EE 2212

EXPERIMENT 8

23 March 2017

Bipolar Junction Transistor (BJT) Measurements, Circuit Analysis, and Amplifier Operation

Report Due: Thursday, 30 March 2017

NOTE 1: We may not have enough background from Wednesday’s class but I will continue the discussion at the beginning of our lab meeting.

COMPONENTS

2N3904 or 2N2222 npn transistor

Resistors: 5 kΩ and 100 kΩ

Note: Use the 2N3904 or 2N2222 npn transistor device models in SPICE rather than the default model. It will be a better match against which to compare your experimental results.

DC Bias Analysis

Construct the circuit in Figure 8.1. Use = 10 Volts for the DC supply. Measure and record the Q-Point values of I_B, I_C, V_BE, and V_CE. Measure the voltage across the RB and RC resistors to obtain the base and collector current rather than inserting an ammeter in series. I suggest this approach since the internal fuse in the multimeter is difficult to replace. Be sure you measure the actual resistor values for your measurement to obtain more accurate results. Compare your results with a SPICE analysis of this circuit. Use the 2N3904 or 2N2222 in the SPICE library. The signal source v_in(t) should be set to zero for this portion of the experiment. You probably will need have to adjust V_BB from the nominal 1.5 volts and perhaps also the value of the 100 kΩ resistor to obtain a Q-Point in the center third of the load line, forward-active region, because of the probable wide variation of BJT β values that are in the bin. You can use a potentiometer to adjust RB also. Observe that you can use the signal generator to provide both the V_BB and v_in(t) by providing an offset to v_in(t).

Also note that SPIC will give you the dc bias point voltage and current values. I will demonstrate this.

Demonstrate Small-Signal and Large-Signal Operation

Now set v_in(t) for a 1 kHz sine wave from the function generator. Do not change the V_BB, that is keep the same dc offset for the Q-Point. Adjust the amplitude initially to 0.5 Volts (1 V_Peak-to-Peak), Measure the voltage gain defined by v_out/v_in. Simulate the circuit in SPICE with your transistor using a transient analysis. Note that SPICE also provides key Q-Point values for small-signal parameters as discussed in Wednesday’s class. Explain your results in the context of a load-line analysis. Use the small-signal model to compute the voltage gain. Also show the transfer characteristic. Adjust v_in(t) to demonstrate clipping in both the saturation and cutoff portions of the load line. Note that saturation in a BJT is defined significantly different than for a FET! However clipping is still clipping whether a FET or BJT circuit.

Reminder that both VBB and RB may need to be adjusted to obtain the Q-Point in the center third of the load line depending upon your actual BJT specifications.

FIGURE 8.1 BJT Circuit

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