lab8

7/21/2019 lab8

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LABORATORY 8

POWER AMPLIFIER

OBJECTIVES 1. To study Class B power amplifier circuits.2. To observe crossover distortion present in Class B power amplifiers.3. To simulate Class B and Class AB power amplifier circuits using MicroCap

software.4. To design and test DC biasing and frequency response of a Class AB audio power

amplifier.

INFORMATION

1. Power Amplifier Class BClass B amplification involves using a dual voltage power supply along with two powertransistors, an NPN, and its complementary PNP device. Such a circuit is shown in Figure8.1 and its operation could be explained as following:

• In the absence of an input signal, neither transistor conducts; both transistors areoff.

• On the positive half of the input cycle, once the input signal is greater than 0.7 V,Q1 will turn on and current flows as shown in Figure 8.1- a. Notice that the base-emitter voltage of Q1 causes Q2 to be held in the off state since Q2’s base-emitteris reverse biased.

• As the input signal swings into the negative half of its cycle and exceeds 0.7V, Q2is turned on and its base-emitter voltage reverse biases the base-emitter junction ofQ1, turning it off.

a) Positive half cycle operation b) Class B output waveformsFigure 8.1 . Class B power amplifier operation

+

I

_

IVin

Q1

B

ON

C

VoE

OFF

Vee

+

Vcc

+RL

_

_

I

C1

FG

Q2

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Typical output waveforms for both Q1 and Q2 BJTs and a Class B amplifier output areshown in Figure 8.1-b.The time required for the input signal to move from zero volts to +0.7 V or to -0.7 V is thetime during which conduction does not occur, consequently the output sits at zero volts forthis interval, producing what is called crossover distortion . Crossover distortion takes itsname from the dead-time distortion occurring when the input crosses over from -0.7 V to

+0.7 V or from +0.7 V to -0.7 V.

Class B has a very low (zero) Quiescent Current, and hence low standing powerdissipation and optimum power efficiency. However it should be clear that in practiceClass B may suffer from problems when handling low-level signals. In the absence of aninput signal, a Class B power amplifier should have zero volts dc on the output terminalwith respect to ground, if the transistors are well matched. Often, they are not wellmatched, so the student should be aware that it is quite possible to have a dc voltage

present at the output. Some output loads, such as speakers, may be damaged by dc. If suchloads are to be used, they must be capacitively coupled to the output in order to block thedc.

2. Power Amplifier Class AB

Crossover distortion could be eliminated in class AB power amplifiers by the addition ofthe diode circuitry shown in Figure 8.2a.

a) Class AB circuit diagram b) Class AB output waveformsFigure 8.2. Class AB power amplifier circuit

Since the diodes in Figure 8.2-a are on all the time, both Q1 and Q2 are held at the edge ofthe conduction mode by the diode voltages (A small but controlled Quiescent Current).When the input goes either positive or negative, very little voltage is required to put Q1 orQ2 into full conduction.Typical output waveforms for both Q1 and Q2 BJTs and a Class AB amplifier output areshown in Figure 8.2-b.

D2

D1

Vee

C2

Vin

D

R2

Q2

Q1

Vo

FG

Vcc

I

C1

RL

R1

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3. Transistors

You will be using the MJE800 NPN and the MJE700 PNP silicon Darlington pair powertransistors. These transistors are a set of complimentary pair silicon power transistors.Two individual transistors connected in a Darlington configuration in each package will

provide a very large short circuit current gain β which is the product of the two β’s of each

internal transistor. For the transistors used here the manufacturer guarantees a minimum β of 750. The transistor diagrams and package are shown in Figure 8.A and the data sheetsare attached in the appendix section of this manual.

a) MJE800 NPN and the MJE700 PNP diagrams b) PackageFigure 8.3. MJE800 NPN and the MJE700 PNP diagrams and package

Note : Two resistors and a diode are integrated internally in the transistor device’s packageand one of the reasons for including these components is to prevent a thermal run-awayfrom occurring. These internal components are not shown on the circuit diagrams inFigures 8.1 and 8.2 however they should be included in the device model in your circuitsimulation.

PRE-LABORATORY PREPARATION

The lab preparation must be completed before coming to the lab. Show it to your TA forchecking and grading (out of 15) at the beginning of the lab and get his/her signature.

1. Calculations

The purpose of this exercise is to design the output stage of an audio power amplifier thatcould be used with one or more of the earlier circuits to complete a power amplifier. Inyour design set the dual DC power supply to ± 6VDC. The amplifier should deliver

approximately 500 mW of sinusoidal RMS audio power to an 8 Ω load, over the standardaudio range of 20 Hz to 20 kHz. In the laboratory you will use an 8.2 Ω resistor. It willmake the lab a lot quieter! Include the basic power amplifier (Figure 8.1) and the diodecompensated circuit (Figure 8.4) in your pre-lab design and simulation. The Figure 8.4circuit must be designed at the edge of the cut-off region. Since we are using a Darlington

pairs instead of single NPN and PNP transistors, the diode compensation group shouldcontain three diodes instead of two, as it is shown in Figure 8.4. The class AB amplifiershave a small I BIAS such that the DC quiescent operating point is just into the start of theconducting region. This will prevent a certain amount of cross over distortion.

MJE700

C (2)

B (3)

Q2 R1

10k R2

600

Q2R2 600

D R1

10k

C (2)

MJE800

B (3)

E (1)E (1)

Q1D

Q1

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The class AB circuit must be designed at the edge of the cut-off region. For the circuit inFigure 8.4.calculate the values of the resistors R 1=R 2 for a diode current of I D=5mA.

Figure 8.4. Real class AB power amplifier circuit.

2. MicroCap simulations

2.1. Use MicroCap to determine the DC biasing voltages and currents with no ac signal forthe class B amplifier circuit in Figure 8.1. You should obtain the following informationthrough the MicroCap simulations:

2.1.1. A table with the expected DC voltages when no AC input signal is applied to thecircuit (Table 8.1).2.1.2. An input and output waveforms for a sinusoidal input signal Vin=4V p (peak) atf=1kHz.

2.1.3. The output waveform for input voltage of Vin=8V p (peak) at 1 kHz. Acomparison with the voltages observed in the lab should be made. Watch for anydistortion occurring in the output waveform.

2.2. Using calculated component values for resistors R 1 and R 2.determine the DC biasingvoltages and currents with no ac signal for the class AB amplifier circuit in Figure 8.3.You should obtain the following information through the MicroCap simulations:

2.2.1. A table with the expected DC voltages when no AC input signal is applied to thecircuit (Table 8.2).2.2.2. An input and output waveforms for a sinusoidal input signal Vin = 4V p for f =1kHz.

2.2.3. The frequency response of class AB amplifier from 10 Hz to 100 kHz. Print theBode plots of the voltage gain and phase frequency response and bring these plots to thelaboratory.

MicroCap simulations tips: • To provide a power supply to the circuit use two “Battery” sources from the

MicroCap library. Connect them as Vcc and Vee voltage sources with commonground and set them to a 6VDC.

• You could simulate the Darlington pair as two separate transistors, each with atypical β = 30 (BF parameter) and |V BE | = 0.7 V (VJE parameter). You need to

Q1

Vo

D3

D

R2

FG

D1

C2

D2

Vee

C1

Q2

Vcc

RL

IVin

R1

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include the internal resistors in your circuit diagram. For NPN transistors you coulduse 2N3904 and for PNP – 2N3905 BJT models from MicroCap library. Adjust BFand VJE parameters within the BJT component data description. Because thesetransistors have low output currents, just for the simulations replace the loadresistor R L=8,2 Ω with R L=500 Ω .

• To obtain the values of all the bias currents and voltages on your schematic from

Analysis menu choose the Probe AC mode and click on Node Voltages andCurrents icons on the toolbar.

• For a sine wave signal source use a 1MHz Sinusoidal Source from the Micro–Cap library. Set the AC Amplitude to A= 4(V) in the model description area of the signalsource. Note that A=4V corresponds to V p=4V.

• To obtain an input and output waveforms you must run “TRANSIENT ANALYSIS”.• To obtain the gain and phase frequency response plots for this circuit you must run

“AC ANALYSIS” for frequency range from 10 Hz to 100 kHz. Note : Set parameter P to plot separate diagram for each curve.

EQUIPMENT

1. Digital multimeter (Fluke 8010A, BK PRECISION 2831B).2. Function Generator Wavetek FG3B.3. Digital oscilloscope Tektronix TDS 210.4. PROTO-BOARD PB-503 (breadboard).5. MJE800 NPN and MJE700 PNP Darlington transistors .6. 1N4148 diodes – 3.7. C=47 µF – 2.8. R=8.2 Ω / 2W.

PROCEDURE

1. You are provided with two heat sinks, which should be attached to the transistorsduring the lab exercise. Connect the heat sink which is electrically connected to thecollector of the transistor. Be very careful you do not 'accidentally' short the heatsink to ground, it has the same effect as shorting the collector to ground.Occasionally check the temperature of the heat sink, if you cannot keep your fingerof the heat sink for more than twenty seconds the transistors may be too hot. Shutthe power off and check your circuit.

2. Connect the class B power amplifier shown in Figure 8.5 using MJE800 NPN andMJE700 PNP Darlington transistors instead of single BJTs. Use the R L= 8.2 Ω resistor to simulate the loudspeaker’s load.

3. Use a dual voltage Power Supply and connect its POS terminal as Vcc, NEGterminal as Vee and COM terminal as a common ground. Set the power supplyvoltage to 6V DC. Measure the DC quiescent point values. Compare the voltagesand currents from simulation with the experimental data in a Table 8.1. If your

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results are significantly different (more than 15%) from your simulated values, tryto find out and eliminate the reason for that discrepancy.

Q1 Q2VCE

[V]VBE

[V] IC

[A] VCE

[V] VBE

[V] IC

[A] Simulation

Experiment Table 8.1. Class B power amplifier DC biasing

4. Once you are satisfied that your circuit is biased correctly, then connect the signalgenerator to the input. Set the signal generator to a frequency of 1 kHz. For theinput signal level of Vin = 4Vp sketch the output voltage across the 8.2 Ω load ontop of your MicroCap simulation plot. Compare the simulated and experimentalwaveforms and explain the differences if any.

5. Increase the input sinusoidal voltage until you notice a clipping in the outputvoltage. For these readings you can use the Fluke meter to measure the AC inputcurrent (Fluke measures the RMS value), measure the input voltage after the Fluke

meter (scope) as it is shown in Figure 8.5. For two settings of the input signal below this maximum signal calculate the input AC power P in, the output AC powerPo, the DC input power from the DC supply P DC . Also calculate the AC voltagegain A V [dB] (Equation (8.1)), the AC power gain [dB] (Equation (8.2.)) and theamplifier efficiency η (Equation (8.3)) of the class B power amplifier.

Figure 8.5. Class B power amplifier measurements .

in

oV V

V dB A log20][ = Equation (8.1)

in

o P P

P dB A log10][ = Equation (8.2)

DC

o

P P

=η Equation (8.3)

Record your measurements and calculations in Table 8.2. Determine if your amplifieris capable of delivering 500 mW of audio power without distortion. If your circuit can

C1

FG

Vee

I

I

Vo

o A

CH1

Vin

I

Q2

CH2

DC

RL

Vcc

in

Q1

CH1

CH2

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not deliver this power, do not lay the sole blame on the DC power supply, themaximum current it can deliver is 200 mA.

AC inputmeasurements

AC outputmeasurements

DC inputmeasurements

Gain calculations

Vin [V]

Iin [A]

Pin [W]

Vo [V]

Io [A]

Po [W]

VDC [V]

IDC [A]

PDC [W]

AV [dB]

AP [dB]

η

24

Table 8.2. Class B power amplifier measurements.

6. Connect the class AB power amplifier in Figure 8.4. Use the calculated values ofR 1 and R 2. Repeat the DC biasing measurements from point 2 and collect all datain Table 8.3.

Q1 Q2VCE

[V]VBE

[V] IC

[A] VCE

[V] VBE

[V] IC

[A] SimulationExperiment

Table 8.3. Class AB power amplifier DC biasing

7. Repeat all measurements from points 3 and 4 and collect all data in Table 8.4.AC input

measurementsAC output

measurementsDC input

measurementsGain calculations

Vin [V]

Iin [A]

Pin [W]

Vo [V]

Io [A]

Po [W]

VDC [V]

IDC [A]

PDC [W]

AV [dB]

AP [dB]

η

24

Table 8.4. Class AB power amplifier AC measurements.

8. Determine the frequency response of the AC sinusoidal voltage gain of thecompensated amplifier over the range of 20 Hz to 30 kHz.

Table 8.5 Frequency response of a class AB power amplifier.

f [Hz] V in [V] V o [V] [deg] Av[dB]2050

100200500

8001k5k

10k20k30k

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9. Plot obtained voltage gain and phase data on top of your simulated Bode plots andcompare the results.

REPORT

Your Lab report is due one week later. Please submit it to your TA in the beginning ofyour next lab session.

Note : You must copy/print the Signature and Marking Sheet from your manualbefore coming to the lab session.

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SIGNATURE AND MARKING SHEET – LAB 8

To be completed by TA during your lab session

Student Name:____________________ TA Name:___________________Student # : _____________________

Checkboxes

Task Max.Marks

GrantedMarks

TASignature

Pre-lab completed 15

Class B Amplifier Test completed 10

Class AB Amplifier Test completed 10

Overall Report Preparation 65

TOTAL MARKS 100

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