Laboratory 3 - Bipolar Transistors

Please read and review the lab guidelines.

This lab does not require a report.

Objective:

The objective of this lab is to characterize the 2N2222A NPN transistor and learn how to operate the curve tracer.

Prelab:

No prelab is required this week.

Equipment and Components:

Procedure and Experiment:

A. Familarization with the Principle of Operation of the Curve Tracer



Figure 6.1 (a) Tektronix Type 576 curve tracer and (b) block diagram


The primary use of a curve tracer shown in Figure 6.1 is to display the current/voltage characteristics of electronic components, e.g., diodes and transistors.


As shown in Figure 6.1 (b), the curve tracer consists mainly of a Base Step Generator and a Collector Voltage Generator. The Base Step Current Generator applies a staircase current waveform to the base of the BJT. The Collector Voltage Sweep Generator applies a current waveform through an internal series resistor (dissipation limiting resistor). The collector current is obtained by measuring the voltage across the collector current sampling resistor (the sampling resistor is calibrated such that its voltage indicates the collector current. This current is displayed on the vertical axis whereas the collector-to-emitter voltage of the BJT is displayed on the horizontal axis of the screen.

To investigate the forward characteristics of the npn transistor, set the switches on the curve tracer to the following nominal settings:

Test panel (Sloping Panel):
OFF
EMITTER GROUNDED
BASE TERM to STEP GEN
Collector Sweep:
Series Resistor = 650W
Polarity = NPN
Peak Volts Range = 15V
Peak Volts = 10V
Base Step Generator:
Step Family = REP
Number of Steps = 4
Rate = NORM
Offset = ZERO
Steps/Pulsed Steps = STEPS
Step/Offset Amplitude = 0.01mA

  1. Adjust the Vertical and Horizontal Divisions for proper viewing.
    Use the ZERO button located in the center of the Curve tracer to locate the zero. If the zero is not already in the left lower corner, then move the zero there by using the vertical/horizontal shift knobs.

  2. Move the switch on the front panel to the LEFT or RIGHT position depending on which side the BJT is connected. There will be a family of curves on the display, each curve corresponding to one value of base current. The base current increment can be controlled by the STEP/OFFSET AMPLITUDE in the BASE STEP GENERATOR.

  3. To investigate the common-emitter characteristics, try adjusting the BASE STEP GENERATOR’s amplitude and # of steps. Return the settings back to the nominal values.

  4. Identify the saturation and active regions on the display (IC vs. VCE). For VCE = 2, 4, and 6 V, find the values of IB, IC, bF. Comment on the variations of bF and ro with respect to IB and VCE. Also, measure bF at VCE = 10V and IC = 2mA. Report this value to the TA. The TA willl collect values from throughout the class to allow mean and standard deviation of the b value to computed.

  5. Estimate the value of the Early Voltage. Remember that IC ~ (1 + VCE / VA). Estimate the early voltage by measuring the current at two different values of VCE and extracting. Does VA change with base bias? Does it change with VCE?

  6. Repeat (4) with the BJT connected backwards, i.e., with its collector lead to E and emitter lead to C. (This will bias the transistor in reverse active). Use somewhat smaller voltages and find a typical value for bR. This is why we normally use the transistor in forward active!

  7. Calculate the values of RB and RC required in the circuit in Fig 6.2 to achieve a Q-point of VCE = 10V and IC = 2mA. Construct the circuit and measure its Q-point. Compare with the design goal.