Lab06

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Orange Coast College *

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A130

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Electrical Engineering

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Apr 3, 2024

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Experiment #6 Bipolar Junction Transistor Biasing Circuits and Bias Point Stability Executive Summary: When working with BJTs it is important to understand how changes in manufacturing techniques and chemistry can affect our circuits. Even BJTs with the same part numbers can have varying specifications and this can lead to undesired behavior. By biasing our BJT circuits we can negate these effects. There are different ways to achieve this with varying levels of complexity and results. By making intelligent component selections we can design circuits that functionally eliminate the effects that BJT variations can cause. This helps with mass production as suppliers of components may change over time. Objective: To study and experiment on three types of DC biasing circuits for BJTs and compare the stability of the bias point in these circuits. Diagrams:

Data: β values for our 2 BJTs. Circuit #1 Data

Circuit #2 Data Circuit #3 Data Circuit #4 Data (Actual Resistor Values same as Circuit #1)

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Data Analysis: Circuit #1 Looking at the voltages on the collector of each of our circuits we can see a clear picture of how the β values of each BJT affect each circuit. Using LTSpice I simulated this circuit using our measured values for the resistors and the β values from our BJTs. Comparing our simulation to our real-world circuit our base voltages are similar, but the collector voltages are off. This can be attributed to simulations using ideal mathematical values while we measure our values using voltmeters and leads with less-than-ideal discrepancies. The biggest takeaway of this circuit is comparing the two different β value BJTs. When we swapped ours the collector voltage dropped by 1.87V. The base currents for both BJTs were extremely close at approximately 21.7μA. The difference

can mostly be seen in the current going into the collector and out the emitter. The Collector-Emitter voltage (Vce) has a large drop when changing to a BJT with a larger β value. This results in an increase in current through the BJT. I can also see this effect when I change the BJT in my simulation to one that better matches the specifications for our second BJT. Just like in our lab testing, my simulation shows about a 1.5V drop at the collector voltage. Circuit #2 For this experiment besides changing resistor values at the base and collector, we also added another resistor at the emitter leg that matched the collector in magnitude. This has the effect of reducing the large change in voltage as measured at the collector. Once again, my simulation shows a similar trend, here is with BJT1 being simulated: We can see there is a half-volt drop between the BJTs. Our lab circuit demonstrated a similar difference as well. The addition of the resistance at the emitter side of the BJT has a cascading effect on the

voltages upstream. The base-to-emitter voltage will remain the same at 0.7V so increasing the voltage at the emitter dramatically increases the voltage seen at the base. This decreases the voltage drop across the 200K ohm resistor, which has a regulating effect on the available current that enters the base. This demonstrates that Circuit 2 is affected less by the β value of our BJT as compared to Circuit 1. Circuit #3 Here are the simulations for both β values used in Circuit 3: Now our collector voltages are very close with only an 80mV difference. This modeled behavior matches our lab results demonstrating that this circuit is nearly immune to differences in our BJT β values. The collector, base, and emitter voltage values in this circuit are within the measurement error of being identical in magnitude despite the differences in our BJTs. In this circuit, the 15K and 3.3K resistors provide the biasing while the 200ohm resister on the emitter provides the stabilization described in Circuit #2. This is known as a voltage divider bias method. Circuit #4 For this circuit, I used a potentiometer to adjust resistance at the 430K location to achieve a Vce value of 4.05V. I will calculate the AC gain of our built amplifier circuit. Gain = Vo Vs = 3.98 V 0.0194 V = 205.15 It is interesting to note that the AC signal is not only amplified but also inverted. This is due to the common emitter configuration of our circuit. As the input voltage increases, current increases through the base. This results in a decrease in the Vce voltage as the current increases from collector to emitter. Likewise, as the input voltage drops to negative values the reverse happens causing a 180° inversion between the input and output. The capacitors are there to ensure that only the AC signal comes through to the output effectively filtering out any effects that the DC voltage source would introduce.

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Post Lab: Here I calculated the percentage change of Q point values for each of our first 3 circuits as a result of different β values from our two BJTs. We can see a clear trend of the change percentage decreasing for Vce and Ic as we transition from one circuit to the next. Circuit #3 has a much more dramatic decrease in change compared to the change from circuits 1 and 2. What is interesting is the increase of change when looking at the base current. This shows the regulating effect of each circuit. The biasing of the BJT is increased with each circuit and this increase in base current is there to reduce the effect of the β.