PH337 Electronics
Experiment #5

TRANSISTOR CHARACTERISTICS LAB

OBJECTIVES

Study how a transistor operates by:
  1. measuring the dc operating characteristics of a typical transistor, and,
  2. choosing the correct operating points (dc bias) and assembling a common emitter amplifier.

DISCUSSION

In the DC part of the lab exercise, you will adjust several DC parameters for a typical transistor and measure how the currents and voltages "interact". What you are to learn is how the transistor responds to changes in the DC voltages and currents through the transistor "base and collector" circuits.
After doing this, you should have a better understanding of the characteristics curve for a transistor.

In the AC part of the lab, you will add an AC signal to the base signal curcuit as an input. Then you can observe and measure the AC signal at the output to determine the gain of the circuit.

APPARATUS

PROCEDURE


  1. Part I: DC Operation

    1. Connect your apparatus as shown in figure 1
    2. Dial in resistances for Rb and Rc such that
      10 uA < Ib < 30 uA, and
      1 mA < Ic < 10 mA
      Note: You will also be able to vary Ic by varying the voltage of the power supply from 2 to 20 volts.

      What you will be doing with this set up is adjusting the base current, Ib, to a low value, initially 10 uA, and then adjusting and measuring the change in Ic as a function of Vce.
      Let's go through this step by step:
    3. Start with Ib = 10 uA
    4. Then vary Ic by adjusting the power supply voltage Vcc. Measure Vce as you do this and record your data.
    5. When you plot this data, you should get a curve that looks something like the following:

      Note that Ic initially increases, then becomes (almost) constant with increasing Vce.
    6. Repeat the previous 2 steps with Ib=20 uA, and Ib=40 uA. (A few data points will do fine; just find the flat part of each curve.)
    7. Calculate hfe from the equation:
      hfe = Ic / Ib
      (pick a constant value for Vce).
      Note: hfe is called the forward current transfer ratio. Interpret and explain what it means from the graph of your data. Wh is the base said to control the amount of current through the collector circuit?

  2. Part II: AC Operation of the Transistor

    1. First you must establish the DC operating range for your circuit. You can continue to use the variable resistor for Rc, but use a variable potentiometer for Rb. Adjust Rc and Vcc so that
      6 V < Vce < 10 V, and
      3 mA < Ic < 10 mA

      see the schematic in figure 3:

    2. Add 0.01 uF capacitors to your circuit as shown.
    3. Now turn on the function generator (using sine waves) and apply an AC signal to the input capacitor. Use your oscilloscope to measure the input and the output voltages.
    4. Adjust the potentiometer from one extreme position to the other while observing the output. What happens? Why? Adjust the potentiometer to give you the largest voltage gain. While in this position adjust the input amplitude. Compare what happens now to the changes your observed when Vin was "close to 5 V" or "close to ground".