for a hypothetical beta=150, if beta was raised to 200 (through replacement with another transistor) the collector current would fluctuate by 20μA and the collector voltage by 6mV.