## Vermont Technical College Electronics I – Laboratory ELT-2051 Lab 07: Transistor Biasing Circuits and Q-point Stability Objectives: • To set an operating point for a transistor using three different bias techniques • To explore amplification of an AC signal • To use MultiSim to verify your experimental data General: In this laboratory, you will be supplied with two NPN transistors with varying ß’s. Prelab: Calculate values of Rb in Figures 1 and 2 assuming ß = 200, VCE = 6V . For Figure 3, calculate R1 and R2 so that their parallel resistance is about 20KΩ or 10% of (ß+1)RE. Also, calculate the critical frequency of the 1uF capacitor in Figure 4. Materials: • 2N3904, 2N4123 NPN TXs (1 high ß, 1 low ß) • (2) 1 k Ohm, 100 k Ohm, assorted resistors • 1uF, 10uF capacitors • Curve Tracer • DC Power Supply • Multimeter • Signal Generator • Oscilloscope • Breadboard Procedure: 1. Use the curve tracer to plot the curves for each of your transistors. From these curves, again using the curve tracer, determine the ßDC for each transistor at the IC currents of 1mA, 3mA, 6mA, and 10mA with VCE = 6V. Of course, be sure to keep track of which transistor goes with which curve. Verify that the ßDC values that you obtain are within the manufacturer’s specifications. Remember– ßDC = hFE ! 2. For each of the three circuits shown in Figures 1-3, using the R values calculated in your prelab, determine the operating points IC and VCE for each of the transistors. Be sure to table your data. In addition, plot ß vs IC for both transistors on a single graph so that the data is meaningful! What conclusions can be reached for the 3 biasing circuits? 3. Lastly – Build Figure 4 and determine the ratio (Gain) of Vout/Vin at 1KHz. Now vary the frequency of Vin to determine at what frequencies this ratio decreases to 0.707 of the value at 1KHz. 4. Use the Bode Plotter feature in MultiSim to verify your data of Part 3. Is the cut-off frequency the same as you measured in the lab? Base Bias: Parameter Calculated Value Simulated Value Measured Value VCE1 (high β) VCE2 (low β) n/a n/a |VCE1 – VCE2| 0 0 IC1 (high β) IC2 (low β) n/a n/a |IC1 – IC2| 0 0 Emitter Bias: Parameter Calculated Value Simulated Value Measured Value VCE1 (high β) VCE2 (low β) n/a n/a |VCE1 – VCE2| 0 0 IC1 (high β) IC2 (low β) n/a n/a |IC1 – IC2| 0 0 Voltage Divider Bias: Parameter Calculated Value Simulated Value Measured Value VCE1 (high β) VCE2 (low β) n/a n/a |VCE1 – VCE2| 0 0 IC1 (high β) IC2 (low β) n/a n/a |IC1 – IC2| 0 0 Laboratory Report: This lab is a semi-formal lab. Be sure to collect all data necessary to make observations and answer questions before you leave the lab. Also, you and your lab partner should discuss the results and outcomes prior to leaving. Take notes, fill in tables and include diagrams as needed. Your report should include: • Data Table • Beta Plot • MultiSim Frequency Response • Comparison of biasing schemes • Comparison of measurements vs. simulations and expectations.

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