BJT Fundamentals and Applications JOR

Purpose:The purpose of this assignment is to design a Pulse Amplifier and a

Common-Emitter Amplifier with voltage divider bias using a 2N2222A NPNbipolar junction transistor(BJT).

This serves as a homework assignment and final laboratoryassignment as a unit.Once you have designed each part use Multisim as your Lab test to verify DCQ-Point voltages and all appropriate input out put waveforms.

Pulse Amplifier:

In this section you will be designing a Pulse amplifier to convert a 0 to5 volt pulse to a 0 to 12 volt pulse. In order to complete the design and construct thecircuit in Figure 1, you will need to calculate the necessary component values. In anypractical design one needs to know the interface requirements in order to design thecircuit that connects the two interfaces together. In the case of Pulse Amplifier, thoserequirements are as follows:

Design a Pulse Amplifier that is driven from a source that is capable ofdriving 5 volts into 50 ohms and the output should be capable of delivering 12 ma whenthe collector-emitter is shorted. The supply voltage is 12 volts and the initial sourcefrequency is 1 Khz. A typical circuit for this is shown in Figure 1.

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Figure 1

1. In Multisim, connect a 2N2222 transistor to the Curve Tracer and obtain the outputcharacteristic curves. Use the grapher to obtain the curves and adjust the curve so itsorigin is at zero and chose appropriate scales. The make a paper copy of the outputcharacteristics so that you can use it to design your circuit( draw load-line, etc.).

2. Use the above characteristics and draw a load-line from the given design criteriaand determine the Saturation point. What is the value of the base current that justbrings the transistor in saturation? Use this to calculate the base resistor R2. R1 isobtained from the given output requirement.

3. Set-up the circuit in Figure 1 with the component values you obtained. Using a 5volt pulse at 1Khz as an input, obtain and record the input/output waveforms. Doesthe output go to zero like the input pulse when it is in it's lowest state? Explain.

4. Assume a base current overdrive factor of 1.5 and re-calculate R2. Insert this newvalue of R2 into your circuit and do part 3 above, again.

5. Adjust the frequency of the generator to 400Khz and record the input/outputwaveforms. Explain.

6. Now connect a lOOpf capacitor C3(speed up cap) across R2 and record the input/output waveforms. Explain.

7. Now connect a lOOpf capacitor C4 across the output to simulate wiring and deviceloading capacitance. Record the input/output waveforms and explain.

8. Do you know why I put O.Olufin parallel with a 100uf across the supply?

Common-Emitter Amplifier with voltage divider bias:

In this section of the laboratory you will be designing a common-emitter amplifier using the most stable bias configuration. This configuration is shown inFigure 2. Before the circuit can be setup, a set of design criteria must be obtained soappropriate component values can be calculated and inserted into the circuit.

Design a CE amplifier to amplify a 40mv pp, sine wave signal, to 1volt pp, into a 1 k ohm load. The available power supply is 12 volts. In your design, usethe "rule of thumb" that Ve:::::0.1 Vcc and that the bias network current 14 = 10 Ib tominimize the BJT loading on the R1, R4 input bias network. This effectively says that theresistor R4 is 1/10ththe BJT input resistance. It is also desired that Vce be Y:! Vcc whichis generally a good starting point unless there are other criteria that need to be met. Thisgenerally yields a low distortion peak to peak output voltage swing provided the outputswing is less than 1/4thof the power supply. .

1. First, calculate the collector resistor R2 utilizing the maximum power transfertheorem. Using the rule of thumb for Ve, calculate the collector quiescent operatingvoltage and then the emitter resistor, R3.

2. Draw the load-line on the output characteristic curve, noting the location of the Q-point based on the Vce criteria.

3. Record the Q-Point collect current, voltage and resulting base current. From thiscalculate the current gain beta and the approximate transistors input resistance Rin.

4. Calculate II, 14, Rl and R4. Now that all the resistor values have been determined,construct the circuit in Figure 2.

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R1 R2

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Vin C1 Vbq5 + 2N2222A*10uF

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R3 330uF 000

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Figure 2

5. With the output load resistor R5 and the signal generator disconnected, measureVcq, Vbq, veq and the supply voltage. Note, before you use the AFG3021generator make sure you have put a 50 ohm load resistor across its outputotherwise the signal coming out of the generator will be larger than what youset on its screen. It is not shown in Figure 1 so as not to make the experimentequipment dependent.

6. Connect the signal generator and record Vcq and Vin on the oscilloscope usingDC coupling on both channels. Move the scope probe from Vcq to Vout and recordthese signals. Now connect the output load resistor R5 and record the resultingwaveforms. Do these waveforms make sense? Explain.

7. Using the input/output waveforms with R5 connected, calculate the voltage gainAv = Vout / Vin.

8. Using the simulator only, do an AC Analysis(frequency response) and FourierAnalysis on the output node. Explain the significance of these plots.

9. Remove the emitter capacitor C3 and calculate Av for the new output voltage.10. Insert a 22uffor the emitter capacitor C3 and record the input/output waveforms.

Calculate the Av and explain any other noticeable differences in the input/outputwaveforms in comparison to those when C3 was 330uf ..

11. Increase RI to approximately 3 times your calculated value and record Vin andVout. Oscilloscope channels should still be on DC coupling. Now decrease RI tooapproximately to 113your calculated value and record these waveforms. Explain.

12. Re-install your calculated value for R1. From your measurements you should havemeasured a peak: to peak: output greater than 1.0 volt pp. If it is desired to have theoutput approximately 1.0 volt pp without changing the other interface requirements,how could you do it with one resistor at the input? Chose a resistor to meet thisrequirement and insert it in your circuit and verify it works. Record the input/outputwaveforms. If the original output signal was a little distorted, did this change haveany effect? Explain.

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