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BJT MULTI-STAGED AMPLIFIER

SYSTEM

EXPERIMENT NO. 1

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CASCADED SYSTEM

- series of amplifier stages where the output of onestage is the input to the next stage, provided that

both stages have the same amplifier configuration

but not necessarily identical

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CASCADED SYSTEM

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BJT CASCADED AMPLIFIER SYSTEM

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AMPLIFIER FREQUENCY

RESPONSE

EXPERIMENT NO. 2

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FREQUENCY RESPONSE The amplifiers frequency response varies dueto the effect of frequency on the capacitive

reactances in the circuit.

BODE PLOT

shows the relative output magnitude, typically

measured in decibels, and the phase variation as

the frequency changes

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FREQUENCY RESPONSE

Cut-off Frequency

It is defined at which the ratio of the output over theinput has a magnitude of0.707 or -3 dB, when

converted in decibels.

At this point, the amount of attenuation due to the

filtering components begins to change rapidly.

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FREQUENCY RESPONSE

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LOW FREQUENCY RESPONSE

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To determine the lower cut-off frequency:

Ifthe two cut-off frequencies are more than a decadeapart (f2 > 10f1 ),the half-power point of the amplifier

is the higher of the two.

If the two frequencies are closer than one decade,

then the actual cut-off frequency of the amplifier is

somewhat larger than either of the two calculated

frequencies.

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To determine the lower cut-off frequency:

If the amplifier has a bypass capacitor, then it can alsoinfluence the cut-off frequency.

NOTE:

- Typically, emitter bypass capacitors are chosento be large enough so that their effects are

negligible.

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HIGH FREQUENCY RESPONSE

The high frequency response of a discrete

transistor amplifier is determined by the internal

capacitances of the transistor itself.

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HIGH FREQUENCY RESPONSE

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DIFFERENTIAL AMPLIFIER

EXPERIMENT NO. 3

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A differential amplifier is a BJT amplifier that

produces outputs that are a function of the

difference between two input voltages.

It has two possible inputs and two possible

outputs, but it is not necessary to use both

inputs and both outputs.

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MODES OF SIGNAL OPERATION

SINGLE-ENDED INPUT

When a differential amplifier is operated in this

mode, one input is grounded and the signal

voltage is applied only to the other input.

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SINGLE-ENDED INPUT

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SINGLE-ENDED INPUT

When the signal voltage is applied to input

1, an inverted amplified signal voltage

appears at output 1.

Also, a signal voltage appears in phase at

the emitter of Q1.

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SINGLE-ENDED INPUT

Since the emitters of Q1 and Q2 are

common, the emitter signal becomes an

input to Q2, which functions as a common

base amplifier.

The signal is amplified by Q2 and appears,

noninverted, at output 2.

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SINGLE-ENDED INPUT

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SINGLE-ENDED INPUT

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DIFFERENTIAL INPUT OR DOUBLE-ENDED

MODE

In this mode, two opposite-polarity (out-of-

phase) signals are applied to the inputs.

This causes the differential signal to be as

twice as large as any either input alone.

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MODE


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MODE

When the input signals are 180 degrees out

of phase, the amplitude of the combined

output signal is equal to the amplitude of

oneinput signal multiplied by two times

the gain of the amplifier.

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MODE

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COMMON MODE INPUT

Two signal voltages of the same phase,

frequency, and amplitude are applied to the

two inputs.

When the two input signals are applied to

both inputs, the outputs are superimposed,

and they cancel, resulting in a zero voltage. This is known as common-mode rejection.

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COMMON MODE INPUT

Common-Mode Rejection

Its importance lies in the situation where an

unwanted signalappearscommonly on both

differential amplifier inputs.

Common-mode rejection means that this

unwanted signal will not appear on theoutputs and distort the desired signal.

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COMMON MODE INPUT

Common-Mode Rejection

Common-mode signals (noise) generally are

the result of the pick-up of radiated energy

on the input lines from adjacent lines, the 60

Hz power line, or other sources.

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COMMON MODE INPUT

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COMMON MODE INPUT

Common-Mode Rejection Ratio (CMRR)

It is a parameter or a measure of an

amplifiers ability to reject common-mode

signals.

It is computed as the ratio of the single-ended

or double-ended gain and the common-mode

gain.

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COMMON MODE INPUT

Common-Mode Rejection Ratio (CMRR)

CMRR =AVs

AVc

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PARAMETERS

Single-Ended or Double-Ended Gain

Common-Mode Gain

AVs = AVd=RC ICQ

52 mV

AVc=RC

26 mV

ICQ+ 2 ( +1) RE

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OPERATIONAL AMPLIFIER

EXPERIMENT NO. 4

Prepared By: Seigfred V. Prado

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OPERATIONAL AMPLIFIER (OP-AMP)

Op-amps are used primarily to perform

mathematical operations such as addition,

subtraction, integration and differentiation

thus the term operational.

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It is a special type of differential amplifier.

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Op-amps are usually high-gain amplifiers

with the amount ofgain determined by the

feedback network.

Three most important characteristics:

High gain

High input impedance

Low output impedance

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Differential Amplifier (Input Stage)

It provides amplification of the difference in the

two input signals.

Special techniques are used to provide the highinput impedance necessary for the operational

amplifier.

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High-Gain Voltage Amplifier (Second Stage)

It is usually made up ofcascaded Class A

amplifier circuits.

This stage may be made from several transistorsto provide high gain.

A typical operational amplifier could have a

voltage gain of up to 200,000. Most of this gaincomes from this stage.

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Output Amplifier (Final Stage)

It is usually a Push-Pull Class B amplifier.

This stage provides low output impedance.

It could be an emitter follower circuit (common-

collector).

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Op-amps are very versatile.

The same op-amp can be used in different

applications by just changing the external

components connected to it.

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NEGATIVE FEEDBACK

Negative feedback is one of the most useful

concepts in electronics, particularly in op-

amp applications.

It is the process whereby a portion of the

output voltage of an amplifier is returned to

the input with a phase angle that opposes

the input signal.

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NEGATIVE FEEDBACK

An op-amp can be connected using negative

feedback to stabilize the gain and increase

frequency response.

Gain-Bandwidth Product An increase in closed-loop gain causes a decrease in the

bandwidth and vice versa, such that the product of the

gain and bandwidth is a constant.

GBP is always equal to the frequency at which the op-amps open-loop gain is unity (unity-gain bandwidth, fT)

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Effect of Negative Feedback on Bandwidth

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INVERTING AMPLIFIER

AV = -Rf

Ri

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NON-INVERTING AMPLIFIER

AV = 1 +Rf

Ri

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SUMMING AMPLIFIER (SUMMER)

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SCALING ADDER

A different weight can be assigned to each input

of a summing amplifier by simply adjusting the

values of the input resistors.

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AVERAGING AMPLIFIER

A summing amplifier can be made to produce the

mathematical average of the input voltages.

This is done by setting the ratio Rf/R equal to thereciprocal of the inputs (n).

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LM741/UA741

Industry standard

General-purpose

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OPERATIONAL AMPLIFIER

COMPARATOR

EXPERIMENT NO. 5


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COMPARATOR

A comparator is a specialized op-amp circuit

that compares two input voltages and

produces an output that is always at either

one of two states, indicating the greater orless than relationship between the inputs.

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COMPARATOR

Because the output is always in one of two

states, comparators are often used to

interface between an analog and digital

circuit.

An op-amp comparator may be used as a

sine-to-square wave converter.

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COMPARATOR

An op-amp running without negative

feedback (open-loop) is often used as a

comparator.

Op-amps have very high open-loop gain,

which enables them to detect very tiny

differences in the inputs.

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ZERO-LEVEL DETECTION

One application of an op-amp used as a

comparator is to determine when an input

voltage exceeds a certain level.

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ZERO-LEVEL DETECTION

Notice that that the inverting input is grounded to

produce a zero level and that the input signal

voltage is applied to the non-inverting input.

Because of the high open-loop voltage gain, a

very small difference voltage between the two

inputs drives the amplifier into saturation, causing

the output voltage to go to its limit.

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NONZERO-LEVEL DETECTION

The zero-level detector can be modified to detect

positive and negative voltages by connecting a

reference voltage source to one of the inputs.

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To compare Vi with Vref, the op-amp would have

an output of +Vcc when Vi is slightly greater than

Vref, and an output ofVEE when Vi is slightly less

than Vref.

Comparators may be non-inverting or inverting. It

is non-inverting ifVi is fed at the non-inverting

terminal and the output has the same polarity asthe input.

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WINDOW COMPARATOR

A window comparator is formed ifboth inverting

and non-inverting comparators are used in a

single circuit, each with its own reference

voltage.

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WINDOW COMPARATOR

The Vref that approaches V+ is called the upper

threshold voltage, while the Vref that approaches

V- is the lower threshold voltage.

Diodes D1 and D2 logically combine the outputs

of the two op-amps.

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WINDOW COMPARATOR

Window comparator determines whether the

input voltage Vi is between the limits of the

upper and lower threshold voltages.

The output voltage is zero if the input voltage is

between the range. Ifnot, the output would be

equivalent to the saturation voltage of the op-

amp.

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WINDOW COMPARATOR

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WINDOW COMPARATOR

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DIFFERENTIATOR

AND INTEGRATOREXPERIMENT NO. 6


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INTEGRATOR

o It is an op-amp circuit that simulates

mathematical integration, which is basically a

summing process that determines the totalarea under the curve of a function.

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IDEAL INTEGRATOR

o In an ideal op-amp integrator, the feedbackelement is a capacitor that forms an RC circuit

with the input resistor.

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OPERATION

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OPERATION

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OPERATION

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OPERATION

o To produce a straight-line voltage rather thanexponential, the charging current must be

constant.

o The key thing about using an op-amp with an

RC circuit to form an integrator is that the

capacitors charging current is made constant.

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OPERATION

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OPERATION

o The constant IC charges the capacitor linearlyand produces a linear voltage across C.

o The voltage on the negative side of the

capacitor (op-amp output) decreases linearlyfrom zero as the capacitor charges (negative

ramp).

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OPERATION

Therefore:

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OPERATION

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PRACTICAL INTEGRATOR

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o The ideal integrator uses a capacitor in thefeedback path.

o But take note that the capacitor is open to dc.

o This implies that the gain at dc is the open-

loop gain of the op-amp.

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o In a practical integrator, any dc error voltagedue to offset error will cause the output to

produce a ramp that moves toward either

positive or negative saturation, even when nosignal is present.

o Practical integrators must have some means of

overcoming the effects of offset and biascurrent.


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o The feedback resistor, Rf, should be largecompared to the input resistor Rin, in order to

have a negligible effect on the output

waveform.o A compensating resistor, Rc, may be added to

the non-inverting input to balance the effects

of bias current.

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DIFFERENTIATOR

o It is an op-amp circuit that simulates

mathematical differentiation, which is

basically a process of determining theinstantaneous rate of change of a function.

IDEAL DIFFERENTIATOR

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o In an ideal op-amp integrator, the capacitor isnow the input element, and the resistor is the

feedback element.

o A differentiator produces an output that is

proportional to the rate of change of the input

voltage.


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PRACTICAL DIFFERENTIATOR

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o Because a capacitor has a very low impedanceat high frequencies, the combination of RfandC form a very high gain amplifier at highfrequencies.

o This means that a differentiator circuit tends tobe noisy because electrical noise mainlyconsists of high frequencies.

o The addition of Rin in series with the capacitorprovides a low-pass filter and reduce the gainat high frequencies.

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TONE CONTROL

EXPERIMENT NO. 8


TONE CONTROL CIRCUIT

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It is a circuit that can either amplify (boost)or attenuate (cut) a certain range of

frequencies in the audio band.

BASS It refers to the range of low frequencies starting

from 20 Hz.

TREBLE It refers to the range of high frequencies, up to

20 KHz.


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AT BASS FREQUENCIES

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AT BASS FREQUENCIES

AT BASS FREQUENCIES

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AT BASS FREQUENCIES

AT TREBLE FREQUENCIES

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AT TREBLE FREQUENCIES

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Class AB

Push-Pull Amplifier

EXPERIMENT NO. 9


AMPLIFIER CLASSES OF OPERATION

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Amplifier classifications are based on the

percentage of the input cycle for which the

amplifier operates in its linear region.

Each class has a unique circuit configuration

because of the way it must be operated.


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CLASS A POWER AMPLIFIER

When an amplifier is biased such that it always

operates in the linear region where the outputsignal is an amplified replica of the input signal

(360o), it is a Class A amplifier.

When the Q-point is at the center of the ac loadline, a maximum class A signal can be obtained.


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CLASS B POWER AMPLIFIER

When an amplifier is biased so that it operates in

linear region for180oof the input cycle and is in

cutoff for 180o, it is a Class B Amplifier.

A primary advantage of a Class B amplifier over a

Class A amplifier is that it is more efficient,hence, you can get more output power for a

given amount of input power.


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CLASS B POWER AMPLIFIER

A disadvantage of Class B is that it is more

difficult to implement the circuit in order to get a

linear reproduction of the input waveform.


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CLASS C POWER AMPLIFIER

Class C Amplifiers are biased so that conduction

occurs for much less than 180o.

They are more efficient than Class A and Class B

amplifiers.

The output amplitude is a nonlinear function of

the input, so class C amplifiers are not used forlinear amplification.


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CLASS C POWER AMPLIFIER


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CLASS AB POWER AMPLIFIER

When an amplifier is biased so that it operates in

linear region forless than 360o but greater than

180o and it cutoff for the rest of the cycle, it is a

Class AB Amplifier.

It is more efficient than Class A but less efficientthan Class B.


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CLASS AB POWER AMPLIFIER


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PUSH-PULL AMPLIFIER

The termpush-pullrefers to a common type of

class B or class AB amplifier circuit in which two

transistors are used on alternating half-cycles to

reproduce the input waveform at the output.


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CLASS B PUSH-PULL AMPLIFIER

It is an amplifier in which a second Class B

amplifier that operates on the negative half cycle

is added in order to amplify the entire cycle.


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CROSSOVER DISTORTION

When the dc base voltage is zero, both

transistors are off and the input signal voltage

must exceed VBE before a transistor conducts.

Because of this, there is a time interval between

the positive and negative alternations of theinput when neither transistor is conducting.


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CROSSOVER DISTORTION


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CLASS AB PUSH-PULL AMPLIFIER

To overcome crossover distortion, the biasing is

adjusted to just overcome VBE of the transistor.

This results in a modified operation called Class

AB.

expt1-9

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