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Emitter and Collector Resistance in Differential Amplifier.

  1. Nov 15, 2011 #1
    Hello experts!

    Image of Differential Amplifier is given below.

    Here in diff amp both transistor have same characteristics. Even +VCC and -VEE are of same magnitude. Everything is same. Hence, their RE and RC are also same.

    1)If both emitter and collector resistance of both transistor are same then why only RE is combined why not RC?
    RC for both are shown individually at both collectors even both collectors have same magnitude of resistance. Why?

    2)And both supply voltages VEE and VCC are same. Is it correct?

    Thanks in advance.
     

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  3. Nov 15, 2011 #2

    sophiecentaur

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    If the collector resistors were common then how would you get a difference signal? The circuit is a differential amplifier because there is a difference in the share of the current (through Re) through the two collector resistors - hence a difference in collector voltages. With your suggestion, the difference in Vc would be zero at all times (connected together).
     
  4. Nov 15, 2011 #3
    But if collector resistance is combined then we can still get differentiated output across collectors of transistors. Let 2Ic is flowing through collector resistance(which is combined) so at output terminals we get IC(half of 2IC). Isn't like that?
     
  5. Nov 15, 2011 #4

    sophiecentaur

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    How, if the two collectors are connected together? The voltage on the collector will be proportional to the SUM of the currents from the two transistors. That would not be a difference, would it?

    In situations like this, it is best to look for reasons that you could be wrong not right. Haha.
     
  6. Nov 16, 2011 #5
    No no. I am still away from my answer. Kindly help me some more.


    Here is my schemetic, this defines what I am trying to say.

    Here if I get collector resistor combined then still I can get output across collectors of transistor. Is this not correct voltage that I would get??? If no then why?

    Thanks for your answer.
     
  7. Nov 16, 2011 #6

    sophiecentaur

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    No schematic on your last post?

    The obvious logical argument against your idea is that you would need to tell me which signal would appear at the collectors - (A-B) or (B-A)? If the two outputs were connected together then there would be no way of knowing. The current through the collectors would be the sum of the currents through the two transistors but pretty much dominated by the current from the transistor with the higher Vbe, whichever it happened to be.
    Do you actually understand how the original circuit functions? It would be a good idea to try to find out before trying to modify it. The difference between the collector voltages is proportional to the difference between inputs. A much better version of a differential amp (an added stage of complexity) would have a current source instead of the shared Emitter resistor. Then, this current would be shared between the two collector loads according to the difference in inputs, whatever their mean value was. This is called common mode rejection and good Op Amp circuits will be very good in this respect.

    I have just thought what your problem could be with this circuit. Any voltage amplifier gives an output signal that is proportional* to the voltage difference between the input pin and some reference (ground etc.). You could either change the reference or the input voltage and still get an amplified difference signal but the input impedance of the (actual) input pin would be very different from the input from the input impedance which you would see if you tried to change the reference volts (i.e. use the ground as 'another input'). What is 'special' about a proper differential amp circuit is that it tends to eliminate any asymmetry of behaviour and both inputs have the same characteristics. This is the 'common mode rejection' I referred to.

    [Edit - I really mean related to rather than strictly proportional - input voltages 'below' the reference may give 'silly results, for instance.]
     
    Last edited: Nov 16, 2011
  8. Nov 17, 2011 #7
    Thanks for description. Thanks once again. I have understood just little bit.

    Here is my schematic for last post.

    Now,
    Our goal in differential amplifier is to find differentiated voltage across collector. However if the collector resistor is not combined still we get differentiated voltage across collectors. Agree?
    If collector resistors is combined still we get differentiated result across collectors. Agree?

    Let us apply 2volt peak to peak signal at input. Then what result would we get in both cases(defined above)?

    Thank you very much.
     

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    Last edited: Nov 17, 2011
  9. Nov 17, 2011 #8

    sophiecentaur

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    "If collector resistors is combined still we get differentiated result across collectors. Agree?"

    I most certainly do not agree. Vout would be ZERO!!!!! because the two collectors are connected together by a short circuit. The two collector currents may be different but your arrangement will not reveal it because the Voltage across R1 will be R1(Ic1 + Ic2).
    Like I said much earlier, you need to approach this from the standpoint that you must be wrong about this - or else all diff-amps would not be made the way they are! You cannot believe that all circuit designers before you have been getting things wrong, surely.

    You seem to have missed the point about what a differential amplifier is supposed to do. Just think about this. If Vin1 is one volt above Vin2, your circuit will produce exactly the same collector volts as if Vin2 is one volt above Vin1. A proper differential amplifier would produce different values of Vc1 and Vc2, and their difference would be proportional to the difference between Vin1 and Vin2. Your circuit can produce no difference signal.
     
  10. Nov 17, 2011 #9
    OK. Thanks a lot. I am very great full to you for this. You alone helped me so so much. Thanks once again.
     
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