Cover Image
close this bookRadio and Electronics (DED Philippinen, 66 p.)
View the document(introduction...)
close this folder1. INTRODUCTION
View the document(introduction...)
View the document1.1. A TRIAL TO STATE A DEFINITION OF ELECTRONICS
View the document1.2. A SHORT HISTORY OF ELECTRONICS
View the document1.3. CLASSIFICATION OF ELECTRONIC DEVICES
close this folder2. PRINCIPLES OF RADIO COMMUNICATION UNICATION
View the document2.1. BASICAL IDEAS ABOUT COMMUNICATION
View the document2.2. DEVELOPMENT OF LONG DISTANCE COMMUNICATION
View the document2.3. FIDELITY AND DISTORTION
close this folder3. TRANSDUCERS
View the document(introduction...)
View the document3.1. MICROPHONES
View the document3.2. LOUDSPEAKERS
View the document3.3. THE TELEPHON SYSTEM
View the document3.4. PROBLEM OF FREQUENCY RANGES
View the document3.5. BANDWIDTH
close this folder4. RADIOWAVES
View the document(introduction...)
View the document4.1. ORIGIN OF RADIOWAVES
View the document4.2. PARAMETERS OF ELECTROMAGNETIC WAVES
View the document4.3. PROPAGATION OF RADIOWAVES
View the document4.4. SPECTRUM OF RADIOWAVES AND BANDS OF RADIOWAVES
close this folder5. MODULATION OF RADIOWAVES
View the document(introduction...)
View the document5.1. THE AMPLITUDE MODULATION (AM)
View the document5.2. FREQUENCY MODULATION (FM)
View the document5.3. SIDEBANDS
View the document5.4. TRANSMISSION OF RADIOSIGNALS
close this folder6. RECEPTION OF RADIOSIGNALS (AM - TYPE)
View the document6.1. AERIAL
View the document6.2. THE TUNED CIRCUIT
View the document6.3. INCIDENTAL REMARK ON BLOCK DIAGRAMS
View the document6.4. DETECTOR OR DEMODULATOR
View the document6.5. POWER SUPPLY
View the document6.6. AMPLIFIER
View the document6.7. SUPERHET RECEIVER (the SUPER)
View the document6.8 INCIDENTAL REMARK ON MIXING FREQUENCIES
View the document6.9. CONSTRUCTION OF A SUPERHETRADIO
close this folder7. COMPONENTS OF MODERN RADIO RECEIVERS
View the document7.1.1. HANDLING OF ELECTRONIC COMPONENTS
View the document7.1.2. HANDLING OF PRINTED CIRCUITS
View the document7.1.3. DIFFERENTIATION OF COMPONENTS
close this folder8. PASSIVE COMPONENTS
View the document8.1. RESISTORS ELECTRICAL CHARACTERISTICS
View the document8.2. CAPACITORS
View the document8.3. INDUCTORS
close this folder8.4. COMBINATION OF PASSIVE COMPONENTS
View the document8.4.1. SERIES CONNECTION OF R AND C, OR R AND L
View the document8.4.2. COMBINATION OF L AND C, RESONANT (TUNED) CIRCUITS
close this folder8.4.3. TUNED CIRCUIT CONNECTED TO AN AC-VOLTAGE
View the document(introduction...)
View the document8.4.4.1. QUALITY OF TUNED CIRCUITS
View the document8.4.4.2. BANDWIDTH
close this folder9. ACTIVE COMPONENTS -1- DIODES
View the document9.1. CHARACTERISTICS OF SEMICONDUCTORS
close this folder9.2. THE PN-JUNCTION OR DIODE
View the document(introduction...)
View the document9.2.1. PN-JUNCTION CONNECTED TO VOLTAGE
View the document9.2.2. CHARACTERISTICS OF A PN-JUNCTION OR DIODE
View the document9.2.3. ZENERDIODE
close this folder10. BLOCKS OF RADIOS / -1- / POWER SUPPLIES
View the document10.1. GENERAL CONSIDERATIONS
View the document10.2. TRANSFORMER
View the document10.3. THE RECTIFIERS.
close this folder10.4. SMOOTHING AND FILTER CIRCUITS
View the document10.4.1. THE RESERVOIR CAPACITOR
View the document10.4.2. FILTER CIRCUITS
close this folder10.5. STABILIZATION
close this folder10.5.1. GENERAL REMARKS
View the document10.5.1.1. LOAD VARIATIONS
View the document10.5.1.2. INTERNAL RESISTANCE OF VOLTAGESOURCES
View the document10.5.1.3. PROBLEMS CAUSED BY THE SMOOTHING CIRCUIT
close this folder10.5.5. METHODS OF STABILIZATION
View the document(introduction...)
View the document10.5.5.1. PARALLEL-STABILIZATION
View the document10.5.2.2. SERIES STABILIZATION
close this folder11. ACTIVE COMPONENTS -2- / TRANSISTORS
View the document11.1. CONSTRUCTION OF A TRANSISTOR
close this folder11.2. CHARACTERISTICS OF TRANSISTORS
View the document(introduction...)
close this folder11.2.1 HANDLING OF CHARACTERISTICS OF TRANSISTORS
View the document11.2.1.1. CONSTRUCTION OF THE STATIC-MUTUAL-CHARACTERISTICS
View the document11.2.1.2. CONSTRUCTION OF THE DYNAMIC MUTUAL CHARACTERISTICS
View the document11.2.1.3. CONSTRUCTION OF THE MAXIMUM-POWER-LINE
close this folder12. AMPLIFIERS
View the document(introduction...)
View the document12.1. STRUCTURE OF A CLASS A AMPLIFIER
View the document12.2. FUNCTION OF A SIMPLE CLASS A AMPLIFIER
View the document12.3. ADVANCED CLASS A AMPLIFIER
View the document12.4. STABILIZATION OF THE QUIESCENT VOLTAGE
close this folder13. CLASS B AMPLIFIERS
View the document13.1. LIMITS OF CLASS A AMPLIFIERS
View the document13.2. CLASS B AMPLIFIERS WITH TRANSFORMERS
View the document13.3. CLASS B AMPLIFIERS WITHOUT TRANSFORMERS
View the document13.4. POWER AMPLIFIER WITH COMPLIMENTARY TRANSISTORS.
View the document14. DETECTOR OR DEMODULATOR
View the document15. AGC-AUTOMATIC GAIN CONTROL
View the document16. IF-AMPLIFIERS
View the document17. FEEDBACK
View the document18. OSCILLATORS
View the document19. FREQUENCY CHANGERS MIXERSTAGE
View the document20. DECOUPLING CIRCUITS
View the document21. MATCHING OF AMPLIFIERSTAGES
View the document22. COUPLING OF AMPLIFIERSTAGES
close this folder23. RADIO SERVICING
View the document23.1. IMPORTANCE AND SUBJECT OF FAULT FINDING
View the document23.2. FAULTS AND FAULT FINDING
View the document23.3. FAULT FINDING METHODS
View the document24. THE USE OF THE OSCILLOSCOPE

13.4. POWER AMPLIFIER WITH COMPLIMENTARY TRANSISTORS.

The following drawings show how a complimentary push-pull amplifier is working.


fig. 163

Fig. 163 shows two circuits. The upper one with a NPN transistor will have current through R1 while there is a positive signal at the input. The lower circuit will have current through R2 always when there is a negative input signal.


fig. 164

Fig. 164 shows the two circuits of fig. 163 combined. Now the two resistors are replaced by a single loudspeaker. Both currents - explained in fig. 163 - flow through the loudspeaker so causing an ac-current in the loudspeaker. This fits to our desire, to have a current flowing only if there is a signal voltage different from quiescence.

The big disadvantage of the circuit in fig. 164 is, that there are two batteries necessary for it.


fig. 165

Fig. 165 shows a circuit which gets rid of that disadvantage.

If there is a positive input signal, there will flow a current via the NPN transistor, the capacitor will have a capacity big enough so that it can be charged completely only during the longest possible half-waves (at lowest frequencies).


fig. 166a


fig. 166b


fig. 166c

So we will find after the positive halfwave at the capacitor a full positive charge as shown in fig. 166b. With a negative input signal the PNP-transistor gets conducting and there will flow a current - originated from the capacitor as a voltage-source - through the PNP-transistor and the loudspeaker as shown in fig. 166c.

The current flowing now through the loudspeaker is flowing backward - which means: there is flowing an ac-current through the loudspeaker.

Improvements of the simple circuit of a complimentary push-pull amplifier.

The circuit derived in fig. 166 has still two main problems which must be solved before it can be used for a receiver.

PROBLEM 1:

It is easy to see, that the voltage at the base of Tr1 can never reach a value higher than that of the supply voltage. But supposed Tr1 is made conducting (by a relatively high base-current) then the voltage at the emitter of Tr1 has a potential which is just 0.2 Volts lower than the supply-voltage.


fig. 167

As we know: to inject a base-current to Tr1 we need at least 0.7 V between Base and Emitter of Tr1.

This shows: With circuit shown in fig. 167 it will never be possible to make Tr1 fully conducting.

Consequence:

We have to find a method to supply point A with a potential about 0.6 V higher than the supply voltage. One possibility to achieve this is shown in fig. 168. But this is a very complicated and inconvenient way, because we need an additional cell for it.


fig. 168

Practical solution:

The most common way of solving that problem is the so called BOOTSTRAP CAPACITOR C2 shown in fig. 169.


fig. 169

Its function is like that: At “NO” input signal the voltage at point A will be about half of the supply voltage because there is flowing a medium current. The voltage at point C through the two collector resistors of Tr.3 will be about 75 % of the supply voltage. If now a negative signal occurs at the input the voltag at point A will be raised. Of course the voltage at point B is increased as well (base-emitter-voltage at Tr1 maximum 0.6 V). So C2 has a rather big capacity the voltage at point C reaches values higher than the full supply voltage.

PROBLEM 2:

If the bases of Tr.1 and Tr.2 are connected like shown in fig. 170 we would face a so called CROSSOVER-DISTORTION as shown in fig. 170 because it takes always at least 0.7V of voltage change until the transistors are starting to get conducting. A first solution would be to insert a resistor R2 as shown in fig. 171. But the dimensioning of R2 is extremly sensitive because:

- if it is too small there will be still a crossover distortion, and
- if it is too big there will be a lot of losses or even a short circuiting through Tr.1 and Tr.2


fig. 170


fig. 171

A better solution is to fit in two diodes like shown in fig. 172. In order to have an automatic adjustment in case of heating up of the transistors, very often is used an additional thermistor connected in parallel to the diodes as shown in fig. 163. This thermistor is mostly fixed to the heat sink of the transistor.

So transistors heat up, the thermistor which will cause a drop of its resistance and therefore a voltage-drop across the two diodes. This again causes a decreasing base-current for both transistors Tr.1 and Tr.2.


fig. 172


fig. 173

In order to avoid lude a crossover-distortion as far as possible, in practical circuits we find at last an adjustable resistor connected in series to our diodes. This adjustable resistor is used to preset the voltage across the two bases exactly to a condition where at quiescence a small collector current just starts to flow.


fig. 175

A rather advanced power amplifier of the COMPLIMENTARY PUSH PULL TYPE is shown in fig. 176.


fig. 176

CHECK YOURSELF:

1. Explain how a Push-Pull amplifier with transformers is working.
2. Explain how a complimentary Push-Pull amplifier is working.
3. Explain what each component in fig. 176 is useful for.