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Multistage Enhancement Mode MOSFET Amplifier Lab


Amplifiers and semiconductor devices laboratory examining the 2N7000 TMOS FET. Theoretical concepts discussed in lecture course Analog and Semiconductor Devices and verified using laboratory experiments, hand calculations, and computer simulations.


Objectives:

1. To examine the voltage gain and bandwidth properties of a multistage common source amplifier which employs enhancement mode MOSFETs.

2. To practice calculating and measuring the DC and AC characteristics of a direct coupled CS-CS MOSFET amplifier.


Equipment and Materials:
1. Oscilloscope
2. Digital Multimeter
3. Power Supply
4. Function Generator
5. Computer with PSPICE software (optional)

6. 2N7000 NMOS FETs, 2 each
7. Miscellaneous resistors and capacitors


Procedure & Data:
1. For the circuit shown in Figure 1, predict the mid band input impedance, output impedance, and voltage gain of the system. (100μF capacitors are used to bypass the source resistors; 1μF capacitors are used to couple the amplifier to the source and load.) (Use the measured Q point gm from Experiment 6 for these calculations.)

2. Use bench instruments to measure the amplifier's DC voltages and drain currents, and the mid band voltage gain at f = 1kHz. (Optional: do a PSPICE analysis for voltage gain.)

3. Use bench instruments to measure the bandwidth of the amplifier. (Optional: use PSPICE to examine the amplifier's frequency response.)



Figure 1: Multistage E-Mode MOSFET Amplifier Circuit





Conclusion & Discoveries:

For the multistage enhancement mode MOSFET amplifier circuit (Figure 1) predictions were made prior to circuit construction. Two devices (provided) were connected where one device was to replace the load of the other. Calculations were made to examine the characteristics of the multistage amplifier using the following equations.

Zin = R11 / R21 (E.1)

Zo = RD2 (E.2)

Avt = AV1 • AV2 where AV1 = -gm(RD) and AV2 = gm(RD / RL) (E.3)

Frequency = 1kHz

VDD = +18v




The circuit was then constructed and values measured using an oscilloscope and DMM. It was found that on measuring the output voltage, the signal was clipping do to the large gain produced by the amplifier. So a voltage divider network of resistors at the AC input stage was constructed to lower the output voltage and allow for proper measurements. This data was gathered and compared to the calculated values (Table 1).

In closing, the usefulness of connecting multiple devices tougher allows one for creating desired gains for needed applications. By adding other components such as an RG in the biasing network other parameters can be accomplished for desired results.



Lab Notes and Graphics
Lab Notes Page 1 of 1


Electrical Engineering lab key words: MOSFET, 2N7000, TMOS FET, gm, common source common collector, Multistage Enhancement Mode, Voltage, current, transresistance, transconductance, and differential amplifiers. Offset voltage, bias current, offset current. Two port models. Frequency response, transfer characteristics and transfer functions, nonlinear distortion and nonlinear devices. Ideal operational amplifiers. Diodes, FETs and BJTs. Current vs. Voltage characteristics, DC large signal models, biasing, load line analysis.

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