LAB 5: BJT I-V CHARACTERISTICS

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1 LAB 5: BJT I-V CHARACTERISTICS LEARNING OUTCOME Students identify the current-control terminal of a three-terminal active device (Bipolar Junction Transistor BJT). The students will use the curve tracer and the scanned-load-line methods to obtain the I-V characteristic of the BJTs and compare the characteristics measured on the curve tracer. MATERIAL AND EQUIPMENT Material 2N2222A npn BJT 2N2907A pnp BJT Resistors Op-amp Equipment Curve Tracer Power supply Signal generator Oscilloscope Breadboard and Analog Discovery Digilent Waveforms software BACKGROUND The BJT is best described as a current-controlled active element while the MOSFET and JFET are voltage-controlled. The base current i B is usually considered the controlling quantity. Although i B in turn depends upon v BE, the change in v BE for a given change in i B is so small because of the exponential relationship that is often neglected. This approximation (v BE =0.7 V) is often a source of confusion for the students who do not realize that a truly constant v BE implies no change in i B. This point should be emphasized in this experiment. MEASUREMENTS There are two methods to measure the I-V characteristic of the BJT using equipments available in the laboratory. 1. Curve tracer method: Use the Tektronix 577 or 575 curve tracer to display and plot the transistor collector characteristics of the 2N2222 transistor. Your instructor will assist you in using the curve tracer. Determine β dc and β ac at VCE = 5 V of the 2N2222 transistor. Carefully sketch the I- V characteristic on the graph paper next page for comparison purpose. Using I C =0 to 10mA and I B =0-60uA (10uA step).

2 2. Scanned-load-line method: The i-v characteristics are measured by the scanned-load-line method. Setup the circuit as shown in Figure 1. Note that you use only ½ of the op-amp TL082, ground the inputs of the un-used op-amp. For power supply, use the ±15V P.S. EE 1035 in 2C V (EE 1035) 0-10V signal generator 100 To horizontal input of the oscilloscope + V CE - 10K pot I B A EE K 2N2222 _ 1K +15V TL082 pin-out (8=+15V, 4=-15V) TL V To vertical input of the oscilloscope Current-to-voltage converter Figure 1: Experiment set-up for npn transistor 2

3 When calculating β F from the measured collector currents and base currents, the students should be sure to use values that correspond to the forward active region. It should be pointed out that i C is practically independent of i B in the saturation region. The data obtained by different students will probably differ significantly because β F is not well-controlled quantity. The data sheet of 2N2222, for example, specifies a range of 30 to 150. The reason is that small changes in i C /i E, or a, cause large change in β F because α is close to unity and is related to β F by: α βf = (1) 1 α where β F varies from 30 to 150, α changes only from to The saturation voltage V CEsat is subject to the same kind of misinterpretation by the students is V f. Its value is often quoted as 0.2 or 0.3 V, but it is actually a monotonically increasing function of i B. Plot the i-v characteristic of the 2N2222 (npn) transistor (similar to the one shown below). The curves should be XY plots of the V CE vs. I C for different base currents i B. (use xy plot in the oscilloscope with persistence option, save the curves in data file and use plotting software to plot). Clearly label the curves. Compare with the result from the curve tracer. 3. Analog Discovery method: Build the circuit shown below on a breadboard. Connect the circuit to the Analog Discovery and run Waveform software similar to the n-channel MOSFET IV characteristic experiment in Lab 3. 3

4 Vcc (AWG1, Scope 2+) R2, 100 Vbb (AWG2) R1, 20K 2N2222 Vc (Scope 2-, Scope 1+) GND (Scope 1-, Analog Discovery GND) Figure 2: Circuit for Analog Discovery method AWG: Channel 1 (AWG1): generates Vcc. A triangle browses the range from (0V 5V) = (Amplitude = 2.5V, Offset=2.5V).There are 10 ramps (rising or falling), each synchronized with one of the steps of AWG2. Each ramp will generate a branch in the Ic(Vc) characteristic for a specific value of the Ib parameter. Channel 2 (AWG2): generates Vbb. There are 10 steps uniformly distributed in the range (0.5V 2.5V). Adjust amplitude and offset of the base voltage (AWG2) to get the base current steps similar to the one in the curve tracer for comparison purposes. Scope channels: C2: Vcc-Vc, the voltage drop across R2 C1: Vc-GND, the collector to emitter voltage drop M1: Math channel calculating C2/R1 = Ic current. Since C2 is expressed in Volts and R2 value is given as Ohm, M1 is expressed in Amps 4

5 Sample of scope window: Compare and comment on the measurement results from 3 different methods. Repeat the curve tracer and Analog Discovery methods for the 2N2907 (pnp) transistor. Keep the transistor 2N2222 and its I-V characteristic for Lab #6. Vcc (AWG1, Scope 2-) R2, 100 Vbb (AWG2) R1, 20K 2N2907 Vc (Scope 2+, Scope 1-) GND (Scope 1+, Analog Discovery GND) Figure 3: Circuit for Analog Discovery method for 2N2907. (Note: the set-up of AWG1 and AWG2 for pnp transistor is different from npn transistor) 5