Using a current sink to bias a transistor

Discussion in 'General Electronics Chat' started by Shagon, Jun 24, 2011.

  1. Shagon

    Thread Starter Active Member

    Aug 19, 2009
    48
    0
    Hey guys I just want to know is it possible to use a simple current sink with a diode and a PNP transistor with only a positive rail to ground (No negative rail) to bias a simple common emitter amplifier with PNP transistor without having to send DC voltage into to base to turn it on?

    Lets say by using the output of the current sink and connecting it to the emitter of the transistor. there is no negative voltage rail... only positive and ground.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Where is your schematic?
    What is a current sink?
     
  3. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    I agree. Too many undefined terms. Need a drawing of some sort.
     
  4. Shagon

    Thread Starter Active Member

    Aug 19, 2009
    48
    0
    Lol Oh come on you guys are being harsh...

    Anyways I just draw up a quick schematic to illustrate what I am talking about please don't pay any attention what so ever to the resistor or capacitor values.

    Schematic.png

    Its just like a normal Emitter Follower but without the dc voltage divider to turn on the transistor. The transistor would be turned on by a current source just like in a differential amplifier.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    There are no PNPs in your schematic.
    The circuit topology you posted will work. Current sinks are sometimes use to set emitter/collector current.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit has a constant current sink, not a current sink.
    It will work only if the upper transistor has its base voltage high enough, usually by two resistors as a voltage divider.
     
  7. Shagon

    Thread Starter Active Member

    Aug 19, 2009
    48
    0
    oops sorry my bad I meant NPN...

    Ok thanks a lot I would like to develop a simple emitter follower amplifier. However I am stuck with a 12v power supply. Down here Electronic Hobbyist are if there are any are scarce like gold. So there aren't any electronic stores where I could purchase components so I gotta salvage what I can.

    Also I don't have a function generator nor an oscilloscope so the only method of testing this circuit is by sending Audio into it and see what I can get from a 0.5 W 8ohm speaker just for the pleasure of having built the circuit.

    However I have noticed that the voltage divider which I would be using to turn on the transistor would make my input impedance extremely low. Which would not allow me to receive a Audio signal from a CD player. SO I have decided to remove the voltage divider and turn on the transistor through a current source.

    Lol sorry I don't have a schematic its on my other computer... but I hope you guys have an understanding of what I am talking about... anyways thanks alot.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The upper transistor is not turned on by anything so it won't work.
    the output level is very low and we think that grannie is dead.
     
  9. Shagon

    Thread Starter Active Member

    Aug 19, 2009
    48
    0
    Then how are differential amplifier like these turned on?
    differential_amplify.gif
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    In the diff amp, V1 and V2 need to be biased at least ≈3V above ground.
    In your circuit, if you want to take the output off the emitter, you should short out the collector resistor, bias the base at about Vcc/2 (through a resistive divider) and couple the input signal to the base through a capacitor. You will also have to couple to the speaker through a capacitor. Be aware that speakers are generally low impedance, and so they require a lot of current. Your current sink will need to be run at a fairly high current level, so both transistors will probably get hot if you want to hear anything out of the speaker.
     
  11. Shagon

    Thread Starter Active Member

    Aug 19, 2009
    48
    0
    If I take the output from the emitter their would not be any gain would there?
    I actually would like some form of gain. If I would have a gain of 5 I would be content.

    The voltage divider biasing is actually what I am trying to avoid to increase my input impedance.

    So I thought that current sinks actually biases the transistors as I don't see any voltage divider biasing in the differential amplifier like this one.

    differential_amplify.gif

    hmmm I actually thought that the speaker rating of 8 ohm is actually the impedance of the speaker. Am I wrong to assume that?
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You are making a "heater" which operates in class-A and gets hot all the time. It draws a high current all the time.
    If its constant current source used a negative power supply voltage then the base of the upper transistor can be connected to ground through the music source and the transistor will be properly biased. Your circuit did not show an emitter-follower.

    Audio power amplifiers usually have a PNP and NPN pair of emitter-followers that operate in class-AB so they do not get hot and draw a very low current when not playing.

    An audio power amplifier has enough transistors so that it has high gain. Negative feedback is used to reduce the gain, reduce the distortion and provide an extremely low output impedance to damp resonances of a speaker.

    Here is a simple class-AB amplifier with a voltage gain of 5. Its input impedance is more than 22k ohms. Its output power at clipping into the 8 ohm speaker is 0.62W. Since it has only one transistor providing voltage gain then its distortion at full output (like shown) is pretty high.
     
  13. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    You need to understand that any transistor needs appropriate potentials at its electrodes in order for it to pass current. Your NPN amplifier transistor needs to have its base potential held positive with respect to its emitter, and the current sink transistor also needs a collector-emitter voltage. To be able to return the input transistor base to zero potential, you would therefore need to use some negative supply for the current sink.

    The differential amplifier you describe is designed to have its inputs returned to a suitable range of potentials, this is described as the common-mode input voltage range for an op-amp. Some op-amps require separate positive and negative supplies to achieve this.

    Since you say that you don't have a fancy power supply, you just need to use voltage dividers, like the rest of the world does. If the input impedance looks like coming out too low because of that, you could always add a buffer stage, probably an emitter follower.
     
Loading...