The George Washington University
School of Engineering and Applied Science
Department of Electrical and Computer Engineering
ECE 20 - Fall 2002
Experiment # 5

Bipolar Junction Transistors:
DC Bias, BJT as Inverter and BJT as Amplifiers








References:

DC Bias(Ref: Sedra & Smith Chp. 4.6)
BJT as Inverter (Ref: Sedra & Smith Chp. 4.14)
BJT as Amplifiers (Ref: Sedra & Smith Chp. 4.7 & 4.11)
 

Equipment:
You must make up a complete equipment list and have your instructor review it before you start.
 

Objectives:


 


Figure # 1












1.- (HW) DC Biasing Analysis

  • Using simple Kirchoff's Laws and Ohm's Law, analyze the circuit shown in Figure # 1 and find VE, VC, VB ,VBE, VCE, , IB, IE, IC, RB, (assume VCC = 30 VDC, b = 180, VT = 26 mV).

    • Label this analysis - "Analysis #1 - DC Bias Calculation"
     
     
  • Assemble this circuit on ORCAD and perform a bias point detail analysis. Use NS3904 for the BJT.  Show the calculated voltages and currents by appropriately placing IPROBEs and VIEWPOINTs on your schematic.  Label the circuit as "Circuit #1 - DC Bias Simulation"

    •  
  • Compare the DC Voltages and DC currents obtained from your hand calculation and ORCAD.  Why there are some differences?

    •  
     

    2.- (HW) Inverter Simulation
     
     


    Figure # 2

    Use SPICE to simulate the BJT inverter shown in Figure # 2. Use NS3904 for BJT.  Plot the value of the output voltage VC vs. the input voltage V2. Label this plot as "Plot #1 -Inverter Simulation". V1 is 5 VDC.  Use "DC Sweep" to set  V2  from 0 to 5 Volts with an increment of 0.5 VDC.  Indicate in the plot when the BJT is in the saturation, linear, or cut-off region.
     
     
     

    3.- Verification for DC Biasing and Inverter

    1. Build and test the circuit shown in Figure #1. Measure VE, VB, VC, VBE, IB, IE, and IC.  Put the measurement in Data Table #1 - DC Bias Measurement.

      2.  
    2. Build and fully test the circuit shown in Figure #2. Measure VC for V2 varying from 0 to 3 Volts with increments of 0.2 Volts. Put the measurement of V2 and VC in "Data Table #2- Inverter Measurement".  Plot the graph of VC vs V2.  Label this plot as "Plot #2 - BJT as an Inverter"

     
     

    4. -  (H/W)  Analysis of BJT as an Amplifier

    In analog circuit, BJT is used primarily in the amplification circuit.  There are three basic amplifier configurations and each configuration has a specific application to electrical circuit.  The configurations are:
    1) Common Emitter
    2) Common Collector
    3) Common Base

    Further information can be read from the textbook in Chapter 4.11.
    Using circuit of Figure #1, derive a formula and find the values for:
         i) Voltage Gain of Common Emitter
         ii) Voltage Gain of Common Collector
         iii) Voltage Gain of Common Base

         Hint:  To derive the voltage gain for each configuration, you need to follow these steps:
                   1) Do a DC Bias Analysis (which you already did in Part 1)
                   2) Draw a small signal analysis corresponding to each configuration.  For example small signal analysis for
                       CEC is different from small signal analysis for CCC.
                   3) Find the expression for Voutput and Vinput.
                     4) Voltage Gain = Voutput / Vinput.
     
     
     

    5. -  Verification of BJT as an Amplifiers

    b)  By applying a sinusoidal signal (small signal) as the input, measure:

        1. Voltage gain Avo (unloaded) for the circuit in Figure # 1 when operating in CEC.  Label the output plot as "Plot A - CEC Voltage Gain (Unloaded)"

          2.  
        2. Voltage gain Avo (unloaded) for the circuit in Figure # 1 when operating in CBC.  Label the output plot as "Plot B - CCC Voltage Gain (Unloaded)"

          3.  
        3. Voltage gain Avo (unloaded) for the circuit in Figure # 1 when operating in CCC.  Label the output plot as "Plot C - CBC Voltage Gain (Unloaded)"
    Important Hints:
    There are several steps that you must do to ensure proper small signal measurement:

    1.  Connect a large capacitor between VCC and ground in order to remove all the noise from the source.
    2.  Use a really small sinusoidal signal as your input.  Vinput < 100mVp.
    3.  Check your DC Bias Voltages and Currents before measuring the small signal.  Make sure that the BJT  is operating in the correct Bias Point (DC) before doing any small signal measurement.  For example, VBE must be at least 0.6 VDC.
    4.  Use Vpp option in the oscilloscope to measure the Voutput and Vinput.
     
     
     

    6.- Conclusion

    Write a clear conclusion establishing the most important facts about the operation of the BJT inverter, the beta stabilized biasing network and particularly its operation in the different configurations.