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

Help With Self-Biases JFET Amplifier

  1. Dec 3, 2011 #1

    I am designing a circuit where i first use a JFET transistor to a provide a voltage gain, the JFET is wired as a self bias. Following the JFET is a BJT transistor which i am using to provide a second amplification. I have an issue though with the output waveform of the JFET transistor. I am not sure why but the output waveform is an amplifier version of the input waveform, except it is also raised by a certain voltage so the sine wave oscillates about a positive voltage and not 0 V. The output of the BJT however oscillates about 0 V. How can i make the output of the JFET oscillate about 0 V?

    Any help is appreciated.
  2. jcsd
  3. Dec 3, 2011 #2


    User Avatar
    Science Advisor

    The output of a BJT amplifier (at the collector) or the JFET amplifier (at the drain) should be about half of the supply voltage with no input signal. This is because you want the output to be able to swing upwards for a negative going input signal and downwards for a positively going input signal.

    Note that this means the output is actually an inverted version of the input.

    Both outputs can have this DC component removed by passing the output through a coupling capacitor with a resistor to ground on the other side of it.

    If you have a bipolar transistor with near zero voltage on the collector, then it is being driven to saturation by having too much DC voltage on its input.
    So, you may need a coupling capacitor between the two stages.
  4. Dec 3, 2011 #3
    ok that sort of worked. I added a 100uF capacitor then after the capacitor a 1k resistor to ground. In doing this the first output waveform had no more DC voltage, but this also messed up my second output of the BJT network. Now then output from the BJT is just 0 V with negative peaks but no positive peaks. So basically a sine wave that is just 0 wherever there should be a positive voltage.
  5. Dec 3, 2011 #4


    User Avatar
    Science Advisor

    OK. A bipolar transistor has to have base current and this is supplied by having a resistor connected to the base and to a source of positive voltage.

    This is one way to do it:


    The resistor shown as Rb would be something like 150 K ohms.

    You can adjust it by measuring the voltage on the collector. If this is less than half the supply voltage, then the resistor Rb is too small. If the voltage at the collector is more than half the the supply voltage, then the resistor Rb is too big.

    Capacitors of about 10 μF maximum would be used in circuits like this. 100 μF is a bit too big.
  6. Dec 4, 2011 #5
    Could you post your circuit? It is very difficult to give advice about a circuit based on what we think it is.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook