Practical - 33 :- Construct and Test JFET and MOSFET Amplifiers

🎯 Objectives

At the end of this exercise, you shall be able to:

1. Construct and test an N-channel JFET common-source amplifier circuit.

2. Construct and test an N-channel MOSFET common-source amplifier circuit.

3. Measure and compare the AC voltage gain of both amplifier types.

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🛠️ Requirements

Tools/Equipment/Instruments

• Regulated DC power supply (e.g.,0-30V 2A) - 1 No.

• Function generator (100 Hz to 150 kHz range) - 1 No.

• 100 MHz Oscilloscope (CRO) - 1 No.

• Digital multimeter with probes ( or DMM/Ohmmeter) - 1 No.

• PCB/Breadboard

• Hook-up wires

Materials/Components

• JFET (N-channel): BF 245B or BFW 10 (or similar) - 1 No.

• MOSFET (N-channel Enhancement type, e.g., IRF 540 or similar for small-signal) - 1 No.

  • Note: IRF 540 is a power MOSFET, a smaller signal MOSFET is generally preferred for an amplifier, but IRF 540 can be used if its characteristics are considered.

• Capacitors (e.g., 5.6nF, 270nF, 6.8muF/24V electrolytic) - Various Nos.

• Resistors (1/4 W) - Various Nos. (as per circuit diagrams/values below)

 

Procedure

Part A: Preliminary JFET and MOSFET Identification

This step ensures the proper device is used and helps identify the terminals.

1. Terminal Identification: Consult the datasheet for the pin configuration (Gate, Drain, Source) of the JFET (e.g., BF 245B) and the MOSFET (e.g., IRF 540).

2. Basic Device Check (Optional for JFET/MOSFET):

• For the JFET, use the ohmmeter to check the resistance between the terminals (Gate, Source, Drain) to quickly identify if the device is damaged ("BAD FET" or "GOOD FET") using the method described in the original JFET exercise (refer to its Table 1).

Part B: Construct and Test a JFET Amplifier

1. Circuit Assembly: Construct the AC voltage amplifier circuit shown in the original JFET exercise (Fig 1 for JFET, BF 245B).

Frequency (kHz)	JFET Vin​ (mV)	JFET Vout​ (mV)	JFET Gain (Av​)	MOSFET Vin​ (mV)	MOSFET Vout​ (mV)	MOSFET Gain (Av​)
40
80
100
120
150

Conclusion

1. Compare the $\text{AC}$ voltage gain of the JFET amplifier and the MOSFET amplifier across the tested frequencies.

2. Comment on the frequency response observed for both amplifiers (i.e., how the gain changes with frequency).

3. Discuss the key difference in input impedance between the JFET and the MOSFET and how this affects the amplifier design (Hint: MOSFET's Gate is insulated from the channel).