GB2100948A

GB2100948A - Push-pull amplifier

Info

Publication number: GB2100948A
Application number: GB08216719A
Authority: GB
Inventors: David A Hotvet
Original assignee: Telex Communications Inc
Current assignee: Bosch Security Systems Inc
Priority date: 1981-06-22
Filing date: 1982-06-09
Publication date: 1983-01-06
Also published as: JPS583405A; FR2508252A1; DE3222931A1; DK257282A

Abstract

A class B power amplifier of the dual-complementary symmetry type is provided with a low impedance voltage multiplier input driver stage (14, 36, 44, 45) to materially reduce the "dead band" and thereby reduce "cross-over" distortion. Internal D.C. feedback (48,49) reduces the input voltage swing required and the non-linearities that may occur before the gain is reduced by overall voltage shunt feedback (18, 19, 50) which stabilizes transresistance. This reduces harmonic distortion. <IMAGE>

Description

SPECIFICATION Power amplifier This invention reiates to improvements in dualcomplementary symmetry class B power amplifier and is more particularly directed to reducing the cross-over distortion typically present in such amplifiers.

Prior art power amplifiers of the class with which my invention is concerned have exhibited undesirable operational characteristics which have reduced their effectiveness as well as the quality of reproduction of input signals.

The dual-complementary symmetry class B is sufficiently well known in the prior art as not to require a specific reference to the numerous sources of information regarding such apparatus, but as an example. Reference is made to US patents 3 374 441, 3 900 790 which relate to complementary symmetry class B type amplifiers but fail to relate or suggest the novel and unobvious relationship of elements as will be set forth in connection with the detailed description of the illustrative embodiment set forth below.

The advantages and purposes of my invention are provided by the combination of the insertion of a low impedance voltage multiplier between a dualcomplementary symmetry class B transistor output stage and an amplifier connected to a source of a signal, typically an audio frequency signal. The novel and unobvious combination results in a dramatic reduction in the "dead band" in the class B amplification phenomenon. Additional internal feedback in the output stage of the class B amplifier is utilized to reduce non-linearities as may exist and an overall voltage feedback is utilized to stabilize the overall amplifier with regard to its transresistance. A further DC feedback may be utilized to maintain the desired level of output voltage.

It will be noted that the introduction of the voltage multiplier results in a very low AC impedance at the input ofthe class B dual-complementary symmetry circuitry providing the substantial reduction in the "dead band" while maintaining a substantial margin of safety in preventing conduction of both of the first stage transistors in the dual-complementary symmetry amplifier.

The invention will now be described by way of example with reference to the accompanying drawings wherein: Fig. 1 is an electrical schematic drawing of an audio frequency power amplifier embodying the principles of my invention; Fig. 2 is an enlarged fragmentary representation of a portion of Fig. 1; and Fig. 3 is a similarly enlarged schematic representation of a prior art circuit.

Referring to Fig. 1 of the drawings, there is shown a dualcomplementary symmetry class B power amplifier 10 including driving stages, having an input terminal 11,adapted for connection to a suitable source or audio signal and having an output connected to a receiver, 41.

The lower left portion of Fig. 1 includes a signal amplifying transistor 14, having base emitter and collector electrodes 15, 16 and 17; a voltage multiplying transistor 36, having emitter, base and collector electrodes 37,38 and 39; and first and second pairs of transistors 28 and 32 and 20 and 24 connected in dual - complementary symmetry configuration.

Transistor 20 includes emitter, base and collector electrodes 21,22 and 23; transistor 24 includes collector, base and emitter electrodes 25, 26 and 27; transistor 28 includes emitter, base and collector electrodes 29,30 and 31; and transistor 32 includes emitter, base and collector electrodes 33,34 and 35.

Transistors 20 and 28 are of opposite conductivity type with respect to transistors 24 and 32.

Transistor 14 is shown having its base electrode 15 connected to input terminal 11 through capacitor 12 and resistor 13. Base electrode 15 is also connected to ground through resistor 18, having capacitor 19 connected in parallel therewith. Emitter electrode 16 on transistor 14 is connected to ground and collector electrodes on transistor 14 is connected to positive terminal 40 through resitors 43,44 and 45.

Transistor 36 is shown having collector electrode 39 connected to base electrode 30 on transistor 28 and to the junction between resistors 43 and 44; base electrode 38 connected to the junction between resistors 45 and 44; and its emitter 37 connected to base electrode 34 on transistor 32 and to collector electrode 17 on transistor 14.

Transistor 28 is shown having its collector electrode 31 connected to positive terminal 40 through resistor 47 and to base electrode 22 on transistor 20 and its emitter electrode 29 connected to emitter electrode 33 on transistor 32, to ground through resistor 49 and capacitor 51 and to collector electrodes 23 and 25 on transistors 20 and 24 through resistor 48.

Transistor32 is shown having its collector elec trode, 35, connected to base electrode 26 on transistor 24 and to ground through resistor 46; and its emitter electrode, 33, connected to collector electrodes 23 and 25 on transistors 20 and 24 through resistor 48.

Transistor20 is shown having its emitter electrode 21 connected to positive terminal 40, and its collector electrode 23 connected to receiver41 through capacitor 52. Transistor 24 is similarly shown having its collector electrode connected to receiver41 through capacitor 52 and to base electrode 15 on transistor 14 through resistor 50. Emitter electrode 27 on transistor 24 is connected to ground.

Table of values The following is atable of values of the components shown in Fig. 1; Reference Character Description 13 12kOhmReisitor 18 100k Ohm Resistor 43 12k Ohm Resistor 44 2.2kOhm Resistor 45 4.7k Ohm Resistor 46 330k Ohm Resistor 47 330k Ohm Resistor 48 33k Ohm Resistor 49 i2kOhmResistor 50 560k Ohm Resistor 19 22p F Capacitor 14 Type BC 239C Transistor 20 Type 2N6003 Transistor 24 Type 2N6012 Transistor 28 Type2N6012 Transistor 32 Type 2N6003 Transistor 36 Type 2N6012 Transistor Fig. 2 illustrates the voltage multiplier represented by a resistance 64 in a voltage divider network of resistors 63, 64 and 65 with resistor 64 connected to the base electrodes 57 and 61 oftransistors 55 and 59 respectively that are shown in configuration simi lay to that of Fig. 1, with respect to transistors 28 and 32. Transistors 55 is shown having emitter, base and collector electrodes 56, 57 and 58 and transistor 59 includes emitter, base and collector electrodes 60, 61 and 62 the relationship to the illustration of Fig. 1 should now be apparent.

Fig. 3 is illustrative of a typical prior art type of driver for a dual-complementary symmetry class B amplifier and includes a transistor70, having emitter, base and collector electrodes 71, 72 and 73, transistor 74, having emitter, base and collector electrodes 75, 76 and 77 and a pair of resistors 78 and 79 connected as shown. The impedance of the equivalent circuit represented by resistor 64 in Fig. 2 may be calculated by use of the formula

and R, Ri. = 11 h )21 for an audio amplifier: he (R111 R2+hie) is approximately 400; h,, is approximately 5,000 R1, (44) = 2.2 k ohms, R2 (45) = 4.7k ohms and R1 11 R2 = 1.5k; R = -203k ohms; and B = 2.2 ohms. Therefore:

ohms.

The low impedance calculated is an approximation of an Ac short circuit. This results in a reduction of the "dead band" of the power amplifier by a factor of 4 and in at least one case, a reduction of 1.2 to 0.3 volts was obtained.

The illustrated embodiment is further provided with an internal DC feedback that is AC coupled to ground and is comprised of a circuit from collector 23 on transistor 20 and collector 25 on transistor 24 through resistors 48 and 49 and capacitor 51. This has the effect of stabilizing the voltage gain of approximately 3.75 as determined by the values of resistors 48 and 49, provides a reduction in the required voltage ofthe signal supplied from the input stage and a further reduction on the nonlinearities associated with a class B type design, this action occurs before the overall feedback of the illustrated amplifier through resistors 50 and 18 is operative.

An amplifier constructed in accordance with the table of values set forth above has been determined to provide a gain of46 which is equal to 34 db and to provide a four volt dc output voltage which will provide substantially equal peak-to-peak output voltage with a typical 8.5 volt power source.

It is contemplated that other variations of the illustrated embodiment may occur to those skilled in the art without departing from the spirit and scope of the invention.

Claims

1. In a dual-complementary symmetry class B amplifier, the combination comprising; an output stage including first and second current controlling means of opposite conductivity type, having input terminals and having output terminals connected to a common load; an input stage including current controlling means having output terminals and input terminals; circuit means connecting the output terminals on said input stage to the input terminals of said first and second current controlling means on said output stage; and circuit means connecting a source of signal to the input terminals of said input stage, said circuit means including low impedance voltage multiplying means.

2. The apparatus of claim 1 in which the low impedance voltage multiplying means includes a current controlling device having input, output and common terminals and first and second impedance means connected intermediate said input terminal and said output and common terminals.

3. The apparatus of claim 2 in which the common and output terminals on the voltage multiplying means are connected intermediate the inputtermi- nals on the output stage and the output terminal is connected to a source of electrical energy.

4. The apparatus of claim 1 in which the output stage is comprised of a first PNP transistor device having base, collector and emitter electrodes and a second NPN transistor device having base, collector and emitter electrodes, the emitter electrodes are interconnected to provide the output terminal and the base electrodes are the input terminals; and the voltage multiplying means is connected intermediate said base electrodes.

5. The apparatus of claim 4 in which the voltage multiplying means comprises a third transistor device having base, collector and emitter electrodes, said emitter and collector electrodes are connected to respective base electrodes on the first and second transistor devices, a first impedance means is connected intermediate said emitter electrode and said base electrode, a second impedance means is connected intermediate said collector electrode and said base electrode and a source of electrical power is connected intermediate the collector electrodes on said first and third transistor devices and the collector electrode on said second transistor device.

6. The apparatus of claim 5 in whichthe first impedance means is of a substantially greater value than said second impedance means.

7 The apparatus of claim 6 in which the first and second impedance means are resistors.