Transistors resistance.

Discussion in 'General Electronics Chat' started by seanlikeskites, Jan 13, 2010.

  1. seanlikeskites

    Thread Starter New Member

    Jan 13, 2010
    I am trying to teach myself how transistors work. I am using this site but also am using a book i have.

    This site talks a lot about current flowing between base and emitter affecting the current flowing between the collector and the emitter. But the book i am reading talks about a voltage across the base and the emitter changing the resistance between the emitter and the collecter.

    I don't see how these two ways of describing things fit together and am very confused.

    In the book the circuit in the picture attached is shown. It says how an increase in voltage at the input increases the resistance of the transistor. As the transistor creates a potential divider with the resistor above it, an increase in its resistance will cause an increse in the output voltage.

    From what i have read on this site however it seems to me that if the voltage was increased at the input more current would flow from base to emitter, so more would flow from collecter to emitter. Surely if there is more current there is less resistance.

    I hope somebody can clear this up for me, and i hope my explenation was not too long winded. (I am not very good at expressing my ideas in words).
  2. hobbyist

    Distinguished Member

    Aug 10, 2008
    The input resistance is increased due to the voltage drop across the emitter resistor.

    Here is how it works,
    without the emitter resistor the base to emitter voltage is around 0.7v. (silicon transistor for sake of argument.)
    This voltage is the voltage reading from base term. to ground.

    Now with the resistor in the emitter lead, under initial conditions, when the input base voltage is just about the threshold voltage to start it conducting, the initial current is zero thru the resistor, so Voltage at the base with respect to the emitter is just below 0.7v.

    The moment the threshold voltage is reached 0.7v. then the base emitter conduct, this allows current to flow, which now causes current to flow thru the resistor, producing a voltage drop, so now the emitter is no longer at zero volts, respect to ground, but some value above ground, which means the voltage at the base with respect to the emitter, is now much less, as well.
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    This first approximation might help understand the workings of a transistor.

    I have added a few things to your picture to illustrate.

    On the left is your transistor with Collector (Ic) and Emitter (Ie) currents shown.

    On the right is a water feed from a tank to a washing machine. There is a simple lever operated valve in the line.
    Water flows from the tank into the washing machine. We control the rate of flow by adjusting the lever valve. We can set the flow to any desired amount by suitable positioning of the valve lever.

    A transistor is a device like that flow control valve that allows us to control the flow of (collector) current. We can turn up or down the current by suitable adjustment of the current control input. There are differences from the washing machine valve, but essentially the more we turn the transistor on the greater the current flow - just like the valve.

    Notice I haven't yet mentioned voltage. In fact this controlled collector current is largely independent of the voltage! ( so long as there is enough, just as there has to be enough water in the tank)

    Now you have shown two resistors and I have labelled them Rc and Re. Just as with the washing machine we can set the valve to allow 1. 2, 5 litres/sec so we can set the transistor control to allow (force) .1, .5 1 amp to flow.

    Ohms Law tells us that if we are causing 1 amp to flow through Rc then it has a voltage of 1*Rc volts across it.

    Now the current through Re very nearly equals the current through Rc so we will also call it 1 amp. This is a characteristic of transistors.
    So the voltage across Re is 1*Re

    So the voltage across the transistor is what's left from the supply. That is

    V - 1*Rc -1*Re.

    The transistor adjusts its own resistance to comply with ohms law and this voltage and the 1 amp flowing.

    As to the control to cause the 1 amp of collector current we can either work in terms of an input voltage or and input current. Usually we work in terms of an input current and the gain or beta of the transistor. If the gain is 100 it means that we need to input 1amp/100 to cause 1 amp of collector current to flow. Or that the collector current is 100 times the input current.
  4. seanlikeskites

    Thread Starter New Member

    Jan 13, 2010
    Ok, i have worked through what you said studiot but it still seems to contradict my book. I dont know if im just missing something really obvious but it doesn't make scense.

    Looking at the diagram you have drawn, if both Rc and Re are 100Ω, V is 250V and the collector current is one amp. There will be 100V across each resistor leavong 50V for the transistor. Giving it a resistance of 50Ω.
    Now if you drop the collector current to 0.5A, you end up with the transistor having a resistance of 300Ω.
    So if as you said there is a gain of 100, you would require 10mA input for the transistor resistance to be 50Ω and a 5mA input for it to be 300Ω.
    Correct me if im wrong, but that makes perfect scence to me.

    In the book i have it says that Rc is 900Ω and V is 10V. It goes on to say that an input of +1V will cauce the series resistance of Re and the transistor to be 900 ohms. Also an input of -1V will cause Re and the transitor to have a resistance of 100 ohms.
    Now if you use ohms law with the 10V and the resistances stated above, you get a 5.5mA collector current when the input is +1V and a 10mA current when the input is -1V.
    This to me seems that there is a smaller collecter current when the input is larger, which goes against what you have said and what i think should happen.

    As i said it is probably just me missing the obvious, but your help would be much appreciated.
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    I'm sure your book says a lot of things but it's difficult to comment without knowing the circumstances.

    Yes although the values are very large for normal transistors they illustrate the point.

    Yes you have got hold of the basic idea that a transistor is a controllable tap for current with a proportional relationship. You can control the current from off to some maximum this way. This is known as the linear or active region of operation.
    Once you reach the maximum, the transistor enters what is known as saturation or switching mode. Here the transistor is considered to be either on or off, like a switch and is the basis for digital circuitry.

    You should realise that the circuits we have drwn are not complete working circuits, they are too illustrate a point. A working circuit requires other components just as the washing machine pipes require pipe clips etc.

    Once again I stress the important concept for transistors is (controlled) current flow.

    So I should just forget your third paragraph an move on to biasing. You are concentrating too much on resistance. Things should become clearer after you study biasing.

    Post again if you need more help.