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    20IM10045 - Sahil Agrawal - Exp 4

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    Sahil Agrawal
    1) Sahil Agrawal aims to investigate the operation of a common-emitter NPN transistor amplifier using various resistors, capacitors, and a BJT transistor. 2) The common emitter configuration provides both voltage and current amplification and operates the transistor in its active region through biasing. 3) Frequency response curves show voltage gain drops off at low and high frequencies but remains constant between the low and high cut-off frequencies.

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    20IM10045 - Sahil Agrawal - Exp 4

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    Sahil Agrawal
    18 views13 pages
    1) Sahil Agrawal aims to investigate the operation of a common-emitter NPN transistor amplifier using various resistors, capacitors, and a BJT transistor. 2) The common emitter configuration provides both voltage and current amplification and operates the transistor in its active region through biasing. 3) Frequency response curves show voltage gain drops off at low and high frequencies but remains constant between the low and high cut-off frequencies.

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    20IM10045_Sahil Agrawal_Exp 4

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    1) Sahil Agrawal aims to investigate the operation of a common-emitter NPN transistor amplifier using various resistors, capacitors, and a BJT transistor. 2) The common emitter configuration provides both voltage and current amplification and operates the transistor in its active region through biasing. 3) Frequency response curves show voltage gain drops off at low and high frequencies but remains constant between the low and high cut-off frequencies.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    18 views13 pages

    20IM10045 - Sahil Agrawal - Exp 4

    Uploaded by

    Sahil Agrawal
    1) Sahil Agrawal aims to investigate the operation of a common-emitter NPN transistor amplifier using various resistors, capacitors, and a BJT transistor. 2) The common emitter configuration provides both voltage and current amplification and operates the transistor in its active region through biasing. 3) Frequency response curves show voltage gain drops off at low and high frequencies but remains constant between the low and high cut-off frequencies.

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    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    Name: Sahil Agrawal

    Roll Number: 20IM10045

    Aim of the Experiment

    To investigate the operation of a common-emitter NPN transistor amplifier.

    Tools Used

    S. No. Apparatus Range Type Quantity

    1 Resistor - 1
    i) 100Ω -1000Ω
    2
    ii)100Ω - 4000Ω
    iii)100Ω-47kΩ 1
    iv)100Ω-15kΩ 1
    2 Voltage Source 50Hz-50MHz i)AC 1
    ii)DC 1
    3 Capacitor 1μF-10μF - 3

    4 BJT - NPN 1

    Background Knowledge-

    The common emitter configuration is widely used as a basic amplifier as it has both voltage and
    current amplification.
    Resistors RB1 and RB2 form a voltage divider across the base of the transistor. The function of
    this network is to provide necessary bias conditions and ensure that the emitter-base junction is
    operating in the proper region.
    In order to operate a transistor as an amplifier, biasing is done in such a way that the operating
    point is in the active region. For an amplifier the Q-point is placed so that the load line is bisected.
    Therefore, in practical design VCE is always set to VCC/2. This will confirm that the Q-point
    always swings within the active region. This limitation can be explained by maximum signal
    handling capacity. For the maximum input signal, output is produced without any distortion and
    clipping.

    Input Capacitor (C1)


    The capacitor C1 is used to couple the signal to the base terminal of the BJT. If it is not there, the
    signal source resistance, Rs will come across R2 and hence, it will change the bias. C1 allows
    only the AC signal to flow but isolates the signal source from R2

    Emitter Bypass Capacitor (CE)


    An Emitter bypass capacitor CE is used parallel with RE to provide a low reactance path to the
    amplified AC signal. If it is not used, then the amplified AC signal following through RE will cause a
    voltage drop across it, thereby dropping the output voltage.

    Coupling Capacitor (C2)


    The coupling capacitor C2 couples one stage of amplification to the next stage. This technique
    used to isolate the DC bias settings of the two coupled circuits.

    CE Amplifier Frequency Response


    The voltage gain of a CE amplifier varies with signal frequency. It is because reactances of the
    capacitors in the circuit changes with signal frequency and hence affects the output voltage. The
    curve drawn between voltage gain and the signal frequency of an amplifier is known as frequency
    response. Below figure shows the frequency response of a typical CE amplifier.

    Frequency Response of Common Emitter Amplifier

    The midband gain is obtained by short circuiting all the external capacitors and open circuiting
    the internal capacitors. Figure 2 shows the equivalent for calculating the midband gain.
    The high frequency equivalent circuit of the common-emitter amplifier is shown in Figure 3.

    Circuit Diagrams:
    Serial Frequency(Hz) Magnitude(dB)
    No.

    1 80 4.77432

    2 96 6.43712

    3 183 12.1336

    4 318 16.92652

    5 727 23.917

    6 1386 28.8692

    7 2197 31.7644

    8 4186 34.393

    9 8746 35.606

    10 20036 35.9562

    11 26413 35.9928

    12 38179 36.0186

    13 79766 36.0352

    14 105153 36.0358

    15 138618 36.034

    16 200364 36.0272

    17 264131 36.0166

    18 381786 35.9884

    19 727478 35.8484

    20 1051530 35.646

    21 2003640 34.7474

    22 4590080 31.5338

    23 9590030 26.5178
    24 45900800 13.4081

    Source Resistance (RS): 100 Ω


    Collector Resistance (RC): 500Ω
    Emitter Resistance (RE): 1000 Ω
    Load Resistance (RL): 2000 Ω
    Base Resistance (RB1): 5000 Ω
    Base Resistance (RB2): 5000 Ω
    Coupling Capacitor (CC1): 10 μF
    Coupling Capacitor (CC2): 10 μF
    Bypass Capacitance (CE): 10 μF

    Graph:

    Midband gain = -63.41 Hz


    Low frequency cut-off = 17882.65323 Hz
    High frequency cut-off= 2.13515e+7 Hz
    1) Frequency Response

    a) Load Resistance is Connected

    Circuit Diagram of CE Amplifier for Frequency Response

    CE Frequency Response

    Low frequency cut-off = 64.874Hz


    High frequency cut-off= 10.841 MHz
    b) Load Resistance is not connected

    Circuit Diagram of CE Amplifier for Frequency Response

    CE Frequency Response

    Low frequency cut-off = 90.331Hz


    High frequency cut-off= 11.415 MHz
    2) Signal Handling Capacity

    a) Load Resistance is connected

    CE Amplifier Circuit for Signal Handling Capacity

    CE Amplifier V(out)/V(in) in Time Domain

    Peak Voltage = 17.5 mV


    Vin = 1mV
    Gain =V(out)/V(in) = 17.5/1 = 17.5
    Vsm = 300 mV
    b) Load Resistance isn’t connected

    CE Amplifier Circuit for Signal Handling Capacity

    CE Amplifier V(out)/V(in) in Time Domain

    Peak Voltage = 18.5 mV


    Vin = 1mV
    Gain = V(out)/V(in) = 18.5/1 = 18.5
    Vsm = 300 mV
    3) DC Analysis

    Circuit Diagram for DC Analysis

    Model of BJT being used - 2N3904

    V(b)=1.08202 V
    V(c) = 7.49125 V
    V(e) = 0.438987 V

    Ic = 0.663051mA
    Ie = 0.665131 mA
    Calculations:
    Discussions:
    At low frequencies (< FL) the reactance of coupling capacitor C2 is relatively high and
    hence very small part of the signal will pass from amplifier stage to the load.
    Moreover, CE cannot shunt the RE effectively because of its large reactance at low
    frequencies. These two factors cause a drop off of voltage gain at low frequencies.
    At high frequencies (> FH) the reactance of coupling capacitor C2 is very small and it
    behaves as a short circuit. This increases the loading effect of the amplifier stage and
    serves to reduce the voltage gain.
    Moreover, at high frequencies, the capacitive reactance of base-emitter junction is low
    which increases the base current. This frequency reduces the current amplification
    factor β. Due to these two reasons, the voltage gain drops off at high frequency.
    At mid frequencies (FL to FH) the voltage gain of the amplifier is constant. The effect of
    the coupling capacitor C2 in this frequency range is such as to maintain a constant
    voltage gain. Thus, as the frequency increases in this range, the reactance of CC
    decreases, which tends to increase the gain. However, we observe that the voltage gain
    drops off at low (< FL) and high (> FH) frequencies, whereas it is constant over the mid-
    frequency range (FL to FH).

    Conclusion:
    Conclusion from Above Curves :
    a) Frequency Response : The voltage gain drops off at low (< FL) and high
    (> FH) frequencies, whereas it is constant over the mid-frequency range
    (FL to FH). Also the gain in case of no load connected is more compared
    to when load is connected.
    b) Signal Handling Capacity : The output voltage is higher than the input
    voltage whether load is connected or not, thus input is amplified in
    both the cases. However on connecting the load resistance, the peak
    voltage decreases. We also observe a shift of 1800
    c) DC Analysis : We see the stabilization of the Q-point.Vcc is greater
    than Vce, thus the amplifier is biased in the active region.
    In Common Emitter Amplifier, Input is applied to B-E Junction and Output is taken from E-C
    Terminal, here the emitter terminal is common for both input and output.
    It is a widely used amplifier circuit because it provides good current gain and good voltage
    gain and it is also known as an inverting amplifier because it gives 180° phase shift from
    input to output. It is widely used in audio amplification and signal amplification circuits. The
    current gain and voltage are moderate. However, in the CE amplifier, there is high thermal
    instability.

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