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Activity 5

Cascaded Systems: RC Coupled Circuit

Objectives:

1.) To observe RC coupled circuit characteristics.

2.) To compare circuit effects of 1st staged and 2nd staged RC coupled circuits.

3.) To measure voltage inputs and outputs Vi1, Vi2, Vo1 and Vo2.

4.) To calculate the voltage gain Av1 and Av2 of each stage.

5.) To compare the input and output phases of the circuit.

Introduction:

One popular connection of amplifier stages is the RC -coupled variety. The name is derived

from the capacitive coupling capacitor Cc and the fact that the load on the first stage is an RC

combination. The coupling capacitor isolates the two stages from a dc viewpoint but acts as a

short-circuit equivalent for the ac response. The input impedance of the second stage acts as a load

on the first stage, permitting the same approach to the analysis.

Materials:

Breadboard Connectors Resistors

Capacitors Function Generator NPN transistor

Multitester Oscilloscope DC Power Supply

Procedure:

1.) Construct the circuit below:

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2.) Perform a DC analysis individually to get IE1, IE2, re1 and re2. Record the values on the

table below.

3.) Perform an AC analysis and obtain Vi1, Vi2, Vo1, Vo2, Av1 and Av2. Record the values on

the table below.

Results:

DC Analysis

Voltage Divider Output at the Emitter

IE 0.545 mA

rE 47.71 ohms

Voltage Divider Output at the Collector

IE 0.423 mA

rE 61.47 ohms

AC Analysis

Mid Frequency

High Frequency at Vo2 = 240 mV Low Frequency at Vo2 = 248.0 mV

High Frequency at Vo1 = 80 mV Low Frequency at Vo2 = 66 mV

f high 1.96 Hz

fmid 1.88 kHz

Vi1 142.0 mV

Vo1 128.0 mV

Vi2 128.0 mV

Vo2 480 mV

Av1 0.9

Av2 -3.75

Av total -3.375

f low 2.91 Hz

Vo1 76.0 mV f high 460.8 kHz

Vo1 122.0 mV

f low 5 MHz

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Graphs:

Mid Frequency at Vo1 = 128.0 V

Mid Frequency at Vo2 = 480 mV

High Frequency at Vo2 = 240 mV

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Low Frequency at Vo2 = 248 mV

High Frequency at Vo1 = 80 mV

Low Frequency at Vo1 = 66 mV

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Conclusion:

Characteristic of an RC Coupled Circuit

The DC conditions of the first stage is isolated from the second stage. Each stage consist of one transistor and an associated circuitry and is coupled to the next stage through the use of a coupling device, this time, a capacitor. The capacitor connects the output of the first stage (Emitter-follower ) to the input of the second stage (Collector-follower) in order to pass the ac signal and at the same time, it blocks the dc bias voltage from the first stage since the capacitor behaves as an open circuit to dc and a short circuit to ac.

Voltage Inputs and Outputs The voltage gain of a multistage amplifier is equal to the product of gains of

individual stages. At the first stage, we expected a smaller output value than the given input since its configuration is an emitter-follower, the input and the output are 142.0 mV and 128.0 mV respectively. At the second stage, we expected a much bigger final output value since its output is at the collector, the input and the output are 128.0 mV and 480 mV , the output is almost four times the input.

Input and Output Phases

For the first stage, the input and the output is in phase since the voltage gain

of an emitter follower is always positive. On the contrary, at the second stage, an

out-of-phase behavior is exhibited by the input and the final output since the

voltage gain at this particular stage is negative.