ECE 1212 – Electronics Laboratory Design

Experiment 4 – BJT Amplifier Design

Notes on Using the Curve Tracer

In Part A-1 of Experiment 4, you are asked to use the curve tracer to evaluate the 2N222 BJTs from your parts kit, by measuring four parameters:VBE(on), VCE(sat), and VA. This document describes methods for making theses measurements.

For all measurements, you should choose the various test parameters so that their values are similar to what the BJT will experience in the amplifier circuits. In particular, the collector current should be in the mA range. The load resistance should be small enough that the voltage drop across it does not limit the collector voltage, and the maximum current and power limits should be chosen so that your transistors are not damaged during the test.

1.Connect the terminals of a 2N222 BJT to the T-shaped connector labeled “Transistor” at the upper right. For the device under test, choose “NPN.” If you choose the test parameters correctly, you should obtain a family of curves like those shown in Sedra and Smith, Figure 5.21 (reproduced below). It is ok if the maximum value for vCE you used is not large enough to show the breakdown region, as we are primarily concerned here with the behavior of the transistor in active region, and at the boundary between the saturation and active regions.

(Suggested parameter values: IC,MAX = 10 mA, VCE,MAX = 10 V, IB/step = 5 A, number of steps = 10, RL = 100 , PMAX = 0.5 W)

Sedra and Smith†, Figure 5.21. Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to saturation region in some detail.

2.Once you have the curves for your BJT, place the cursor on one of the curves in the active region, such that the collector current and collector-emitter voltage are near the values you expect when the BJT amplifier is in operation. The curve tracer measures  directly for you; it is displayed on the screen as hFE.

3.The saturation voltage VCE(sat) is the value of the collector-emitter voltage at the edge of saturation, as shown in the figure above. Measure this quantity by placing the cursor on one of the curves at the boundary between the linear portion that defines the active region, and the nonlinear curve of the saturation region. It may be impossible to tell precisely where this boundary lies, which makes precise measurement of this parameter difficult.

4.The early voltage can also be measured using the curves you have already obtained. As shown in Sedra and Smith, Figure 5.19 (also reproduced below), if the linear portion of each curve is projected back to the vCE-axis, all of the projected lines should intersect at vCE = -VA. Take two points from the linear portion of one of your curves, and use algebra to calculate where the projection of the line will intersect the vCE-axis. In practice, you will find that your calculated value of VA will vary significantly depending on which curve you use for the projection.

Sedra and Smith†, Figure 5.19. (a) Conceptual circuit for measuring the iC-vCE characteristics of the BJT. (b) The iC-vCE characteristics of a practical BJT.

5.Finally, to measure the BJT turn-on voltage VBE(on), you must connect the BJT to the terminals labeled “Diode” on the curve tracer. Connect the emitter to the cathode terminal, and connect both the base and collector to the anode terminal. Set the curve tracer to test a diode, and run the test. If you have chosen your test parameters appropriately, you should obtain a curve something like that shown in the figure below.

In this figure, the blue dashed line represents the data acquired by the curve tracer. As the current iB increases exponentially, the exponential curve quickly becomes approximately linear. Record two data points from this part of the curve using the cursors on the curve tracer (black dots in the figure). Then use algebra to calculate where a line passing through these points will intersect the vBE-axis (red dot). This is your measurement of VBE(ON).

Source:

†Sedra and Smith, Microelectronic Circuits, 5th edition, Oxford Press, 2004.