push-pull amplifier

Discussion in 'General Electronics Chat' started by mik3ca, Feb 5, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I have seen many schematics on how to create a power amplifier by combining a push-pull amplifier and a common-emitter amplifier together.

    such a schematic is shown:

    [​IMG]

    I was wondering how I can modify this so that the first NPN transistor is a common-collector amplifier, or do I have to add an additional transistor?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why do you want to do that?
     
  3. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    because I found on http://www.eix.co.uk/Articles/Radio/Welcome.htm a way that I can improve my receiver, and it suggested that I should use what appears to be a common-collector amplifier instead of the common-emitter one that I already have.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The circuit you show has a common-emitter transistor at its input. When it is driven from a fairly low impedance then it has plenty of voltage gain.

    A common-collector transistor is an emitter-follower that has no voltage gain.

    The circuit already has two common-collector transistors (emitter-followers) at its output.
     
  5. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I know what my shown circuit is.

    The one you want to see is fig 3c of:

    http://www.eix.co.uk/Articles/Radio/image032.gif

    then I can somehow embed that circuit into another circuit so that I can make a power amplifier similar to the first circuit. that I showed in the thread.

    why embed that circuit? because I read that the circuit of fig 3c allows for more sensitivity which is what I want. Currently, my circuit is using almost the same circuit as figure 3B.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The article shows a super-regenerative receiver transistor similar to the other circuit by Andersen on the web. They have the tank at the collector for high gain. The emitter of the RF transistor is made into a quenching oscillator for automatic sensitivity adjustment.

    Your receiver transistor doesn't doesn't have the tank at the collector where it must be connected for high gain. Your receiver transistor also doesn't have the quenching oscillator.

    The article shows in figure 3b an NPN common emitter transistor that has a fairly low input impedance that must quickly charge the C5 coupling capacitor when the RF begins to oscillate but before the quenching oscillator stops the RF oscillation.
    Figure 3c also shows a common-emitter transistor (not common-collector) but it is a PNP so C5 doesn't need to charge quickly, it cuts off the PNP transistor (driven out of conduction) when the RF begins to oscillate. Then the PNP transistor is turned on slowly when the quenching oscillator stops the RF oscillation.

    Replace the NPN common-emitter transistor in your push-pull amplifier with a PNP transistor and swap where it connects to the output transistors.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    How much sensitivity do you need?

    How sensitive is your circuit in the first post?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The radio is a super-regen. Its sensitivity was ruined by the circuit changes that were made to it.
    Since it has only a single transistor it doesn't have the very high gain of a real multi-stage FM radio and it doesn't have automatic-gain-control. It doesn't even have an FM detector since it is an AM radio.

    Judging by the very high gain of Andersen's 6 transistors audio amplifier it is not sensitive.
     
  9. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I'm in Hamilton and I am trying to connect to Z103.5 in Orangeville. Both places are in Ontario.

    after I added that transistor in the NPN configuration (before I read the replies), and tied a 220uF capacitor across the feedback resistor, I was able to pick up Z103.5 some more.

    I never thought about trying it with a PNP transistor.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    A capacitor across the feedback resistor of the transistor reduces its gain to zero.

    The PNP transistor audio amplifier is designed for connection to the emitter of a super-regen receiver. Your circuit has the audio output at the collector of a regen receiver.
     
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