ECE - 340 EXPERIMENT XI

MOSFET COMMON SOURCE AMPLIFIER

Purpose:

In this experiment, we will examine the voltage amplification property of a MOSFET common source amplifier by measuring the voltage gain Av = and comparing it with the theoretical values.

Parts:

2 - 2N 7000 - NMOS Enhancement type device.

Assorted resistors and capacitors.

Theory:

A MOSFET behaves as a voltage amplifier if it is biased to operate in the "saturation mode." For a given MOSFET [NMOS] device, the voltage-current relationship in the saturation mode is given by the following expression.

ID = K [VGS - VT]2,

Where K - current gain factor, VT threshold voltage, [K and VT are characteristic parameters of the MOSFET device] ID-drain current flowing through the device, and VGS-applied gate-to-source voltage. For the small-signal model of an NMOS device shown in Figure 2, the transconductance gm, defined as the ratio of the small-signal AC drain current and the small-signal AC gate-to-source voltage, is given by:

gm = 2K[VGS - VT].

The circuit diagram of an NMOS common source amplifier is shown in Figure 1. All component values are marked on the Figure.

Procedure:

1) Construct the NMOS common source amplifier as shown in Figure 1.

2) Measure the DC operating point [Q-point] of this amplifier, and compare it with the calculated value. Also, verify the mode of operation of the NMOS.

3) Now connect an AC source at the amplifier input, and observe the output voltage waveform as a function of the amplitude of the input signal, and note the values [of both input and output] at which distortion becomes apparent.

4) Set the input frequency to 50kHz while keeping the signal amplitude less than the maximum [as measured in step 3], and determine the nominal voltage gain of this amplifier. Compare this value with the calculated value.

5) Determine the frequency response [variation of gain with frequency] of this amplifier in the range 50Hz to 5MHz. Plot the response on a semi-log paper and comment on it.

Report:

1) Find the smallest and the largest input voltages at which the output is undistorted. What would be the expected voltage swing at the output, and why?

2) Compare your experimental results form step 2 and step 4 with the calculated theoretical values. Comment on discrepancies, in any.

3) Calculate the gain G(f) and phase-shift ?(f) as a function of frequency. Plot the curves on a semi-log paper against frequency, and comment on them.