electronics ii lab manual 4

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EECE 322 Lab 4: MOSFET Amplifiers

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Laboratory oals

This project will show the biasing, gain, frequency response, and impedance properties ofthe MOSFET common source and common drain amplifiers.

!ea"i#g

Student Reference Manual for Electronic Instrumentation Laboratoriesby Stanley olf

and !ichard Smith, "opyright #$$%.

Oscilloscope &ser's (uide )"opies of this reference boo* are a+ailable in the lab, or at

the website

Tektronics 571 Curve Tracer Manual

-S#% Transistor /ata Sheet

!ead the pre0lab introduction below

E$%ipme#t #ee"e"

1ab noteboo*, pencil

Oscilloscope )2gilent or Te*tronics 3 oscilloscope probes )already attached to the oscilloscope

-4"5E6 7oo* test leads

Te*tronics 8# "ur+e Tracer 9-08%: 9roto0-oard or*station 9", with 9S;"E application

Parts #ee"e"

34%%% MOSFET

Lab safety &o#&er#s

Ma*e sure before you apply an input signal to a circuit, all connections are

correct, and no shorted wires e


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1' Pre(Lab )#tro"%&tio#

Two of the most popular configurations of small0signal MOSFET amplifiers are thecommon source )"S and common drain )"/ configurations. These two circuits are

shown in

Figures =0# and =03 respecti+ely. The common source and common drain amplifiers, li*eall MOSFET amplifiers, ha+e the characteristic of high input impedance. The +alue of theinput impedance for both amplifiers is basically limited only by the biasing resistors !(#

and !(3. >alues of !(# and !(3 are usually chosen as high as possible to *eep the

input impedance high. 7igh input impedance is desirable to *eep the amplifier fromloading the signal source. One popular biasing scheme for the "S and "/ configurations

consists of the +oltage di+ider !(# and !(3. This +oltage di+ider supplies the MOSFET

gate with a constant /" +oltage. This is +ery similar to the -?T biasing arrangementdescribed in 9roject $. The main difference with the -?T biasing scheme is that ideally no

current flows from the +oltage di+ider into the MOSFET.

The "S and "/ MOSFET amplifiers can be compared to the "E and "" -?T amplifiersrespecti+ely. 1i*e the "E amplifier, the "S amplifier has a negati+e +oltage gain and anoutput impedance approT is aparameter of each particular MOSFET and is temperature sensiti+e. This parameter

sensiti+ity to temperature is one reason for establishing a stable /" bias. The 34%%%

MOSFET data sheet lists the minimum and maT as %.@ > and :.% >respecti+ely.

2' *esig#

/esign a common source amplifier as shown in Figure =0# with the following

specificationsA

#. use a 34%%% MOSFET and a 3% +olt /" supply

3. midband gain >O5>; B.%

:. low cutoff frequency F1 #%% 7C


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=. >O symmetric swing :.% +olts pea* )B > p 0 p

8. load resistor !1 D 8 *

B. source resistance !; D 8% )this is in addition to the Te*troni< function generators

internal resistance

/esign a common drain amplifier as shown in Figure =03 with the following

specificationsA

#. use a 34%%% MOSFET and a 3% +olt /" supply

3. midband gain >O5>; %.8

:. low cutoff frequency F1 #%% 7C

=. >O symmetric swing 8.% +olts pea* )#% > p 0 p

8. load resistor !1 D 3%%

B. source resistance !; D 8% )this is in addition to the Te*troni< function generators

internal resistance

Figure = 0 #A "ommon Source MOSFET 2mplifier


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Figure = 0 3A "ommon /rain MOSFET 2mplifier

Figure = 0 :A Small Signal FET Model

3' Lab Pro&e"%re

)steps # 3 may be omitted if done prior to this lab period and the same FET is used

4oteAThe MOSFET can be easily damaged by static electricity, so careful handling is

important

#. Find the +alue of the threshold +oltage >T and conducti+ity parameter G from the

digital cur+e tracer )remember the relation ;/ D GH>(S 0 >TI3 in the saturation region.

3. /etermine the +alue of rds from the digital cur+e tracer. The slope of the transistor

;/0>/S cur+es in the acti+e region is #5rds.

:. "onstruct the "S circuit shown in Figure =0#. !emember !; is installed in addition to

the internal 8% resistance of the function generator.


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=. >erify that the specifications ha+e been met by measuring the J0point, midband

+oltage gain, and pea* symmetric output +oltage swing. 4ote any distortion in the output

signal.

8. 2djust the output signal to obtain the maerify that the input impedance specification has been met.

$. "onstruct the "/ circuit shown in Figure =03. !emember !; is installed in addition to

the internal 8% resistance of the function generator.

#%. !epeat steps =0@.

4' A#alysis

#. "ompare the gain, frequency response, input impedance, and output impedance of

the "S amplifier with the results from the "E amplifier. "ompare the "/

amplifier with the "" amplifier. "omment on the differences5similarities.

electronics ii lab manual 4

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