Elex Amplifier

8/13/2019 Elex Amplifier

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Batangas State University

College of Engineering, Architecture and Fine Arts

Electronics Engineering, Instrumentation and Control Engineering and Mechatronics

Engineering Department

ECE 351: Electronic Circuits And Devices

Progress Report #1

Group Members:

Baldrias, Rommell M.

Olan, Denisse T.

January 24, 2014


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I. Minutes Of Meeting

The meeting was held at school, Batangas State University Main II Alangilan, Batangas,

last January 23, 2014.

II. FILA Table

Facts Ideas Learning

Issues

Action

Amplifier I Research

abouot

amplifier

III. Pictures


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RESEARCH:

The JFET was first theorized in 1925 by Julius Lillenfield but wasnt built until

much later. As much as the JFET was theorized many years before many bipolar

junction transistors that you see today, it wasnt built until much later. In amateur,

they dont have a very large amount of uses (unlike MOSFETs and BJTs).

In JFET Based amplifiers, the input impedance is pretty much determined by the

gate resistor which comes after the input capacitor to ground.

Step 1: Choosing the design parameters
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The parameters we need to design ourselves a JFET amplifier are:

Vcc (Positive supply voltage) Minimum Rds On of the JFET (resistance of the JFET when fully on biased)

Ids (Current flowing through the JFET from drain to source)

Cut off frequency of the JFET preamplifier

Vgs Cut off voltage

From these parameters, we can calculate the values for Cin (Input capacitor), Rg

(Gate resistor), Rs (Source resistor), Rd (Drain resistor) and Cout (Output

capacitor).

When designing the circuit, you must ensure you dont exceed the voltage andcurrent rating of the JFET. With the JFET I am using (2N5457), the maximum

VDS (maximum voltage across the drain and source) is 25v, the Vgs (off) is -2.5v

and the maximum Ids (maximum current across the drain and source) is 3mA.

On the data sheet, the Ids is called Idss. All that Idss means is the current flowing

through the drain of the JFET with the gate grounded. This value should not be

exceeded and as a general rule, you should not exceed about 60% of the
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absolute maximum rating, this will prevent any broken devices and failures.

Step 2: Choosing values for Rd and Rs

Choose the value of the resistors Rs and Rd.

The first parameter we need to calculate is the total resistance of Rs and Rd.This requires the Vcc, Ids, Rds(on) and Vgs(Cut off).

So:Rs + Rd = ( Vcc( Rds(on) x Ids )) / Ids

Firstly, we must calculate the source resistor:

Rs = Rs + Rd x ( -Vgs(off) / 10 )

Calculating Rd from this is a the very easy task of:

(Total resistance of Rs and Rd)Rs = Rd
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From this simple maths and use of Ohms Law, we have calculated the vitalparameters required for a JFET amplifier.

Step 3: Choosing the values for the input filter and output filter.

Now that we have calculated the vital parameters of the Jfet amplifier, we can

now calculate the cut off frequencies of the input and output filters. A capacitor

with a resistor in this configuration produces a high pass filter. This can be

calculated to work with another equation.

The equation for an RC filter (Resistorcapacitor filter) is:

1

---------------- = Frequency cut off at -3dB

2 x Pi x R x C

Now we know what the input impedance needs to be 100k and the cut off

frequency needs to be


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Step 4: Designing a JFET amplifier using the load line technique
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Step 5: Designing JFET amplifier using a constant voltage

source... on the source!
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The final method about JFET amplifier design is using a constant voltage on the

source. This will maximize the gain but will not produce a perfectly symmetricalwaveform depending on what JFET to use.

Step 6: Now you have finally designed your JFET Amplifier!
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Step 7: Building the JFET amplifier on breadboard
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Building a JFET amplifier on a breadboard is also an incredibly easy task as it

generally required no more than 7-8 components

Step 1:

Assemble the components you are going to use and make sure they are of

correct value and suited to the job.

Step 2:

Place the JFET in the breadboard and ensure to note which leg is what (this can

be found on the data sheet,

Step 3:

Add resistors

Step 4:

Add the capacitors
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Step 8: Adding more gain

.Step 9: The end.
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Documents

Elex Amplifier