NPN transistor help please

Discussion in 'Homework Help' started by akyra83, Nov 11, 2008.

  1. akyra83

    Thread Starter New Member

    Sep 23, 2008
    im trying to draw a circuit in which you have a signal that fluctuates between 1.0V and 1.2V that you wish to amplify using an npn transistor, so the 0.2V change appears as a 2.0V change. Define all values assuming load resistance of 10k ohms

    im like hours reading and studying npn transistors and i need someone to guide me please.
    Last edited: Nov 11, 2008
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Can you supply any additional specifications for the NPN transistor that you will be using? Parameters that would be helpful to know are Beta and Vbe for a start.

  3. steveb

    Senior Member

    Jul 3, 2008
    It sounds like you can use a common emitter (CE) amplifier without a bypass capacitor on the emitter resistor. If you are reading up on this subject, one of your books should show you the circuit and the basic design approach. The CE amplifier has negative gain, but the wording of your statement seems to allow either positive or negative gain.

    One thing that is not entirely clear from your wording is if you are interested in an AC or DC coupled amplifier. This would have an impact on the design and the performance. Usually a problem like this implies AC coupling, but you need to be sure.

    Basically you have given very limited information, but if you start to work on the problem as post some results, people can guide you better.
  4. hobbyist

    AAC Fanatic!

    Aug 10, 2008
    As was said you have given very vague information, However by gleaning the info given we can work out certain parameters and see what we come up with.

    1. you said "an NPN" that indicates one transistor being used.
    2. Fluctuation in input voltage, 1v - 1.2v. output 2V.
    3. load = 10K ohms.

    So now we have some parameters to work in.

    Alright 2v.out / 0.2v in = Av (signal voltage gain)
    Now we know that Rc / Re = Av also.
    so we can choose a value for Rc. lets begin by making Rc = to RL = 10K.
    Now Rc // RL = total Zout.
    Zout / Re = Av. therefore Re = Zout / Av.
    Now choose a supply voltage. Vcc Arbitrarily make Vcc. = 5 X v.out (signal voltage)
    For proper biasing in amplifier design it is good to make Vc = to Vcc/2. (voltage at collector with respect to ground)
    Ok we can now solve for Ic by using Vc/Rc assuming this = Ie also we can now solve for VRe, so Ic X Re = VRe.
    Now assuming NPN is silicon than Vbe = around 0.7v.
    Now we will solve for our Base resistor to ground. (RB1) ... VRe + Vbe. = VRB1
    Now from here we have a choice to make since Zin is not specified we can approach this a couple ways. We can just make RB1 around (20 X Re) or we can choose a base divider current.
    We will go with the 20 X Re approach. So we make RB1= 20 X Re.
    We now have Rc and Re and RB1 established.
    Now to solve for the resistor from base to the supply terminal. (RB2)
    We need to solve for the divider current to do this we take VRB1 / RB1 = IB1
    Now (Vcc - VRB1) = VRB2.
    So RB2= VRB2 / IB1

    From there you would breadboard (prototype the circuit) and then adjust values as needed.
    This design may or may not work without tweaking the values due to Beta of the transistor as well as other factors. But this is a basic approach to get you in the ballpark of biasing an transistor amp.