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

Objectives:

• To verify the operating point for a Beta Stabilized Biasing Network (Bias Point)

• To verify the operation of BJT inverter

• To introduce BJT as an amplifier using CEC, CCC and CBC.

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).

Hint: See the handout - DC Bias for assistance.

 

Assemble this circuit on ORCAD and perform a bias point detail analysis. Show the calculated voltages and currents by appropriately placing IPROBEs and VIEWPOINTs on your schematic.

 

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

2.- (HW) Inverter Simulation
 
 

Use SPICE to simulate the BJT inverter shown in Figure # 2. Plot the value of the output voltage Vo vs. the input voltage Vi. Label this plot as "Plot #1 - ORCAD Inverter Simulation". V1 must remain at 5 VDC while V2 varies from 0 to 5 Volts. 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. Build and fully test the circuit shown in Figure #2. Measure Vo for Vi varying from 0 to 3 Volts with increments of 0.2 Volts. Put the measurement of Vi and Vo in Data Table #2- Inverter Measurement.  Plot the graph of Vo vs Vi. Label this plot as "Plot #2 - BJT as an Inverter"

4. - Introduction to BJT as an Amplifier

One of the famous BJT application is as an amplifier.  There are three basic configurations and each configuration has a specific application to electrical circuit.  The configurations are:
1) Common Emitter
2) Common Collector
3) Common Base
 

a)  (HW) 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.
       Check the textbook : Chapter 4 under BJT as an amplifier for further assistance.
 
 
 

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

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.
4.  Use Vpp option in the oscilloscope to measure the Voutput and Vinput.
 
 
 

4.- Conclusion

1.  BJT has been shown to operate as an inverter.  Can BJT with a dc power supply (VCC) be used to control appliances that has ac power supply?  Elaborate.

2.  BJT must stay at one operating point (DC Bias) to operate as an amplifier.  Why?