EE 2212

EXPERIMENT 8

18 November 2021

Note: Thursday, 25 November, is Thanksgiving and obviously no lab. Please submit your Experiment 8 Lab Report no later than noon on Wednesday, 24 November.

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

COMPONENTS

2N3904 or 2N2222 npn transistor

Resistors: 5 kΩ and RB to be determined Design Value

Capacitor: 1 μF (Not critical). Functioning simultaneously as a DC blocking capacitor and also an AC coupling capacitor

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. As an added comment, based on the characteristics of the latest batch of transistors from the latest vendor we ordered from, I would start out using the 2N3904. Note the different C, B, E pin connection possibilities.

DC Bias Analysis

Ø Start by using SPICE to design for an RB that provides a Q-Point in the center third of the DC Load Line. I suggest using the 2N3904 model but a 2N2222 will also work well.

Ø Construct the circuit in Figure 8.1. Use = 10 Volts for the DC supply. Use the value for RB from your SPICE design as a starting point. You want the Q-Point to be in the center third of the DC Load Line. The actual RB may differ significantly from the SPICE simulation design value since the β values could be quite different in the actual transistors in the drawers. Correct your SPICE simulation to reflect the RB you use. RB could be large, on the order of a MEGohm. You will have to experiment with different RB values so that your Q-Point is in the center third of the load line; that is from VCEQ from 3 to 7 volts. To increase VCEQ-you need to lower IB which means increasing RB, and conversely, to lower VCEQ, you need to increase IB which means lowering RB. Refer to the DC load line for this Q-Point placement. 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. Verify that the signal generator is set to high impedance, not 50 Ω.

Also note that SPICE Output file will give you the dc bias point voltage and current values directly. I will demonstrate this feature in Wednesday’s class.

Demonstrate Small-Signal and Large-Signal Operation

Ø Now set v_in(t) for a 100 Hz sine wave from the function generator. Adjust the amplitude initially to 0.5 Volts. 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 class. Explain your results in the context of a load-line analysis. Use the small-signal model to compute the voltage gain. Av = -g_m(R_C). Recall g_m = I_CQ/V_T. 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 current saturation in a a FET! However clipping is still clipping whether using a FET or BJT in a circuit.

FIGURE 8.1 BJT Circuit

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