CASCODE AMPLIFIER

I. AIMTo plot the frequency response of Cascode amplifier and to calculate the gain and bandwidth

II. APPARATUS REQUIRED i. APPARATUS

CRO (Dual Channel) – 1 No.sFunction Generator, Regulated power supply (0 - 30V)

ii. Components Resistors 1KΩ-2, 33KΩ-2, 3.3KΩ-2, 2.2KΩ-1, 560Ω-1Capacitors 10µf -4, 0.01µf - 1

III. THEORYThe cascade amplifier consists of a common emitter amplifier stage in series with a

common base amplifier stage. It is one approach to solve the low impedance problem of a common base circuit. Transistor, T1 and its associated components operate as a common emitter amplifier stage, while the circuit of Ts functions as a common base output stage. The cascade amplifier gives the high input impedance of a common emitter amplifier, as well as the good voltage gain and high frequency performance of a common base circuit.

IV. CIRCUIT DIAGRAM

V. PROCEDURE

1. Make the circuit connections as shown in the figure.2 Set Vin= 50mv (say), using 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

*CASCODE AMPLIFIERVIN 1 0 AC 50MVVCC 4 0 DC 24VRS 1 2 1KC1 2 3 10UR3 4 3 33KR4 3 0 3.3KR1 4 5 33KR2 5 0 3.3KC2 5 0 10URc 4 6 2.2KRE 8 0 560CE 8 0 10Ucc 6 9 10uRl 9 0 1KCSH 9 0 0.01UQ1 6 5 7 BC107.MODEL BC107 NPN(BF=100)Q2 7 3 8 BC107A.MODEL BC107A NPN(BF=100).AC DEC 10 10HZ 100MEGAHZ.PRINT AC V(9).PLOT AC V(9).PROBE.END

VI. CALCULATIONS

Band width = fH - fL Mid frequency gain AVM =VII. GRAPH

VIII. INFERENCE Emitter bypass capacitors are used to short circuit the emitter resistor and thus increase the gain at high frequency. These coupling and bypass capacitors cause the fall off in the low frequency response of the amplifier because their impedance becomes large at low frequencies. In the mid frequency, the-large capacitors are effective short circuits and the stray capacitors are open circuits, so that no capacitance appears in the mid frequency range hence the midband gain is maximum. At the high frequencies the bypass and coupling capacitors are replaced by short circuits and stray capacitors and the transistors determine the response

IX. PRECAUTIONS1. Note down the type number of the transistor. Note the important specifications of the

transistor from the data book.2. Do not exceed the maximum values specified in data book3. Do not install a transistor or remove one from a circuit with power ON. 4. Identify the terminals of the transistors.5. Check .the transistors.6. Set the function generator just below the point of distortion, so that the maximum

undistorted sine wave appears. 7. Adjust the oscilloscope for proper viewing.

X. TROUBLE SHOOTING:1. Check the transistor.2. Check the battery connections.3. Measure VBE and VCE (=VCC/2) with the DMM.. 4. Check for undistorted input signal can be viewed on the CRO.5. Check the CRO probe.

XI RESULT / CONCLUSION1. Frequency response is plotted 2. Bandwidth was calculated.

XII. EXTENSIONMultistage amplifier.

XIII. APPLICATIONS1. Amplifiers.2. Radio transmitter and receiver circuits.

XIX. QUESTIONS1. What is meant by Q point?2. Which multistage configuration results in cascade amplifier?3. What is the need for biasing a transistor4. What factors are to be considered for selecting the operating point for an amplifier?5. What is thermal runaway? How it can be avoided.6. What three factors contribute to thermal instability?