aet_8_transistor amps.pdf

8/14/2019 AET_8_Transistor Amps.pdf

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TRANSISTOR AMPLIFIERSAET 8

First Transistor developed at Bell Labs on December 16, 1947


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TRANSISTOR AMPLIFIERS

Objective 1a

Identify Bipolar Transistor Amplifier Operating

Principles


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(1) Dynamic Operation(2) Configurations(3) Common Emitter(4) Common Collector(5) Common Base(6) Temperature Stabilization(7) Coupling

Overview


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TRANSISTOR AMPLIFIERSTypical Amplifier


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(A). Output taken from A toB:

Reduce the resistance of R2,voltage from A to Bdecreases.

Increase the resistance ofR2, voltage from A to Bincreases.

(Voltage follows resistance)!


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Resistor (R2) is replaced withtransistor (Q1)

(B). Output taken from A to B:

Reduce the resistance of Q1,voltage from A to Bdecreases.

Increase the resistance of Q1,

voltage from A to B increases.(Voltage follows resistance)!


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(C)An input signal from G1 isapplied to the basethrough C C. The inputsignal changes the Bias onthe base of the transistorcontrolling the current flowthrough the transistor.

The output, taken from A toB, will be a reproduction ofthe input signal only muchlarger.


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Amplification: The ability of a circuit toreceive a small change of input voltageor current ( signal ) and produce a largechange in the output voltage or current( signal ).

Amplification depends on the change inthe transistors resistance caused by aninput signal.


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TRANSISTOR AMPLIFIERSCONFIGURATIONS


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TRANSISTOR AMPLIFIERSCommon Emitter

Common Emitter issometimes called theGrounded Emitter.

Input signal is appliedto the base.

Output signal is takenfrom the collector.

The common line,(not used for signal)is connected to the

emitter.


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TRANSISTOR AMPLIFIERSCommon Collector

Common Collector (CC)is sometimes calledGrounded Collector.

The input signal isapplied to the base.

The output signal istaken from the emitter.

The common line, (notused for signal) isconnected to thecollector.


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TRANSISTOR AMPLIFIERSCommon Base

Common Base (CB) issometimes calledGrounded Base.

The input signal isapplied to the emitter.

The output signal istaken from thecollector.

The common line, (notused for signal) is

connected to the base.


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TRANSISTOR AMPLIFIERSCommon Emitter Amplifier

The purpose of the commonemitter amplifier is to providegood current, voltage, andpower gain.

180 phase shift


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TRANSISTOR AMPLIFIERSCommon Emitter Amplifier Components

R 1 determinesforward bias

R 2 aids in

developing biasR 3 is the collectorload resistor used todevelop the outputsignalR 4 is the emitterresistor used for

thermal stability


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TRANSISTOR AMPLIFIERSCommon Emitter Amplifier Components

Q1 - transistor

C1 is the inputcoupling capacitor


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Current paths andpercentage of flow

I E = 100%, I C = 95%,I B = 5%

NPN Current flowsfrom Ground to +VCC


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Signal path: When a signal is applied toan amplifier, four things occur.

Base, emitter & collector currentschange at the rate of the input signal

Collector voltage changes at the rate of

the input signalPhase shift of 180

There will be signal gain!


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Static or Quiescent Operation

By definition, bias is defined as theaverage DC voltage (or current)used to establish the operatingpoint in transistor circuits for a

static or quiescent condition. Astatic condition means the circuitdoes not have an input signal and isfixed in a non-varying condition.


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TRANSISTOR AMPLIFIERSTypical Amplifier with Bias - Static Condition

Transistor CurrentPath

600mv (.6v) Bias(emitter to basevoltage) causesemitter current (IE),base current (IB),and collectorcurrent (IC) to flow.


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TRANSISTOR AMPLIFIERSTypical Amplifier with Bias - Static Condition

Current enters theemitter and exitsthe base.

Current enters theemitter and exitsthe collectorthrough R1 to V CC.


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Dynamic Operation

The varying condition of a circuit iscalled its dynamic condition oroperating condition. This occurswhenever an input signal is applied.


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Typical Amplifier with Bias - Dynamic

Dynamic condition: DC Bias with signal added(Varying condition)The output voltage has a much larger voltagechange than the input.


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Typical Amplifier with Bias - Dynamic

Notice the .2V Pk-Pk signal at the input is using the.6v DC as its reference and the output 10V Pk-Pksignal is using the 15V DC as its reference.


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TRANSISTOR AMPLIFIERSNPN Common Emitter Amplifier Operation

The negative alternation of the input signalapplied to the base of the transistor causesforward bias to decrease and collector

current to decrease.The voltage drop across R 3 decreasesbecause I C decreased

The collector voltage (V C) increasesThe bias decrease caused an increase inoutput voltage and produced a 180 phase

inversion



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TRANSISTOR AMPLIFIERSNPN Common Emitter Amplifier Operation

The positive alternation of the input signalapplied to the base causes forward bias toincrease and collector current to increase

The voltage drop across R 3 increasesbecause I C increased

The collector voltage (V C) decreases

The bias increase caused a decrease inoutput voltage and produced a 180 phaseinversion



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NPN with conventionalpower connection V CC to base andcollector usingrespective resistors

(R 1 & R 2 ).


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NPN with alternatepower connection V EEto emitter withcurrent path throughQ1 out the base andcollector through R 1& R

2to ground.


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PNP with conventionalpower connection

-V CC to base andcollector usingrespective resistors(R

1& R

2).


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PNP with alternatepower connection

+V EE to emitter withcurrent path in thebase and collectorthrough R 1 & R 2 out

the emitter toground.


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Characteristic Curve Graph

A transistor CHARACTERISTIC CURVE is agraph plotting of the relationshipsbetween currents and voltages in atransistor circuit.

The graph is then called a FAMILY ofcurves.


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Characteristic CurveGraph

Graph shows basecurrent (I B) changesvs. collector current(I C).

Graph shows achange in V CC vs. I C


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Amplifier Gain A ratio of the change inoutput to the change in input expressed asa formula:


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Collector Load Resistor Changes

Increasing the resistance of R3 will cause a

corresponding increase in the amount of changein collector voltage and increase in voltage gain.

Gain is directly proportional to the resistancevalue of R3.


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Increasing R2 to 6K changes the load lineand gain increasesfrom 10V to 12.5 volts.

Point B-Saturation


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TRANSISTOR AMPLIFIERSCommon Emitter Amplifier Class A

Class A amplifiershave an exact

reproduction of theinput in the output.Conducts 100% ofthe timeThe collector currentwill flow for 360degrees of the input

signal


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Class A Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA80 uA90 uA

Saturation

Cutoff

Q-Point



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TRANSISTOR AMPLIFIERSCommon Emitter Amplifier Class AB

Class AB amplifiershas some amplitude

distortion andconducts 51% to99% of the time.


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Class AB Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA

80 uA

90 uA

Saturation

Cutoff

Q-Point



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TRANSISTOR AMPLIFIERSCommon Emitter Amplifier Class B

Class B amplifiers hasamplitude and

crossover distortion.Conducts 50% of thetime.

The collector currentwill flow for 180degrees of the inputsignal.

l l f


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Class B Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA

80 uA

90 uA

Saturation

Cutoff

Q-Point


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Fidelity The degree to which a deviceaccurately reproduces at its output thecharacteristics of its input signal.

Class A has the best fidelity

Efficiency The ratio between theoutput signal power and the total inputpower.

Class C has the best efficiency.



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Amplitude Distortion The result ofchanging a waveshape so its amplitudeis no longer proportional to the originalamplitude.

Amplitude distortion caused by too largeinput signal, excessive bias, orinsufficient forward bias.



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TRANSISTOR AMPLIFIERSClass of Operation Chart



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TRANSISTOR AMPLIFIERSCommon Base Amplifier

The common base amplifier is also known as thegrounded base amplifier.

The common base amplifier has a voltage gain

greater than one, but it has a current gain lessthan one.

It is normally characterized by a very small inputimpedance and a high output impedance like thecommon emitter amplifier.

The input signal is in phase with the outputsignal.



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R 1 provides forwardbias for the emitter-base junction

R 2 aids in developingforward bias

R 3 is the collector loadresistorR 4 develops the inputsignal



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C1 places the base at AC ground

C2 is the input couplingcapacitor

C3 is the Outputcoupling capacitor

Q1 NPN transistor



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TRANSISTOR AMPLIFIERSCommon Base Amplifier A positive alternationapplied to the emitter of thetransistor decreasesforward bias and causesemitter current to decrease.

A decrease in emittercurrent results in a decreasein collector current. Adecrease in I C decreases E R3 ,

causing V C to become morepositive. The collectorwaveform is an amplifiedreproduction of the positiveinput alternation .


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TRANSISTOR AMPLIFIERSCommon Collector Amplifier

Common collector amplifier has a largecurrent and power gain, excellent stabilityand frequency response.

The output impedance of this circuit isequal to the value of the emitter resistor,this circuit is used for impedance matching.

The input and output signals are in phase.



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Uses degenerative or negative feedback.

Degenerative feedback is the process of

returning a part of the output of anamplifier to its input in such a manner thatit cancels part of the input signal.

As a result, the common collector amplifierhas a voltage gain of less than 1.



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R1 Determines amplifier forward biasR2 Aids in determining forward bias

R3 Emitter load resistor-develops theoutput signal & degenerative feedback C1 Input coupling capacitor

C2 By-pass capacitor, places collectorat AC ground

C3 Output coupling capacitor

Q1 Transistor amplifier



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As the voltage on thebase goes in a positivedirection, the voltageon the emitter goes in apositive direction.This positive voltagereverse biases thetransistor decreasing I Cresulting in an increasevoltage drop across thetransistor.



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As the voltage dropacross the transistorincreases the voltagedrop across the loadresistor R 3 decreases,thus gain less than one.


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Appraisal



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1. In the common emitter configuration,the input is applied to the _______ and theoutput is taken from the ______ .

a. emitter; collector

b. base; collector

c. emitter; base

d. base; emitter



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2. What is the purpose ofresistors R1 and R2?

a. Amplify the input signal

b. Develop the outputsignal

c. Develop forward bias

voltage for Q1d. Block DC from the baseof Q1



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3. In a common collector amplifier,degenerative feedback is ______ out ofphase with the input signal.

a. 0 degrees

b. 90 degrees

c. 180 degrees

d. 270 degrees



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4. The common base amplifier has avoltage gain ______, but a current gain______ .

a. less than one, less than one

b. greater than one, less than one

c. less than one, greater than one

d. greater than one, greater than one



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5. In the common emitterconfiguration, R3primarily affects

a. gain.

b. forward bias.

c. degeneration.

d. temperaturestabilization.



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6. What is the purpose of R4 inthe common base amplifier?

a. Couple the output signal

b. Develop the input signal

c. Develop the output signal

d. Keep the base at AC ground



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7. In the amplifier circuitshown, the purpose of C2 is to

a. couple the output signal.

b. develop the output signal.

c. place the collector at ACground.

d. provide regenerativefeeback.



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8. The amplifiers class of operation isdetermined by

a. fidelity.b. efficiency.

c. output waveform.

d. amount of forward bias.


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10. In the transistor amplifiershown, what is the phaserelationship between the input

and output signals?a. 0 degree phase shift

b. 90 degree phase shift

c. 180 degree phase shift

d. 270 degree phase shift


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TRANSISTOR AMPLIFIERTEMPERATURE STABALIZATION


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TEMPERATURE STABALIZATION


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Negative Temperature Coefficient

Transistors have a negative temperaturecoefficient

This means that as temperature increasesthe resistance of the transistor decreases.



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Collector Current (I C) vs.Temperature GraphNon-stabilized circuits

As temperatureincreases I C increases

due to the resistance ofthe transistor decreasing.

This causes the

transistor I C to moveabove its operating

point.


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TEMPERATURE STABALIZATIONSwamping Resistor Stabilization



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TEMPERATURE STABALIZATIONSwamping Resistor Stabilization

Placing a resistor (R 3) in the emitter fortemperature stabilization is referred to as a

Swamping resistor. Using swampingresistor (R

3) for temperature stabilization

results in degeneration feedback.

An increase in I C flows through the emitterresistor and develops an increase in voltageon the emitter.

This voltage opposes forward bias andreduces I B and I C.



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TEMPERATURE STABALIZATIONSwamping Resistor with Bypass

Capacitor



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TEMPERATURE STABALIZATIONSwamping Resistor with Bypass Capacitor

C2 is referred to as the emitter bypasscapacitor.

By placing a large capacitor (C 2 ) acrossR 3 , a signal ground is established andcompensates for signal degeneration



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TEMPERATURE STABALIZATIONThermistor Stabilization



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TEMPERATURE STABALIZATIONThermistor Stabilization

A thermistor has a negative temperaturecoefficient of resistance.Bias is established by R 1 and R T1 the

thermistor. As temp. increases, resistance of R T1decreases, causing bias and I C to decreaseThis compensates for the change in I

Cdue

to temp. variations.


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Reverse Bias Diode Stabilization

Used to reduce the effects of I CB on collectorcurrent.

As the reverse current of CR 1 increases, it willcause a larger voltage drop across R 1 .

This will reduce the voltage across the base-emitter junction (V EB), causing base current to

decrease, causing collector current willdecrease.


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TEMPERATURE STABALIZATIONDouble Diode Stabilization



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Double Diode Stabilization

Forward biased diode CR 1 compensatesfor changes in the resistance of the forwardbiased emitter-base junction due totemperature.

The reverse biased diode CR 2compensates for the effects of I CB in thereverse biased collector-base junction.



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TEMPERATURE STABALIZATIONHeat Sink

Heat sinks dissipate heat generated by high currentthrough transistors

The transistor is connected directly to the heat sink andthe fins dissipate the heat away from the junctions.


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TEMPERATURESTABILIZATION

Appraisal

TEMPERATURE STABILIZATION


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11. The negative temperature coefficientof a thermistor means that as temperatureincreases resistance ________ .

a. increases

b. decreases

c. Remains the same



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12. Using the swamping resistor forthermal stability, what type of feedback isdeveloped to control the amplifier?

a. Regenerative

b. Degenerative

c. Positive feedback



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13. Which of the following devices is usedto dissipate heat into the air?

a. Heat sink

b. Circuit board

c. Swamping resistor

d. Coupling capacitor


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TRANSISTOR AMPLIFIERCOUPLING

TRANSISTOR AMPLIFIER COUPLING


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Purpose:

To achieve the high gain needed

by most circuits, amplifiers areconnected in series (or cascadedtogether) to form cascadeamplifiers. The signal is couplefrom one amplifier stage toanother.



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Low gain amplifiers do not amplify theinput signal enough to be of practicaluse. Using cascade amplifiers, highgain is achieved without incurringdistortion.

The overall gain of a cascade amplifieris equal to the product of theindividual gains, or: A V(TOTAL) = A V1 x

A V2 x A V3

Cascade Amplifier Voltage Gain


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There are numerous types of circuitsused to connect (or couple) oneamplifier to another. The mostcommonly used are:

Direct Coupling

RC Coupling

LC Coupling

Transformer Coupling


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LOW FREQUENCY GAIN LOSS


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Three types of components cause lowfrequency gain loss in a cascadeamplifier: coupling capacitors, inductorsand transformers.

Coupling Capacitors cause low frequency gainloss because of their high capacitive reactance atlow frequencies.

Inductors and Transformers cause low frequencygain loss at low frequencies for the oppositereason. They have low reactance at lowfrequencies.

HIGH FREQUENCY GAIN LOSS


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Components that cause highfrequency gain loss are transistors,

inductors and transformers.

Stray capacitance caused byvarious wires and components willalso cause high frequency gain loss.

Transistors Interelement CapacitanceHIGH FREQUENCY GAIN LOSS


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Transistors - Interelement Capacitance

The emitter-base and collector-base junctions forman effective capacitance at high frequencies.

The signal bypasses the transistor thus preventingamplification.

Interelement capacitance causes loss of signal at

high frequencies. HIGH FREQUENCY GAIN LOSS


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Inter-winding Capacitance

High frequency loss is due to inter-winding capacitance in which the

individual coils can act as capacitors.Only occurs at very high frequencies.

HIGH FREQUENCY GAIN LOSS


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Stray Capacitance

Capacitance which exists betweencircuit components and wiring. Thestray capacitance between two

conductors could cause output signalamplitude to decrease.

Stray capacitance is not normally a

problem in audio amplifiers. Thehighest frequency involved is about20kHz and the capacitive reactance isstill high.


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DIRECT COUPLING


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Has a poor highfrequency response dueto interelementcapacitance of

transistors.R3 - Collector loadresistor for Q 1 and the

base bias resistor for Q 2 .

DIRECT COUPLING


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Observe the loss ofsignal at the highfrequencies which is

due to thetransistors inter-elementcapacitance.

Frequency Response Graph

high frequencyloss

DIRECT COUPLING


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When amplifying AC signals, direct

coupling isnormally used toamplify the audiorange offrequencies (20Hzto 20kHz).

Frequency Response Graph

high frequencyloss


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Resistive-Capacitive (RC)


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Has a poor highfrequency responsedue to transistorinterelementcapacitance.

Poor low frequencyresponse due to X Cof the capacitor.

Resistive-Capacitive (RC)


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Low frequency lossdue to the highreactance of C 1 , C 2

and C 3High frequency lossdue to theinterelementcapacitance of Q 1and Q 2

low frequencyloss

high frequencyloss

Frequency Response Curve

Impedance Coupling (LC)


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LC coupling is used toamplify much higherfrequencies.

R L

is replaced with aninductor.

The gain of theamplifier is nowdetermined by theinductors inductivereactance (X L).



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The main disadvantage ofimpedance coupling isthat it is limited to highfrequency use.

The reactance of theinductor at lowfrequencies is not largeenough to produce goodvoltage gain.



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Loss of amplitude atlow frequency is dueto low reactance of theinductor.

Loss of amplitude athigh frequency is dueto inter-elementcapacitance of Q 1 andQ 2.

low frequencyloss high frequency

loss



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The specific point (inthis case 30MHz) that

the amplifier will peakat is determined bythe resonantfrequency of the LC

circuit.

low frequencyloss high frequency

loss



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The advantages of transformer coupling isthat it provides isolation between stages sothat one stage does not feedback andinterfere with another stages, and is also usedfor impedance matching between stages.



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Has a poor lowfrequency responsedue to low XL of

transformer windings.Transistor interelementcapacitance and inter-winding capacitance ofthe transformer causeshigh frequency loss.


low frequency

losshigh frequency

loss


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TRANSISTOR AMPLIFIERCOUPLING

Appraisal



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14. What type of amplifier coupling is usedthat can amplify DC voltages as well as ACsignals?

a. LC

b. RC

c. Direct

d. Transformer


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a. C1, C2, and C3b. R1 and R4

c. R2 and R5

d. Q1 and Q2

16. In the RC coupled amplifier, the lowfrequency gain loss is due to the reactance ofcomponents



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a. C1, C2, and C3b. R1 and R4

c. R2 and R5

d. Q1 and Q2

17. In the RC coupled amplifier, the highfrequency gain loss is due to the inter-elementcapacitance of components



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a. LC

18. In the following figure, what type ofcoupling is used between Q3 and Q4?

b. RC c. Direct d. Transformer

Appraisal Answers1 B 10 A


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1. B

2. C3. C

4. B

5. A

6. B

7. C8. D

9. B

10. A

11. B12. B

13. A

14. C

15. D

16. A17. D

18. A


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