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8 JFET Ampl ifying Circuits

8.1 Objectives Understand the principles of JFET amplifier circuits.

8.2 Basic Description

8.2.a Terminology

The three most important parameters of JFETs are described as follows:

1) gm(trans conductance)fixedVds

Vgs

id

=

=

2) rd(drain resistance)

fixedVgsid

Vds

=

=

3) (amplification factor)

fixedidVgs

Vds

=

=

8.2.b Basic Principle

8.2.b.1 Bias Arrangement

Self b ias arrangement

Source self-bias arrangement for JFET is shown in Fig 8.1 (a). When the single

power supply Vdd is applied to drain, the self-bias can be established at gate and

souce so as to result in adequate operating point. There is no need for a second

power supply in order to have a negative VGSvoltage value

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Since RGis very large,

0, 0.G R GG

I V V = =

We can also write:

0.G GS RS GS D

V V V V I Rs= + = + =i

This equation yields:

.GS D

V I Rs= i

Fig. 8.1 JFET Self biasing

Plotting the load line:

a.From the KVL of output circuit:

dd D D DS D S V I R V I R= + +

b.When Id = 0, VDS= Vdd = 12V (point A)

c.When VDS= 0,

124

3D

D S

Vdd V I mA

R R K= = =

+ (point B)

d.The straight line connected between point A and B is the DC load line. The

operating point lies in the intersection of this load line and the curve for VGS.

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Voltage dividing bias arrangement

Fig 8.3 shows a voltage-dividing circuit for JFET, wherein except no longer

setting Vg as zero, the solutions to Vsg and Id are same as those of self-bias.

21 2

G

RV Vdd R R

=

+ ,

GS G D S V V I R= and

2(1 )GS Q

DQ DSS

VI I

Vp=

.

Fig. 8.2 Voltage dividing bias arrangement

8.3 Experiment Equipments

1. KL- 200 Linear Circuit Lab. Device

2. Experiment Module: KL-23004

3. Experiment Instruments: Oscilloscope, Multimeter, Function Generator

4. Connection cables and short-circuit clips

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8.4 Procedures

Procedure : Common Source Amplifier Self Biasing

1 )Locate the block marked 23004-block c.2 )Insert the short-circuit clips by referring to Fig 8.4.

Fig. 8.4

3 )Use voltmeter (DCV) to measure VGSand VD, then make records.4 )Connect signal generator in the input terminal (IN) and connect

oscilloscope to the output terminal (OUT).

5 )Adjust the output of signal generator to 1 kHz sine wave and graduallyincrease the amplitude so that the oscilloscope can display maximum non-distorted output waveform, and make records. At the same time use theoscilloscope to measure the waveform in the input terminal (IN), and makerecords on Table 8.1.

6 )Change R12 to R16 (6.8K), then repeat Step (3), (4) and (5).7 )Resume R12 to 3.3K and disconnect C3, then repeat Step (5).

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Table 8.1 CS Self Biasing

VDS

VGSVD

AV

R12

Vinmax

VDS

VGS

VD

AV

R16

Vinmax

VDS

VGS

VD

AV

C3

Disc.

Vinmax