Basics of Common Emitter amplifier.

  1. Hello experts!

    I have some question related to this configuration of BJT.

    1)Why we use capacitor at the base of the circuit?(Answer: To decouple DC so that our signal that is reaching to base of the transistor is not to be DC shifted that's why it is called "Coupling capacitor". If it is not then we get our signal DC shifted from the ground level). And why we use capacitor at the emitter of the circuit i.e. CE?(Answer: Because we want signal to be ground easily. If capacitor is not placed there then we will have less gain.)

    2)2)Why we use resistors at collector and at emitter?(Answer: So that we could get stability if it is not here then we get distortion at the ouput signal).

    Are my three answers right?

    Thanks in advance.
  2. jcsd
  3. First two are OK (unlike your numbering)

    But think a bit harder about the collector and emitter resistors and ohm's law.
  4. Emitter resistor RE affects the input impedance and the gain by the same factor, in opposite magnitude, by [1+gmRE]. The smaller RE the larger the gain, but the lower the input impedance. Everything in the amplifier is a design tradeoff dependent on the functional circuit requirements.
  5. Usually the CE amplifier is arranged so that the steady voltage at the collector is roughly half the supply voltage Vc≈ Vs/2. The output of the amplifier is Vc and this means that Vc can 'swing' symetrically up to Vs and down to 0 (roughly).
    So if Vs is 10V you would want Vc to be about 5V.
    If you decided on a collector current Ic of 1mA then this means that Rc would be 5kΩ.
    The way to set Ic to 1mA is by means of the emitter resistance Re in conjunction with a bias voltage set at the base. If Vb is set to 2V then the voltage across Re will be (2-0.7) for a silicon transistor = 1.3V. To have an emitter current of 1mA therefore needs a resistor Re of 1.3kΩ.
    The voltage at the base of the transistor can be set by means of a potential divider across the power supply (10V)
    When a small AC signal voltage is applied to the base we want it to be applied between the base and the emitter and therefore an AC 'short circuit' is needed across Re.
    This is the purpose of the 'bypass' capacitor across Re. It is selected to have a low resistance (reactance) compared to Re at the frequencies to be amplified.
    eg if Re is 1.3kΩ and the lowest frequency to be amplified is 100Hz then we want 1/ωC to be less than (about 1/10) Re
    We want Xc to be about 130Ω at100Hz means C of about 12μF

    Hope this helps !!!!
  6. sophiecentaur

    sophiecentaur 14,324
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    You need the transistor to be conducting current all the time if your AC signal is to be undistorted on the way through. The input signal needs to vary the current in the output about a non-zero value. A good way to ensure this is to ensure that the DC conditions (bias) and the AC conditions (the signal) are set up separately - hence the use of Capacitors to couple the signal into the amplifier and the use of resistors to set the current.
  7. Thanks for all who replied.

    Coupling capacitor is to set the configuration for undistorted output. Is it? and if it is not there then what will we get at output?

    Similarly I have known by here that resistors at emitter, collector & base is to limit current. Is it. If they are not at their places then excessive amount of current can smoke my device.

    Are these answers right??
  8. sophiecentaur

    sophiecentaur 14,324
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    You presumably realise that there needs to be a standing current through the transistor and the AC signal consists of a variation around this mean value. If you don't have an appropriate value for the standing current then you can saturate the transistor or it may 'turn off' before achieving a full amplitude of AC output variation. The Collector Volts are determined by the current through the Collector load resistor.

    The resistor in the emitter does two things for you: it is basically a feedback mechanism because the emitter volts will 'follow' the base volts (current will flow out of the Emitter through the Emitter load until Vbe reaches about 0.7V). It ensures that the standing current is what you want and it also defines the voltage gain of the amplifier - which is given by -Rc/Re.
    Without an emitter resistor (Emitter just grounded), Vbe can be very high and damage the junction. Or, if there is a high resistor in series with the base to limit the base current, the transistor can still saturate and the linearity is poor. Feedback of some kind is essential for making a linear transistor amplifier and an emitter resistor is very suitable, although not the only way it can be achieved.
  9. But the purpose of coupling capacitor is to separate DC from the AC signal. If it is not there then I will get output shifted from reference and may be it gets saturate. Hence, therefore we put a capacitor between collector and load. Am I right.

    Now as you said that RE perform a job of a feedback. But how? I can't realize it. And two type of feedback are; degenerative feedback(+ve feedback) and regenerative feedback(-ve feedback). Does RE performs anyone of these feedback's job? If so then how?

    But it is the appreciable explanation. Thanks sophiecentaur.
  10. shayaan, we need to clear up a few things.

    First and most important:

    The input and output capacitors are there to block the DC, not to prevent 'signal shifting'.
    If the capacitors were not there you would simply get a loud bang and smoke when you connected things together. The signal would not be shifted it would disappear altogether.

    The capacitors allow an alternating signal to be transferred between two circuits or parts of a circuit that are operating at different direct voltages, without damage. Proper choice of capacitor values permits transfer without distortion - you can't just bung in any old capacitor you happen to have.

    The input and output capacitors are called coupling capacitors, (not decoupling capacitors).

    Now you have asked about the common emitter configuration and I suggested you consider the collector and emitter resistors in the light of Ohm's law.

    Did you do this?

    For a CE configuration the output is across the collector resistor.
    The transistor tries to drive beta times the base current through the collector resistor, regardless of the value of that resistor. It can do this so long as the circuit remains within parameters - we say in the active region.
    Ohm's law says that the greater the current through the collector resistor the greater the voltage across it and that voltage equals the current times the resistance.
    The greater the voltage the greater the voltage gain.
    Clearly if there is no collector resistor (ie it is zero) zero times the current equals zero, ie the output and gain are zero - not a useful situation.
    So we make the collector resistor as high as practicable for the circuit concerned.

    If you have followed this we can move on to deal with the emitter resistor and feedback.
  11. Lets do a design exercise about what we want !!
    1) specification : a voltage amplifier to produce a power output of 10mW from a 10V supply.(this is a low power amplifier.... it would drive headphones....just)
    2) voltage variation is from +5V to -5V therefore current variation is +/- 2mA
    3) this 2mA is to be drawn from a supply of some sort.... lets make certain it can supply 2mA......make it capable of supplying 10 x this ie 20mA
    4) If we decide to build a CE amplifier using one transistor then the output is from the collector and needs to vary by +/-5V
    5) The collector resistor is chosen so that Vc =5V (0.5Vs) So 5V appears across Rc.
    We want Ic to be 20mA (this is where the power current is drawn from)
    This gives a value of Rc = 5/20mA = 250Ω
    6) How to get Ic =20mA?... well Ic is more or less equal; to Ie (the emitter current) and the voltage across any emitter resistance is more or less = voltage at the base -0.7 (for a silicon transistor). If we make the base voltage 1.7V (using a potential divider) then V across Re = 1V
    we want the current to be 20mA therefore Re = 1V/20mA =50Ω
    This combination of Rc and Re will give a single transistor amplifier capable of delivering 10mW. There is an endless combination of resistors for the base bias combinaton.
    If we want this amplifier to amplify audio signals with a lowest frequency of say 100Hz then a bypass capacitor across Re is required. The reactance (Xc = 1/ωC) of C must be less (X10) than Re so that it behaves as a short circuit at this frequency. So Xc must be less than 50Ω at a frequency of 100Hz gives a value for C as greater than 30μF
    The coupling capacitors are another matter but as long as they are there they will do the job of isolating the DC conditions in each stage of the amplifier. Basically coupling capacitors need to have as low a resistance (reactance) as possible for the frequencies to be amplified..... there is a lot of lee way in this.
  12. sophiecentaur

    sophiecentaur 14,324
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    Some of that is a bit oversimplified. There are many DC coupled amplifiers; a capacitor is only necessary when the input DC conditions are not suitable. AS we are not referring to any particular amplifier then you cannot assert "a loud bang" would occur. With large enough coupling resistors there will be no excessive base current; look at the basic RTL logic gate configuration, for instance. (Of course, a logic gate is not a good linear amplifier but that is another issue).

    A few issues here. You seem to have the feedback terms the wrong way round. I would have used the word regenerative feedback for positive feedback and vice versa.
    Coupling Capacitors are only appropriate when you do not need to have a DC response. Many laboratory amplifiers need to have a flat response right down to DC. (Take the amplifiers in an oscilloscope for instance). It is just more convenient to use AC coupling, sometimes so you can treat the DC and AC conditions separately..
    I said that Re provides negative feedback and described what happens: current will flow through Re until the Vbe stabilises at 0.7V. This feedback keeps Ve 'following' Vb-0.7V. If Ie goes too high or too low (by a small amount), then Vbe changes by a little and causes the value of Ie to change, returning Ve to Vb-0.7V again.
    So the volts across Re determine the current through Re, which is the same (plus a minute amount of base current) as Ic. The volts across Rc are then equal to IcRL which is why I said the voltage gain is equal to the ratio of RL/Re (This voltage comprises the DC value due to the base bias and the AC variation of the base current) . Without the emitter resistor, the voltage gain would be much higher because the collector current would be Beta times the base current - but the output would be distorted.
  13. I have never had any 'loud bangs' or 'smoke',
    I think that I must have done something right
  14. sophiecentaur

    sophiecentaur 14,324
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    HAHA - just keep all your resistors high enough and the transistor will survive anything!

    Your arm waving / verbal amplifier design seems just fine to me. You have the same attitude to this as I do - start with as many simple assumptions as possible and design around them first. If you need to tidy things up then do it later.

  15. Consider two CE connected transistors, both with a beta of 100 and a base current of 100μA.

    Transistor 1 has a collector resistor of 1k and an emitter resistor of 100.

    Transistor 2 has a collector resistor of 1k, but no emitter resistor.

    What are the collector and emitter currents for each transistor? (assuming an adequate voltage rail in each case).
  16. sophiecentaur

    sophiecentaur 14,324
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    Gold Member

    How would you ensure that (and is it a relevant question at this stage)? With a constant current bias circuit? The input resistance with 100Ω would be something like 10kΩ so the base current, through whatever bias resistor was used, would be reduced accordingly. Did you not know that or were you just being 'controversial'? There's always another factor that can be introduced into a crude, practical circuit that has not actually been drawn out in full or treated rigorously. I should say that your question is more about the curves you find in a Transistor Spec Sheet than about your average circuit.
  17. at sudiot

    As you said capacitor at input and output is to block DC. I know the purpose of capacitor is to block DC and allows AC to pass.
    So now let an AC signal [itex]\pm5V[/itex] with 1V of DC is at collector. Now if capacitor is not present here then obviously it is DC shifted because AC signal [itex]\pm5V[/itex] has ground level but now without capacitor DC also adds to AC and output becomes +6V and -4V at output. Is not it?

    And yes I have considered that in the light of Ohm's law and reach at the same place i.e. same to your answer.

    I have just one question that I asked above after that we will move to emitter resistor and feedback.

    That was really a good reply.
  18. at technician

    Yes my original purpose is to design a CE amplifier.
    But someone said me that before designing it you should know and understand that how every component works. And if any component is not at the place where it should be then what will be the output under these type of conditions.
  19. at sophiecentaur

    OK. I got something i.e. RE provides negative feedback.

    Now let me think how then I will discuss it more with you here.

    Thanks for hint.
  20. sophiecentaur

    sophiecentaur 14,324
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    Gold Member

    You seem to have ignored any voltage gain that will be there if Rc is greater than Re.

    It really is time to have a diagram that we can discuss. I am not sure where your voltages are being measured. How can you get a -4V at the output if you are operating between 0V and some Vcc that is not specified?
    I have a feeling that you are making all this up on your own instead of getting the information from a book. Why not look at some practical amplifier circuits and descriptions and try to figure out how they are operating? I googled "simple transistor circuit" and found enough pictures to wallpaper my living room.
  21. Hello shayan, I'm glad you are finding my input useful.

    I wholeheartedly endorse this.

    However when you first meet transistor circuit theory it can be very confusing. The purpose of each component can be hard to see since they all interact.

    That is why I like to start with something simple. To see what a couple of components are doing and then see why we might need to add more. Finally to build up to a useful circuit.

    IMHO what SC and T have offered are to complicated to start with and beyond your original questions.

    If you would like to follow my trail I will draw some diagrams, starting from the simple and moving on to the complicated, giving reasoning.
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