Exp. # 7: RC Coupled Multistage Amplifier

Dept. of Communication and Electronics Engineering

Second Semester: 2009/2010

Group Members:

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Objectives:

Electronics Laboratory

Experiment 7

Amplifier Frequency

Response

Experiment 7

RC Coupled Multistage Amplifier

7-1

Course Title: Electronics Lab Lecturers:

Course No.:

Exp. # 7: RC Coupled Multistage Amplifier

To determine the range of linear operation of an RC-coupled two-stage amplifier.

To observe the phase relationships at the input and output of each stage in the

amplifier.

To troubleshoot an audio amplifier

Reference Readings:

1- Electronic Devices, THOMAS L. FLOYD, Fifth edition.

2- Electronic Devices “a design approach”, Ali Aminian & Marian Kazimierezuk.

Theory

A single stage of amplification is often not enough for a particular application. The overall

gain can be increased by using more than one stage, so when two amplifiers are connected

in such a way that the output signal of the first serves as the input signal to the second, the

amplifiers are said to be connected in cascade.

The most common cascade arrangement is the grounded-emitter to grounded-emitter

configuration. As you will recall, grounded-emitter amplifiers exhibit high voltage, high

current, and high power gains, so they are used in sound-reproducing systems as audio

amplifiers, in TV receivers as video (picture) amplifiers, and in many other applications.

Multistage amplifiers can be used either to increase the overall small signal voltage gain, or

to provide an overall voltage gain grater than 1, with a very low output resistance.

Coupling Methods

Transformer Coupling

Transformers make it possible to match the output impedance of the first stage to the input

impedance of the next. Proper impedance matching ensures maximum transfer of power

from one stage to the next. Transformers are frequently used in coupling amplifier stages

because of the bulk and cost of the transformers themselves.

Electronics Laboratory 7-2

Exp. # 7: RC Coupled Multistage Amplifier

FIGURE 7-1 Cascaded transistor amplifier with transformer coupling.

Direct Coupling

Direct coupling is also used in cascaded transistor amplifiers. In this method both dc and ac

voltages at the output of one stage appears at the input of the next stage. An advantage of

direct coupling is the savings possible in components and the improvement in frequency

response. This technique is used by default in circuits like IC op-amps, since large coupling

capacitors cannot be fabricated on-chip.

FIGURE 7-2 Direct-coupled transistor amplifier.

RC Coupling

Figure 1-3 shows an RC-coupled cascaded amplifier. Capacitors C1 and C3 couple the

signal into Q1 and Q2, respectively. C5 is used for coupling the signal from Q2 to its load.

If the operation of coupled amplifiers is considered, a complicating factor appears. The

addition of a second stage may alter the characteristics of the first stage and thus affect the

level of signal fed to the second stage.

Electronics Laboratory 7-3

Exp. # 7: RC Coupled Multistage Amplifier

FIGURE 7-3 RC-coupled transistor amplifier

Computing the overall gain of RC Coupled Multistage Amplifier

To compute the overall gain of the amplifier, it is easier to calculate unloaded voltage gain

for each stage, then including the loading effect by computing voltage dividers for the

output resistance and input resistance of the following stage. This idea is illustrated in

figure 1-4. Each transistor is drawn as an Amplifier consisting of an input resistance, an

output resistance, along with its unloaded gain, .

FIGURE 7-4: Two-Stage Amplifier

Then, the overall loaded gain , of this amplifier can be found by:

(1)

For the RC Coupled(C-E _ C-E) multistage amplifier:

(With Emitter bypass capacitor) (2)

(Without Emitter bypass capacitor) (3)

Electronics Laboratory 7-4

Exp. # 7: RC Coupled Multistage Amplifier

(With Emitter bypass capacitor) (4)

(Without Emitter bypass capacitor) (5)

Rout 1 = RC1 (6)

Rout 2 = RC2 (7)

Note that if a load resistor was added across the output, an additional voltage divider

consisting of the output resistance of the second stage and the added load resistor is used to

compute the new gain.

Equipments Required:

Resistors (1/4 W): 27k, 10 k, 5k, 2.7 k, 1k, 470, 220, and 100.

5-k potentiometer

Capacitors (25 V): Two 10 μF.

Two 2N3904 NPN silicon transistor.

0-15 V dc power supply.

DMM (Digital Multimeter).

Signal generator.

Dual trace oscilloscope.

Bread boarding socket.

Procedure

Electronics Laboratory 7-5

Exp. # 7: RC Coupled Multistage Amplifier

1- Setup the circuit shown in Figure7.5, omitting the signal generator and the power supply.

FIGURE 7-5 RC-Coupled Two -Stage Amplifier

2- Apply the 15-V supply, then using the DMM measure the required parameters in Table 7-1 below & compare these values with the calculated values, assuming that VBE = 0.7 V.

Table 7.1

3- Using the measured value for the dc emitter voltage obtained in Step 2, calculate the following:

Electronics Laboratory

DC

Parameter

Computed

Value

Measured

Value

VB(Q1)

VE(Q1)

IE(Q1)

VC(Q1)

VCE(Q1)

VB(Q2)

VE(Q2)

IE(Q2)

VC(Q2)

VCE(Q1)

7-6

Exp. # 7: RC Coupled Multistage Amplifier

Transistor-1 ac emitter resistance, re1 = ……………. Ω

Transistor-2 ac emitter resistance, re2 = ……………. Ω

For steps from 4-6, record your measure in table 7.2

4- Connect the signal generator to the circuit after setting to 0.4 V p-p sine wave at 1KHz. as shown in Figure 7-5, and then measure the output voltage of the first stage

5- Apply the input voltage to the second stage, then measure the output voltage of the second stage.

6- Connect the two stages together, and apply the input to the first stage, and measure the output of the second stage. Then calculate the overall voltage gain of the circuit.

Table 7.2

7- Set the oscilloscope to the following settings, then sketch both the input and output waveforms for the circuit.

Channel 1: 0.1 V / division

Channel 2: 2V/ division

Time base: 0.2 ms/ division

8- Is there any phase shift between the input and the overall output of the circuit?

9- Measure the input resistance of the multistage amplifier, which is equal to the input resistance of the first stage, and can be measured as following:

Insert a 5 k potentiometer in series between the function generator and the input coupling capacitor.

Adjust the potentiometer until drops to one-half the value noted prior to inserting the potentiometer.

Electronics Laboratory

AC ParameterComputed

ValueMeasured

Value

Vout(Q1) (Step 4)

AV (NL) Q1 (Step 4)

AV (NL) Q2 (Step 5)

Vout (Q2) (Step 5)

Vout (Step 6)

AV (Step 6)

7-7

Exp. # 7: RC Coupled Multistage Amplifier

Power down, and remove the potentiometer from the circuit without disturbing its setting.

Measure the adjusted resistance of the potentiometer, and this resistance will be equal to the input resistance.

R in (measured) = ……………… Ω

10- Measure the output resistance of the multistage amplifier, which is equal to the output resistance of the second stage, and can be measured as following:

Connect a 5 KΩ potentiometer in parallel between the collector resistance and the ground.

Adjust the potentiometer until V out drops to one-half the previous value. Remove the potentiometer and measure its resistance. By the voltage divider

role, the resistance of the potentiometer equals the output resistance of the amplifier.

R out (measured) = ……………… Ω

Conclusions

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Electronics Laboratory 7-8