EE321 Lab #11


Prelab for Lab 11

The purpose of this lab is to investigate the characteristics of MOSFET's, and to use them in some simple circuits. For simplicity we will use only n-channel devices.

Static Characteristics

1. The CMOS 4007 integrated circuit contains 6 enhancement MOSFET's, 3 n-channel and 3 p-channel. The n-channel bodies(p-silicon) are connected to pin 7 and must be kept at the most negative voltage used in the circuit. The p-channel bodies (n-silicon) are connected to pin 14 and must be kept at the most positive voltage used in the circuit. The drain and source are interchangeable on Q2 and Q5.

2. Does the MOSFET behave as a variable resistor for small drain-source voltages (both positive and negative vDS)?

Voltage Controlled Switch

3. Construct the following "chopper" circuit, which uses a square wave across the gate-source to turn the MOSFET on and off. The path from the drain to source acts as a resistor in a simple voltage divider. The resistance is very high for off and low for on.

Variable Gain Amplifier

4. The gain of the following amplifier can be controlled by the gate voltage on the FET, which is supplied by a voltage divider. Construct the circuit and apply a small input voltage (less than 50 mV p-p) at 1 kHz. How much can the gain be varied, and does this agree with the range of resistance values for the FET?

5. With the gain of the amplifier at approximately 5, increase the input signal amplitude until the output distorts noticeably. Can you figure out what causes the distortion? Sketch a distorted waveform.

6. Extra Credit. The distortion can be reduced dramatically by feeding half of the drain voltage back to the gate with Rf, as shown. Connect a 1 V p-p sine wave as shown to vary the gate voltage. The amplifier output should be an amplitude modulated signal.

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