Common Source Amplifier Experiment

Experiment No. 2. COMMON SOURCE AMPLIFIER

I.

AIM: To plot the frequency response of the FET amplifier and to calculate the mid frequency gain and bandwidth

II.

EQUIPMENT AND COMPONENTS:

III.

i.

Apparatus: 1. Regulated power supply 0-30 v/1 Amp - 1 2. Function generator - 1 3. CRO – 1 4. Multimeter – 1 5. Digital Multimeter

ii.

Components: 1. FET BFW 10/11 2. Bread board, wires and probes 3. Resistors –1K-3, 10M-1, 2.2K 4. Capacitors – 10 μf – 3, 0.01μf-1

iii.

Specifications: BFW11

BFW10

IDSS

4-10 mA

8-20 mA

VGS

2-4 v

4-8 v

gm

3.9 mA/V

3.2 mA/v

THEORY:In a FET, the conduction of current through the device is controlled by the electric field between the gate and the conducting channel of the device. The output current is controlled by the input voltage. Thus, FET is basically a voltage controlled device. In common source JFET amplifier there is phase displacement of 180 degrees between the output and input voltage the mid frequency voltage gain of the amplifier is given by A = gm x Rd The voltage gain and phase angular are constant in mid frequency range. Design: Self Bias Method: Capacitor C1, C2 are open circuited and the resistor RG acts as short –circuit, since IG=0. Therefore VRG=IG RG=0V The gate voltage is zero and the current through RS is equal to the drain current. i.e. IS =ID RS = VRS = ID RS Applying KVL to the gate source loop we obtain, -VGS – ID RS = 0  VGS = -VRS = -ID RS Where VGS is used to the output current ID and not fixed in the magnitude. The value of output drain current ID can be calculated using equation as. 2

I D  I DSS

 VGS  1   I DSS  VP 

 VD  VS 1 VP 

  

2

Summing the voltage drop across RD and applying KVL to output loop VDD-ID RD – VDS – ID RS =0  VDS =VDD – ID (RS +RD) Since the gate current is practically zero, the drain resistance R D is given by

RD  IV.

CIRCUIT DIAGRAM:

VD  VDD - VDS  VRS     ID ID  

V.

PROCEDURE: 1. Make the circuit connections as shown in the figure. 2 Set Vin= 50mv from the signal generator. 3. Keeping the input voltage constant, vary the frequency from 50 HZ to 1 MHZ in regular steps and note down the corresponding output voltage. 4. Plot the graph of gain v/s frequency. 5. Find the bandwidth. PROGRAM *FET AMPLIFIER VDD 5 0 DC 12V VS 1 0 AC 50MV R6 1 2 1K C1 2 3 10UF R7 3 0 10MEGA RS 6 0 1K CS 6 0 10UF R3 5 4 5K C3 4 7 10UF R5 7 0 1K C4 7 0 0.01UF J 4 3 6 BFW10 .MODEL BFW10 NJF(BETA=1E-1) .AC DEC 10 10HZ 10MEGAHZ .PRINT AC V(7) .PLOT AC V(7) .PROBE .OP .END

VI.

OBSERVATIONS: Sl.No. 1 2 3 4

VII.

Frequency 50Hz 100Hz 200Hz ....

GRAPH:

VIII. CALCULATIONS: Bandwidth = fH-fL Mid frequency gain AVM =

Output voltage Vo

Voltage gain Av = Vo/Vi

Voltage gain Av dB = 20 log Vo/Vi

IX.

RESULT: 1. Frequency response curve is plotted. 2. Bandwidth is calculated.

X.

INFERENCE: 1. 2. 3. 4.

XI.

The maximum voltage gain occurs at mid band. The gain at the half power point is 3 dB less than the mid band gain. The gain remains constant throughout the mid band. The gain decreases both at low frequency and high frequency.

PRECAUTIONS: 1. 2. 3.

Identify the terminals of FET properly. Set the function generator just below the point of distortion. So that maximum Adjust the oscilloscope for proper viewing.

undistorted sine wave appears.

XII. APPLICATIONS: 1. In balanced modulator 2. Multi Channel Switch driver and amplifier séquences XIII. EXTENSION: Designing cascaded FET amplifier XIV. TROUBLE SHOOTING: Sl No. Fault

XV.

1

If there is no output

2.

If the output is distorted

Diagnosis Check for loose connection in the circuit Check the CRO probes Check the position of operating point.

QUESTIONS: 1. What is the difference between FET and BJT? 2. Explain the construction and working of JFET? 3. Briefly describe some applications of FET. 4. What is the difference JFET and MOSFET? 5. What do you mean by frequency response? 6. Why FET is called unipolar device? 7. What is the typical value for the input impedance Zi for JFET? 8. Which BJT amplifiers configuration is similar to common source amplifiers? 9. Why thermal runaway is not possible for FET? 10. What is the amplification factor for FET transistor amplifier.