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question about transistor design

Discussion in 'General Electronics Discussion' started by samy555, Oct 20, 2012.

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  1. samy555


    May 11, 2010
    I often see many electronic circuits using a simple fixed biased common emmiter transistor like Q2 in the following
    and transistor T3 here:
    Why those designers didn't use a more strong configurations like voltage divider or collector feedback?

    Thank you very much
  2. duke37


    Jan 9, 2011
    The simple bias uses fewer components and has the advantage that there is no negative feedback. It needs to be set up accurately in the audio circuit, taking into account the gain of the transistor.

    In the transmitter circuit, the smallest feedback is best and the highest supply voltage is needed for efficiency.

    Sometimes the collector - base resistor is split and the junction bybassed to reduce feedback and so increase gain.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The other thing you may see is a voltage divider biased transistor with collector and emitter resistors. This has a stable DC operating point and a fairly easily predicted (but very low) gain. Then you see the emitter resistor bypassed bay a capacitor which increases the AC gain almost to the gain of the transistor at the bias point without upsetting the DC characteristics.
  4. Zork


    Oct 26, 2012
    On the second circuit, for the T3, the transistor needs a small dc current on base, this is an class C amplifier, that has much less than 180 degree conduction periods.

    This circuit isn't linear like the other on the first picture, the only advantage is to have a greater efficiency than the class A amplifier (T1), the output of this circuit has a high harmonic distortion, usually on commercial projects this output is filtered by a low pass filter.

    As said above, to put two resistors on the base terminal causes to lower the ac resistance and lowers the gain too, one advantage of doing this is to cause a lower dependancy of the bias by the temperature (Vbe decreases with a higher temperature), causing a more stable bias, this is one great concern on RF small signal amplifiers and oscilator circuits.
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