Chapter 3

Pulse Width

Modulator

Pulse Width Modulator

3-1: Curriculum Objectives

1. To implement the pulse width modulator by using µA741.

2. To understand the characteristics and basic circuit of LM555.

3. To implement the pulse width modulator by using LM555.

4. To measure and analyze the pulse width modulation circuit.

3-2: Curriculum Theory

Pulse width modulation (PWM) is a modulation method in between the digital and analog,

which can be used to process the digital and analog data transmission. The amplitude of pulse width

modulator is fixed, but the pulse width will be varied and controlled by the input audio

signal amplitude. If we control the time variation of the electric level, then this means that we can

control the width of the pulse. When the amplitude of the audio signal is getting larger, then the

pulse width will become wide; on the' other hand, when the amplitude of the audio is getting

smaller, then the pulse width will become narrow. Therefore, the PWM can be applied in the fast

and slow of the rotation rate of the motor, the strong and weak of the light source of the light

bulb and so on. The relationship between audio and pulse width modulation signal is shown in

figure 3-1.

Generally, pulse wave modulation can be classified as pulse amplitude modulation (PAM), pulse

posit ion modulat ion (PPM), pulse width modulation (PWM) and so on. Table 3-1 shows the

comparison between each modulations and figure 3-2 shows the output characteristics diagram of

PAM, PPM and PWM modulations.

Figure 3-3 is a square wave oscillation circuit, the output signal pulse width is controlled by R2,

C2 and V in(+) input terminal voltage. The op-amplifier μA741 is the comparator in this circuit.

The Vin(+) input (pin 3) reference voltage is decided by the resistor R1 and variable resistor VR1. R2

and C2 are constructed to become a charge/discharge path. When no signal supply to the audio

signal input terminal, if we adjust VR1, then the Vin(+) input terminal operation voltage will

change, which means the reference voltage of comparator will change, thus, the output signal of

pulse width will also change too.

Figure 3-1 Signal waveforms of audio signal and PWM signal.

Table 3-1 Comparisons between three different types of modulations.

Modulation

Types

Features Pulse

Amplitude

Pulse

Width

Pulse

Interval

PAM

Pulse amplitude will be varied

with the amplitude of the input signal.

Varied

Constant

Width

Changeless

PPM

Pulse position will be varied

with the amplitudeof the input

signal.

Constant

Amplitude

Constant

Width

Varied

PWM

Pulse width will bevaried with

the amplitudeof the input signal.

Constant

Amplitude

Varied

Changeless

Figure 3-2 Output characteristics diagram of PAM, PPM and PWM modulations.

Figure 3-3 Circuit diagram of PWM by using µA741.

Figure 3-4 Waveforms diagram of the charge and discharge of uA741.

If the VR1 is fixed, it means that the operation voltage of Vin(+) input terminal is fixed. If input an

audio signal to the audio signal input terminal, then the audio signal voltage will add to the

operation voltage of the Vin(+) input terminal. Besides, by following the path of charge and

discharge of Rand C2, the operation voltage of V in(-)will change as well, as shown in figure

3-4. However, when we change the bias point by tuning the variable resistor VR1, we can change

the level and the width of the output square wave of Vin(+) and Vin(-) at the same time. At this

moment, the reference voltage of the comparator will be varied and controlled by the voltage of the

audio signal. Therefore, the output signal of pulse width will also change with respect to the voltage

of input audio signal,then the pulse width modulation signal is produced.

The circuit diagram of LM555 astablemultivibrator is shown in figure 3-5. In figure 3-4, the circuit

can be divided into 5 important parts, which are lower comparator, upper comparator, flip-flop

(FF), discharge transistor and output driver. If the controlled voltage terminal (pin 5) does not input

any signal, then the upper comparator reference voltage is 2VCC/3 and the lower comparator

reference voltage is VCC/3. If we add the controller voltage to the control voltage terminal (pin

5), the comparator reference voltage can be externally controlled. When the controlled voltage

termingdoes not use, then we can make the controlled voltage terminal connects with a capacitor

0.01 μF to the ground to avoid the interference of noise.

Figure 3-5 Circuit diagram of LM555 astablemultivibrator.

Figure 3-5 is the circuit diagram of astablemultivibrator by using LM555 IC. The output signal of

this circuit is a square wave. The oscillation frequency is determined by R1, R2 and C1. The charge

time (t1) of capacitor 0.693 x(R1+R2) x C1 ; the discharge t ime (t 2) of capacitor C1 is

0.693 x R2 xC1. So the period T is the charge time t1 plus the discharge time t2 equa ls t o

0 . 693x( R 1 + 2xR 2 ) x C 1 . Figur e 3- 6 sho ws t he o utput waveforms of LM555

astablemultivibrator at different points.

Figure 3-7 is the circuit diagram of monostablemultivibrator by using LM555 IC. When the trigger

input changes from high (+12 V) to low (0 V), the output terminal will produce a pulse. This pulse

width T is determined by R1 x C1 actually is approximately 1.1 x R1 x C1. If R1 = 10 kΩ and

C1= 0.01 μF, then the pulse width is about 110 µs. If the frequency is less than 9.1 kHz at the

trigger signal input terminal (pin 2), (refer to the waveforms of astablemultivibrator in figure 3-6),

then the output will be the 50 % duty cycle pulse signal. The audio signal is inputted by the

controlled voltage terminal. Therefore, this will produce the PWM signal.

Figure 3-8 is the circuit diagram of PWM by using two LM555 ICs, which U, which U1 is the

astablemultivibrator and U2 is the monostablemultivibrator. By combining these two parts, we will

obtain a PWM circuit. Monostablemultivibrator (U2)needs the trigger pulse from the

astablemultivibrator (U1output terminal (pin 3), the audio signal is inputted at the controlled voltage (pin

5) of the monostablemultivibrator (U2). The PWM signal is outputted at the output terminal (pin 3)

of the monostablemultivibrator.

Figure 3-6 Output waveforms of LM555 astablemultivibrator at different points.

Figure 3-7 Circuit diagram of monostablemultivibrator by using LM555 IC.

Figure 3-8 Circuit diagram of PAM by using two LM555 ICs.

3-3: Experiment Items

Experiment 1: µA741 pulse width modulator

1. Refer to figure 3-3 or figure DCT3-1 on GOTT DCT-6000-02 module, let J1 be open circuit,

that means R1 is unused.

2. Adjust the variable resistor VR1 so that Vin(+) input terminal voltage is 0 V. Then let J1 be short

circuit, that means let R1 is used.

3. At the audio signal frequency input terminal (Audio I/P), input a 3 V amplitude and 500 Hz

frequency waveform.

4. By using oscilloscope, observe on the signal waveform of audio signal. Input terminal and output

terminal (pin 6). Then record the measured results in table 3-2.

5. Let J1 be open circuit, then record the audio input signal. Adjust VR1 so that Vin (+) voltage of

input terminal is 6 V.

6. Let J1 be short circuit, that means let R1 is used. Then input the audio signal terminal into the

original audio signal.

7. By using oscilloscope, observe on the signal waveforms of the audio signal input terminal

and output terminal (pin 6). Then record the measured results in table 3-2.

8. Let J1 be open circuit, that means R1 is unused and record the audio input signal. Adjust VR1 so

that Vin (+) voltage of input terminal is -6V.

9. Let J1 be short circuit, then input the original audio signal into the audio signal input

terminal.

10. By using oscilloscope, observe on the signal waveforms of the audio signal input terminal and

output terminal (pin 6). Then record the measured results in table 3-2.

11. Let J1 be open circuit and record the audio input signal. Adjust VR1 so that Vin(+) voltage of the

input terminal is 0 V, then let J1 be open circuit.

12. Change the audio signal amplitude to 5 V, the others remain the same. Repeat step 4 to step 10

then record the measured results in table 3 -3.

Experiment 2: LM555 pulse width modulator

1. Refer to figure 3-8 or figure DCT3-2 on GOTT DCT-6000-02 module.

2. By using oscilloscope, observe on the test point TP3 and the output Signal waveform, at the

same time adjust variable resistor VR1 until when the square wave signal of test point TP3 at

differences voltage level, the output square wave signal has different pulse width. (i.e. different

duty-cycle).

3. At the audio signal input terminal (Audio I/P), input a 2.5 V amplitude and 1 kHz frequency

square wave. Then record the measured results in table 3-4.

4. By using oscilloscope, observe on the output signal waveforms of the discharge capacitor TP1,

critical point TP2, trigger signal TP3, critical point of the discharge capacitor TP4, and PWM

O/P.

5. By using oscilloscope and switching to DC channel, observe on the output signal waveforms

and record the measured results in table 3-5.

6. Change the input signal to triangular wave, the others remain the same, repeat step 5.

7. Change the input signal to sinusoidal wave, the others remain the same, repeat step 5.

8. Change the input signal amplitude to 1.5 V, the others remain the same, repeat step 6 to step 7,

then record the measured results in table 3-6.

9. Repeat step 3 to step 5, then record the measured results in table 3-7.

3-4: Measured Result

Table 3-2 Measured results of µA741 pulse width modulator.

(Vm = 3V, fm = 500 Hz)

DC Bias Voltage at

Vin (+)

Input Signal Waveforms Output Signal Waveforms

0V

6V

-6V

Table 3-3 Measured results of μA741 pulse width modulator.

(Vm =5 Vin, fm = 500 Hz)

DC Bias Voltage

at Vin (+)

Input Signal Waveforms Output Signal Waveforms

0V

6V

-6V

Table 3-4 Measured results of LM555 pulse width modulator. (Vm = 2.5 V, fm = I kHz)

Input Signals Input Signal Waveforms

Square Wave

Triangular wave

Sinusoidal Wave

Table 3-5 Measured results of LM555 pulse width modulator.

(Vm= 2.5 V, fm=1 kHz Square Wave)

Input Signals Input Signal Waveforms

Square Wave

Triangular Wave

Sinusoidal Wave

Table 3-5 Measured results of LM555 pulse width modulator.

(Vm = 2.5 V, fm =1 kHz Triangular Wave)

Test Points Output Signal Waveforms

TP1

TP2

TP3

TP4

PWM

O/P

Figure 3-5 Measured results of LM555 pulse width modulator. (Continue)

(Vm = 2.5 V, fm =1 kHz Sinusoidal Wave)

Test Points Output Signal Waveforms

TP1

TP2

TP3

TP4

PWM

O/P

Table 3-6 Measured results of LM555 pulse width modulator.

(Vm = 1.5 V, fm = 1kHz)

Input Signals Input Signal Waveforms

Square Wave

Triangular Wave

Sinusoidal Wave

Table 3-7 Measured results of LM555 pulse width modulator.

(Vm =1.5 V, fm = 1 kHz Square Wave)

Test Points Output Signal Waveforms

TP1

TP2

TP3

TP4

PWM

O/P

Table 3-7 Measured results of LM555 pulse width modulator. (Continue)

(Vm =1.5 V, fm =1 kHz Triangular Wave)

Test Points Output Signal Waveforms

TP1

TP2

TP3

TP4

PWM

O/P

3-5: Problems Discussion

1. What are the functions of VR1 in figure 3-3 and figure 3-8?

2. If we change the capacitor C6 to 0.1 µF in figure 3-8, the others remain the same, does the

output still show the PWM waveform? Why?

3. For output voltage polarity, what are the differences of PWM signal between experiment 1

and experiment 2?