EXPERIMENT No.8

EXPERIMENT No. 8

Summary: Vary the base resistor and measure changes in voltage and current for Vce, Vbe, Ic, and Ib. Then plot a load line.

Set up the following circuit on a bread board. Use a 470R for Rc and a BC547 NPN transistor.

Pick five resistors between 2K2 and 1M for Rb. You want a range of resistors that allow you to see Vce when the transistor is the saturated switch region and when it is in the active amplifier region. I used 47K, 220K, 270K, 330K and 1M, but this can vary depending on your transistor. Some may need to use 2K2. Put one resistor in place, and measure and record voltage drop across Vce and Vbe. Also measure and record the current for Ic and Ib. Then change the Rb resistor and do all the measurements and record the new readings. Do this for each of the resistor values above.

  Record here:

            Rb 47K            Vbe 0.69V       Vce 0.08V       Ib 0.02mA        Ic 6.41mA

Rb 220K          Vbe: 0.67V      Vce: 0.15V      Ib: 0.03mA       Ic: 6.24mA

Rb 270K          Vbe: 0.69V      Vce: 0.20V      Ib: 0.03mA       Ic: 6.21mA

Rb 330K          Vbe: 0.67V      Vce: 0.30V      Ib: 0.03mA       Ic: 6.01mA

Rb _____         Vbe: _____     Vce: _____     Ib: _____         Ic: _____

Your voltage drop measurements across Vce should vary from below 0.3 v (showing the transistor is in the saturated switch region) to above 2.0 v (showing the transistor is in the active amplifier region) If this is not the case, you may have to try a smaller or bigger resistor at Rb. Talk to your teacher to get a different size resistor, and redo your measurements.

 Discuss what happened for Vce during this experiment. What change took place, and what caused the change? 


During this experiment the different resistors caused different voltage drop readings at collector to emitter (Vce). The base resistors take up most of the voltage, which is 4.3 volts and the supply voltage is 5 volts. So the base resistors (Rb) take up 4.3 volts, so the voltage are low at 'Vce'...

Discuss what happened for Vbe during this experiment. What change took place if any, and what caused the change?

During this experiment the voltage at base to emitter (Vbe) got around .7 of a volt like a diode in its forward bias of 0.7 volt drop across it. So the resistors took up 4.3 volts and 0.7 volt remaining is used to push current from the base to emitter. So the resistors voltage and the voltage - base to emitter added up to the supply voltage which was 5 volts.

Discuss what happened for Ib during this experiment. What change took place, and what caused the change?

 Discuss what happened for Ic during this experiment. What change took place, and what caused the change? 

Plot the points for Ic and Vce on the graph below to create a load line. Plan the values for so you use up the graph space.  Use Ic as your vertical value, and Vce as your horizontal value.

Using Vbe on the Vce scale, plot the values of Ib so the finished graph looks similar to fig 13

Calculate the Beta (Hfe) of this transitor using the above graph.

  

 Explain what the load line graph is telling you. Discuss the regions of the graph where the transistor is Saturated, Cut-off, or in the Active area.