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    What is the pupose of diode in the diagram??

    I was wondering what is the use of adding 1N4148 diode in the circuit.Cant we directly connect the emitter to ground. This kind design has been used in many circuits.



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    Re: What is the pupose of diode in the diagram??

    Perhaps the author likes to start from 1.8V than 1.2V.
    One may think the diode is added as a protection from a possible reverse polarity since the Veb (from E to B) breakdown is usually low. In this case it is better to use a diode with a higher breakdown voltage than 100V.

    On the other hand, it is better to add a resistor at the entry base as 1K or 2K2 for example.

    Kerim

    Edited:
    Since the 4 limiting current resistors have the same value, one may notice a slight brightness differences. LED_1 has the highest brigthness, LED_4 the lowest due to the higher voltage at the collector of its transistor.
    Last edited by KerimF; 27th July 2011 at 08:54.



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    Re: What is the pupose of diode in the diagram??

    It should work with it shorted out. The only reason I can see for it is to give it an extra 0.6V start threshold. Whether this is to overcome the effects of leakage in the transistors or to linearize the steps is uncertain.

    Brian.



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    Re: What is the pupose of diode in the diagram??

    What is the purpose( application) of adding diode D1 in the figure. IS it to protect from noise




    ---------- Post added at 10:00 ---------- Previous post was at 09:59 ----------

    What is the purpose( application) of adding diode D1 in the figure. IS it to protect from noise





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    Re: What is the pupose of diode in the diagram??

    Here the diode is to block a possible high reverse voltage to protect the B-E junction of the npn transistor. As you see the input signal is AC. While its negative peak voltage likely doesn't exceed the breakdown voltage of the base-emitter junction, it is good to assume it may happen :)

    Of course we can't be sure if it has the second function since we have no idea about the input signal (like a math problem misses some variables). So in case the signal voltage is relatively high, the amplifier will run in class C and the clipping voltage at the first transistor base will be higher (the sum of forward voltages; B-E and the diode). I think this doesn't apply here, the amplifier looks as it runs in class A (linear).
    Last edited by KerimF; 27th July 2011 at 13:17.



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    Re: What is the pupose of diode in the diagram??

    I have read a patent that tells adding diode in current branch to improve EMI.



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    Re: What is the pupose of diode in the diagram??

    Sorry iDidnt upload the circuit This query is with ref to circuit given


    Now if we have a circuit anlaysis as shown D1 is used .Do we need to add this diode in circuit if yes why.

    Last edited by angy; 28th July 2011 at 12:41.



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    Re: What is the pupose of diode in the diagram??

    Yes, one needs to add it in the real circuit, unless he likes to redesign the amplifier since removing D1 will change the bias point of the first transistor.

    Kerim

    Added:
    Do you know what is expected for the circuit to do?... for which sort of input signal?... for which sort of load at its ouput?
    Last edited by KerimF; 28th July 2011 at 12:27.



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    Re: What is the pupose of diode in the diagram??

    Sorry i Didnt upload the circuit This query is with ref to circuit given



    Now if we have a circuit anlaysis as shown D1 is used .Do we need to add this diode in circuit if yes why.



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    Re: What is the pupose of diode in the diagram??

    The circuit operats on 1.5 volts. Biasing shows it is switching application. Diode raises Ve to 1.2 volts wile Vb is less than .5 volts. So .7 volts for noise margin is available.
    Sorry i Didnt upload the circuit This query is with ref to circuit given

    Student may have to more carefull while calculating current through biasing resistor with 1.2 volts on base.
    Last edited by ALERTLINKS; 28th July 2011 at 13:08.



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    Re: What is the pupose of diode in the diagram??

    Hi again,

    I don't think I will use 1N1183 (rated for 35A) in such a circuit

    But a diode here is good for protection since it is possible for the 27V to be applied by mistake with a reversed polarity.
    The B-E junction cannot block more than about 5 volts while the diode can.

    On the other hand, I don't see anything on the circuit which helps the coil current to be discharged (when the transistor is off) to avoid the temporary voltage breakdown at the transistor's collector.

    Kerim


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