ENGR325 Electronics II

Lab #4 MOSFET Amplifiers I

 

Objectives:

                      To study the operation of a typical MOSFET amplifier circuit.

                      To see how the dc bias can set the Q point around which we amplify the ac input.

 

Procedures:

Last week we did this:

 

1. Use the same setup we used to measure the JFET I-V characteristics in Electronics I to obtain the I-V characteristics for either the 2N7000 or the BS170 N-Channel MOSFET. Carefully determine the threshold voltage at which drain current just begins to flow. Save the I-V curves for a number of values of VGS and include a copy of these characteristics in your report.

 

2. From the characteristics determine the value of the transconductance gain gm = ΔID/ΔVGS. Compare your value with the data sheet for your transistor. Also, you should be able to calculate a value for Kn for your transistor in the vicinity of the Q point (ID@5mA and VDS @5V)

 

3. Design a dc bias circuit to meet these specifications (similar to problem 3.28).

IDQ @5 mA

VDSQ @5V

VDD = 10V

VTN = your measured value

Kn = your computed value from measurements

Voltage across RS is approximately equal to VGS

 

4. Build the bias circuit that you designed using standard values of 5% resistors, and take measurements to determine how close you are to the above specifications. Show your design Q point and the measured Q point values in a table and comment on their agreement.

 

This week I want you to continue with the same MOSFET and circuit:

 

5. Connect a 100mVpp 1kHz sinewave from the Wavetek through a 1μF capacitor to the gate of the MOSFET. This will be the input to the amplifier. Connect a 100kΩ load resistor through a 0.1μF capacitor to the drain of the MOSFET. The voltage across the 100kΩ load will be the output of the amplifier.

 

6. Measure the gain of your amplifier. Then connect a 100μF capacitor across RS and measure the gain of your amplifier again. Calculate the gain that you would expect in each case using the gain equations in Design Examples 4.4 and 4.5 on pp. 222-226. Show your measured and calculated values in a table and comment on their agreement.

 

7. With RS bypassed increase the input voltage slowly and note the kind of distortion you begin to see in the output waveform. Using your characteristic curves with the load line, account for distortion that you see. A record of the distorted output waveform would add to your discussion.

Relate your findings to the linearity requirement in Equation 4.4 on page 210.