Module 8: ac amplifiers based on jfets (multisim)




This experiment explores the uses of JFET as AC signal amplifiers. It also describes techniques for measuring the input and output impedance of circuits.

Remember that your lab report will need to include your measurements, calculations, screenshots, etc. as indicated at the end of this outline.




1. Common Source Amplifier


1.1 Build the common source amplifier shown in Figure 8.1










Figure 8. 1: Common source amplifier

1.2 Using the oscilloscope, measure the voltage gain of the amplifier defined as Av = Vout/Vin.


2. Measurement of Input impedance


In this section we will learn a very useful technique to measure the input impedance of any circuit. This technique is based on placing a known resistor in series with the input of the circuit and measuring the voltage drop across the new resistor. This technique can also be used in live circuits and not just




Figure 8. 2: Circuit to measure input impedance


2.1 Build the circuit shown in Figure 8.2 You will notice that this is the same circuit used in Figure 8.2 with the extra R4 resistor added to the input.


2.2 Measure with the oscilloscope the voltage at node V1 and the voltage at node V4.


2.3 Calculate input impedance as follows:


3. Measurement of output impedance



The following technique can be used to measure the output impedance of a circuit. In practical circuits, the best value of the load resistor must be selected by trial and error.


3.1 Measure Vout as shown in the circuit from Figure 8.1. We will name this voltage Vout


3.2 Connect a load resistor of 1 kΩ at the output of the same circuit. Measure the voltage across the load. We will call this voltage Vload


3.3 Calculate output impedance as:



(in the case of using a different value for the load resistor, change the 10 kΩ value in the equation to the appropriate value of resistor used).

Laboratory Report



Create a laboratory report using Word or another word processing software that contains at least these elements: – Introduction: what is the purpose of this laboratory experiment? – Results for each section : Measured and calculated values, calculations, etc. following the outline. Include screenshots for the circuits and waveforms as necessary — You can press Alt + Print_Screen inside Multisim or if using Windows 7, you can use the “Snipping tool”. Either way, you can paste these figures into your Word processor.


– Conclusion : What area(s) you had difficulties with in the lab; what did you lean in this experiment; how it applies to your coursework and any other comments.





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