Basics of Electronic CircuitsExperiment 8 Characteristics of Basic Active Devices 2005-06/I

In this experiment, we will examine the characteristics of the two basic active devices used in electronic circuits – the Bipolar Junction Transistor (BJT) and the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). Unlike the two-terminal passive devices studied in Experiment 7, which could be characterised simply by their terminal i-v behaviour, both these devices are active two-ports, and hence have to be represented by appropriate controlled-source models. As you will observe in this experiment, a BJT fits into the current-controlled current source (CCCS) model, while a MOSFET fits into the voltage-controlled current source (VCCS) model. The two-port configurations for these devices in terms of their physical terminals – Collector (C), Base (B) and Emitter (E) for BJT, and Drain (D), Gate (G) and Source (S) for MOSFET – are shown in Fig. 8.1.

(a) pnp BJT (b) npn BJT (c) n-channel (d) p-channel MOSFET MOSFET

Fig. 8.1 Port Configurations of BJTs and MOSFETs

Port Variable npn BJT pnp BJT n-channel MOSFET p-channel MOSFETv1 Positive (vBE) Negative (vBE) Positive (vGS) Negative (vGS)i1 Positive (iB) Negative (iB) 0 0v2 Positive (vCE) Negative (vCE) Positive (vDS) Negative (vDS)i2 Positive (iC) Negative (iC) Positive (iD) Negative (iD)

Moreover, these devices can be active only for one polarity of each of the port variables. The polarities of the two-port voltages and currents associated with these devices are given in Table 8.1.

Table 8.1 Nomenclature and Polarities of the Two-port Variables in BJT and MOSFETThe circuit you will use to display the output i-v characteristics of BJT and MOSFET is given in Fig. 8.2. This is basically the same circuit you had used in Experiment 7, with the addition of a potential divider consisting of two 10-k resistors and one 10-k potentiometer to provide the desired control input to the input port of the DUT, which is now a two-port. Note that the DUT control input is the current i1 for a BJT and the voltage v1 for a MOSFET. As in Experiment 7, R = 1.00 k, and hence the opamp output voltage vo (V) = – i2 (mA), where i2 is the current flowing through the output port of the DUT, assuming i1<<i2. Thus the setup gives a display of –i2 versus v2 (i-v characteristic of the output port) for any particular value of the DUT control input.

Part A. BJT Identification and Output Characteristics

1. Two BJTs – CL100 and BC557 are given to you. You have to identify the type (npn or pnp) of each of these two BJTs, and also the base terminal for each. The identification will be based on the property of a pn junction that it has a low (up to a few hundreds of ohms) forward resistance and a high (in the range of hundreds or thousands of k) reverse resistance. The emitter and collector terminals cannot be identified by simple measurements, and they are indicated by markings on the device. Measure the resistances between different pairs of leads of each BJT by the multimeter, taking two readings for each pair of terminals by reversing the multimeter polarity. A low resistance indicates that the red (+) terminal of the multimeter is connected to the p side of a pn junction. On the other hand, a high resistance indicates that the multimeter is either connected across a pn junction with the n side connected to the red (+) terminal of the multimeter, or connected between the collector and emitter terminals of the transistor.

i1 + C + i2 v1 B v2

E

+ D + i2 v1 G Sub v2

S

i1 + C + i2 v1 B v2

E

+ D + i2 v1 G Sub v2

S

Fig. 8.2 Circuit for displaying the output i-v characteristics of BJT/MOSFET2. Set up the circuit given in Fig. 8.2 without a DUT, following the breadboard bus organisation

done in Experiment 7. Switch on the FG and the Power Supply, and set the FG controls to give a 100-Hz sine wave with a peak-to-peak value of 8 V. Click on the D-C Offset control of the FG and adjust the D-C Offset of the FG so that the minimum and maximum points of the FG output waveform are at –8V and 0V respectively. Insert the pnp BJT as the DUT in the circuit of Fig. 8.2 according to the port definition given in Fig. 8.1(a). Adjust the potentiometer to obtain a proper collector-emitter i-v characteristic of the BJT. Note the region of the characteristic where the curve becomes practically flat, indicating a nearly constant current i2.

3. Set the potentiometer at five different points one by one to obtain i2= 1, 2, 3, 4 and 5 mA in the flat region of the characteristic, and measure the values of vR and v1 at each setting with the multimeter. Note that as vR is the voltage across a 1.00-k resistor carrying current i1, i1

(mA) = vR (V). Verify that the assumption i1<<i2 is valid. Sketch all the five curves on the same coordinates, labelling each curve by the corresponding value of i1. Plot the flat-region value of i2 against i1 and verify that a BJT indeed behaves like a linear CCCS with i1 (=iB) as the control input. Determine the value of the control parameter (=h21) of the BJT from the slope of this plot.

4. Remove the pnp BJT from the position of the DUT and change the D-C Offset of the FG so that v2 is a sine wave having minimum and maximum values equal to 0V and +8V respectively. Now insert the npn BJT as the DUT and repeat step 3.

Part B. MOSFET Output Characteristic5. Remove the npn BJT from the DUT position and recheck the waveform of v2. Note that the

MOSFET to be tested is not an isolated device, but is one of several MOSFETS forming part of a 14-pin IC CD4007. The MOSFET you are going to use is an n-channel device having its Gate, Source and Drain terminals available at pins 3, 4 and 5 of the IC. All MOSFETs have a common substrate for all the n-channel MOSFETs in the IC; this substrate is brought out at pin 7 of this IC. Connect the MOSFET as the DUT, following the two-port configuration given in Fig. 8.1(c). Connect the substrate (pin 7) to Ground.

6. Set the potentiometer at five different points one by one to obtain values of i2= 0.2, 0.4, 0.6, 0.8 and 1 mA in the flat region of the characteristic, and measure the values of v 1 at each setting with the multimeter. Verify that i1 = 0, as given by the value of vR, for all values of v1

and i2. Sketch all the five curves on the same coordinates, labelling each curve by the corresponding value of v1, and verify that the MOSFET does behave like a VCCS with v1

(=vGS) as the control input for a MOSFET. Plot the flat-region value of i2 against v1 and note the difference between the control action of a BJT and that of a MOSFET.

10.0 k + vR +12V i1 i2

1.00k i2

v1 v2 R +12V CRO 10 k _ _ potentiometer 2 7

3 + 4 6 + I(Y) II(X)+ 10.0 k vo v2 12V 12V _ _

i1 i2

+ DUT +v1 (3-terminal v2

_ two-port) _

FG