GB2261785A - Reducing amplifier distortion by comparison of input and feedback from output - Google Patents
Reducing amplifier distortion by comparison of input and feedback from output Download PDFInfo
- Publication number
- GB2261785A GB2261785A GB9203043A GB9203043A GB2261785A GB 2261785 A GB2261785 A GB 2261785A GB 9203043 A GB9203043 A GB 9203043A GB 9203043 A GB9203043 A GB 9203043A GB 2261785 A GB2261785 A GB 2261785A
- Authority
- GB
- United Kingdom
- Prior art keywords
- signal
- input
- amplifier
- output
- feedback
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
- H03F1/34—Negative-feedback-circuit arrangements with or without positive feedback
Abstract
An amplifier circuit comprises a main amplifier (12) having an input (16) for receiving an input signal and an output (18) for delivering an output signal, a first feedback circuit (14) and a second feedback circuit (R1, R2). The first feedback circuit (14) comprises an attenuator (R3, R4,) which attenuates the output signal to the level of the input signal, a phase correction circuit (R8, C2, R9 C3) which compensates the input signal for phase changes occurring in the main amplifier (12) and a difference amplifier (22) which subtracts the attenuated output signal from the phase corrected input signal. The output of the difference amplifier (22) is amplified in an operational amplifier (23) and supplied to the main amplifier (12) as a negative feedback signal. In operation, the first feedback circuit (14) greatly reduces the level of distortion components in the output signal. <IMAGE>
Description
AMPLIFIER CIRCUIT
This invention relates to an amplifier circuit and particularly, but not exclusively, to an amplifier circuit for amplifying signal components in the audio frequency range.
The output signal of an amplifier usually includes unwanted distortion components. These distortion components are mainly harmonic components caused by non-linear characteristics within the amplifier and inter-modulation components caused by intermodulation between signals within the amplifier. In order to reduce the distortion components in the output signal, it is well known to provide a feedback circuit comprising an attenuator, the output of which is supplied to the input of the amplifier as a negative feedback signal. Where such a feedback circuit is provided, both the gain and the distortion components in the output signal of the amplifier will be determined primarily by the attenuator. As an attenuator may be constructed from components having substantially linear characteristics, such a feedback circuit is effective to reduce distortion components to a low but measurable level.It would, however, be desirable to reduce distortions still further below the levels achievable with conventional feedback circuits.
It is an object of this invention to provide a new or improved amplifier circuit in which the distortion components in the output signal are reduced below the levels achievable with conventional feedback circuits.
According to this invention, there is provided an amplifier circuit comprising a main amplifier section having an input for receiving an input signal and an output for delivering an output signal, and a feedback circuit comprising means for comparing the input signal with the output signal to produce a feedback signal and means for supplying the feedback signal to the input of the main amplifier section as a negative feedback signal.
Preferably, the comparing means comprises means for attenuating the output signal to produce an attenuated signal, means for subtracting the input signal from the attenuated signal to produce a difference signal, and means for amplifying the difference signal to produce said feedback signal.
Preferably, the feedback circuit includes means for correcting the phase of the input signal prior to supplying the input signal to the subtracting means.
Conveniently, in the feedback circuit, the gain of the amplifying means expressed as the ratio of output voltage to the input voltage is greater for DC signal components than for signal components in the audio frequency range.
Desirably, the amplifier circuit includes a second feedback circuit comprising means for attenuating the output signal to produce an attenuated signal and means for supplying said attenuated signal to the input of the main amplifier as a negative feedback signal.
This invention will now be described in more detail, by way of example, with reference to the drawings in which:
Figure 1 is a circuit diagram of an amplifier circuit according to a first embodiment of this invention; and
Figures 2 and 3 together form a circuit diagram of a second amplifier circuit according to a second and preferred embodiment of this invention.
Referring now to Figure 1, there is shown an amplifier circuit for amplifying signals in the audio frequency range and comprising a main amplifier section in the form of a single power amplifier 12, a first feedback circuit 14 and a second feedback circuit comprising resistors R1, R2. The non-inverting input of amplifier 12 is connected to a terminal 16 receiving an input signal. The output of amplifier 12 is connected to an output terminal 18 for delivering an output signal and the terminal 18 is connected to the input of a loudspeaker 20.
In the second feedback circuit, resistors R1 and
R2 are connected between the output of amplifier 12 and ground and the junction of these two resistors is connected to the inverting input of amplifier 12. Thus, resistors R1, R2 function to attenuate the output signal and to supply the attenuated signal to amplifier 12 as a negative feedback signal. Consequently, the gain of amplifier 12 is determined by the attenuator comprising resistors R1, R2.
The first feedback circuit includes a pair of resistors R3, R4 connected between terminal 18 and ground and which also function as an attenuator. The junction of these two resistors is connected through a variable resistor R5 and a further resistor R6 to the inverting input of operational amplifier 22. The output of amplifier 22 is connected through a capacitor C1 and a resistor R7 to its inverting input. The resistors R3 and R4 are arranged to attenuate the output signal by a factor which is equal to the amplification factor of amplifier 12 as determined by resistors Rl, R2.
The first feedback circuit 14 also includes a phase correction circuit comprising resistor R8, capacitor C2 and resistor R9, connected in a series between terminal 16 and earth, and variable capacitor C3 connected and parallel with resistor R9. The junction of capacitor C2 and resistor R9 is connected to the noninverting input of amplifier 22. Resistor R8 has a small resistance value compared with resistor R9 and capacitor C2 has a low impedance in the audio frequency range. Thus, the signal supplied to the non-inverting input of amplifier 22 is close in magnitude to that of the input signal. The phase correction circuit is designed to compensate for phase changes occurring in amplifier 12 so that the signals supplied to the noninverting and inverting inputs of amplifier 22 are in phase. The phase change occurring in amplifier 12 is only a few degrees.Capacitor C3 may be adjusted so as to bring the two signals as closely into phase with each other as possible. The resistor R5 may be adjusted to ensure that the levels of the two signals are as close as possible.
The amplifier 22 is configured as a difference amplifier and its output signal represents the difference between the input signal and the output signal attenuated to the level of the input signal. This difference signal represents the distortions present in the output signal and introduced by amplifier 12.
The output of amplifier 22 is connected through a resistor R10 to the inverting input of an operational amplifier 23, the output of which is connected through a feedback resistor R11 to its inverting input. Thus, the output of amplifier 23 also represents distortion components present at the output of amplifier 12.
Preferably, at the output of amplifier 23, the distortion components are greatly amplified in comparison with the distortion components present in the output signal at terminal 18. The output of amplifier 23 is supplied as a feedback signal through a resistor R12 to the inverting input of amplifier 12. Thus, the output of amplifier is supplied to amplifier 12 as a negative feedback signal.
In operation, the first feedback circuit 14 functions to greatly reduce the level of the distortion components appearing at the output of amplifier 12.
The amplifier circuit 10 is designed to be DC coupled both to the input signal and to the loudspeaker 20. As will now be explained, the first feedback circuit 14 also functions to reduce any DC bias in the output of amplifier 12 to a negligible level. In the phase correction circuit, capacitor C2 blocks any DC component present in the input signal. Thus, there is no DC component in the signal supplied to the noninverting input of amplifier 22. The capacitance value of capacitor C1 is selected so that amplifier 22 has a much greater gain for DC components than for components in the audio frequency range. When the gain is expressed as the ratio of the output to input voltages, the combined gain of amplifiers 22 and 23 for DC components is at least 100 and, preferably, in excess of 1000.Thus, any DC bias present in the output signal of amplifier 12 is supplied to its input after great amplification as a negative feedback signal.
Consequently, any DC bias in the output of amplifier 12 is reduced to a negligible level.
Referring now to figures 2 and 3, there is shown an amplifier circuit which is also suitable for amplifying signals in the audio frequency range. The amplifier circuit has an input terminal 30 for receiving an input signal, an output terminal 32 for delivering an output signal and which may be connected to a loudspeaker, a main amplifier section comprising a first stage 32, second stage 34 and a third stage 36, a first feedback circuit 38, and a second feedback circuit 40.
The amplifier circuit has a positive supply terminal 42 connected through a fuse 44 to a line 46.
Line 46 is connected to earth through a capacitor C10 and to a positive supply line 48 through a diode D1. The amplifier circuit also has a negative supply terminal 50 connected through a fuse 52 to a line 54. Line 54 is connected through a capacitor C11 to earth and through a diode D2 to a negative supply line 56.
The positive and negative supply lines 48 and 56 are connected, respectively, to earth through capacitors
C12, C13.
The first stage comprises a pair of FET type transistors Q1, Q2, five further transistors Q3 to Q7, resistors R20 to R31, capacitors C15, C16, diodes D3 to
D5, and a 12 volt zener diode ZD1. The input signal is connected through a resistor R21 to the base of transistor Q1. The first amplifying stage is configured as a differential amplifier and the base of transistor Q1 is the non-inverting input. The base of transistor of
Q2 is the inverting input. The output of the first stage 32 is produced between the collectors of transistors Q3 and Q6 and this output is supplied to the second stage 34.
The second stage comprises transistors Q8 to Q13, resistors R32 to R38, capacitors C17, C18 and a 15 volt zener diode ZD2. The output of the second stage is produced between the anode and cathode of zener diode ZD2 and supplied to the third stage on lines 57, 58.
The third stage comprises transistors Q14 to Q25, resistors R39 to R58, capacitors C19 to C25 and diodes D6 to D9. Transistors Q24 and Q25 are of the MOSFET type.
The third stage is configured as a push-pull amplifier and transistors Q22 and Q23 drive the output transistors
Q24 and Q25. The output signal is present at the junction of resistors R57 and R58 and this junction is connected to the output terminal 32. Transistors Q14 to
Q19 together with associated components function as an output biasing arrangement. Transistors Q20 and Q21 together with associated components provide protection against a short circuit across the output terminal 32.
In the second feedback circuit, the output terminal 32 is connected through resistors R59 and R60 and capacitor C26 to earth. Resistor 59 is bridged by capacitor C27. The junction of resistors R59 and R60 is connected to the base of transistor Q2 of the first stage 32. Thus, the second feedback circuit 40 attenuates the output signal and supplies the resulting attenuated signal to the first stage as a negative feedback signal.
Consequently, the overall gain of the stages 32, 34, 36 of the main amplifying section is determined by the second feedback circuit 40.
In the first feedback circuit 38, the output terminal 32 is connected through a variable resistor R61, resistor R62, resistor R63 and a capacitor C28 to earth.
Resistors R61 to R63 together with capacitor C28 function as an attenuator and function to attenuate the output signal present at output terminal 32 to the level of the input signal at terminal 30. The attenuated signal, which appears at the junction of resistors R62 and R63, is connected to the inverting input of an operational amplifier 60. The output of amplifier 60 is connected to its input through a capacitor C30 and resistor R64, resistor R64 being bridged by a capacitor C29.
The first feedback circuit 30 includes a phase correction circuit comprising a resistor R65, a capacitor
C31 and a resistor R66 connected between the base of transistor Q1 and earth. R66 is bridged by a capacitor
C32 and a variable capacitor C33. The phase correction circuit is arranged to compensate for the phase change occurring in the main amplifying section 32, 34, 36.
The phase change is only a few degrees. Resistor R65 has a small resistance value compared to that of resistor
R66 and capacitor C31 has a low impedance in the audio frequency range. Thus, the signal appearing at the junction of capacitor C31 and resistor R66 is close in level to that of the input signal supplied to the base of transistor Q1 and this signal is supplied to the noninverting input of amplifier 60 on a line 63.
Capacitor C33 is adjusted so as to ensure that the two signals supplied to the two inputs of amplifier 60 are as closely in phase with each other as possible.
Resistor R61 is adjusted to ensure that their levels are as close as possible.
The output of amplifier 60 is connected through a resistor R67 to the inverting input of an operational amplifier 62, the output of which is connected to the inverting input through a resistor R68. Thus, the amplifiers 60 and 62 together with their associated components function to subtract the output signal after attenuation to the level of the input signal from the input signal and to amplify the resulting signal. This resulting signal, which appears at the output of amplifier 62, is supplied through a resistor R69 and a line 64 to the base of transistor Q2. Thus, this resulting signal is supplied as a negative feedback signal to the first stage 32.
The first feedback circuit 38 functions in a similar manner to the first feedback circuit of the amplifying circuit described with reference to Figure 1.
Thus, in the output signal present at output terminal 32, the distortion components and any DC bias are reduced to a very low level.
In an amplifier, the distortion may be defined by the distortion factor. When expressed as a percentage, the distortion factor is defined as one hundred times the square root of the ratio of the sum of the squares of the amplitudes of the distortion components in the output signal to the square of the amplitude of the fundamental component in the output signal.
In a typical high quality audio amplifier, the distortion factor lies in the range 0.1 to 0.5 percent. In the circuit described with reference to Figures 2 and 3, the distortion factor is below 0.001 percent.
In the circuit of figures 2 and 3, the components have the values set out in the table below.
R20 100k# R30 8.2k# R40 10k# R50 2.7k# R21 1k# R31 8.2k# R41 10k# R51 2k# R22 100kQ R32 150 # R42 10k# R52 2kQ R23 100k# R33 22kQ R43 47n R53 2.7k# R24 100 # R34 22k# R44 500# R54 150# R25 100 # R35 30k# R45 470# R55 47# R26 22kn R36 30k# R46 470n R56 47# R27 100k# R37 220# R47 500# R57 47# R28 280 # R38 220# R48 47# R58 47# R29 1.5k# R39 10k# R49 220# R59 18k# R60 2kn C10 470nF C21 470pF C31 100 F
R61 10k# C11 470nF C22 33pF C32 68pF
R62 15kQ C12 100 F C23 33pF C33 10-50pF
R63 2.2kn C13 100 F C24 47pF
R64 47kQ C15 220pF C25 22pF
R65 1k# C16 22 F C26 10pf
R66 66kQ C17 68pF C27 100 F
R67 4.7k# C18 10 F C28 100 F
R68 10k# C19 220pF C29 22pF
R69 12kn C20 220pF C30 22 F
Claims (5)
- CLAIMS 1. An amplifier circuit comprising a main amplifier section having an input for receiving an input signal and an output for delivering an output signal, and a feedback circuit comprising means for comparing the input signal with the output signal to produce a feedback signal and means for supplying the feedback signal to the input of the main amplifier section as a negative feedback signal.
- 2. An amplifier circuit as claimed in Claim 1, in which the comparing means comprises means for attenuating the output signal to produce an attenuated signal, means for subtracting the input signal from the attenuated signal to produce a difference signal, and means for amplifying the difference signal to produce said feedback signal.
- 3. An amplifier circuit as claimed in Claim 2, in which the feedback circuit includes means for correcting the phase of the input signal prior to supplying the input signal to the subtracting means.
- 4. An amplifier circuit as claimed in Claim 2 or Claim 3, in which, in the feedback circuit, the gain of the amplifying means expressed as the ratio of output voltage to the input voltage is greater for DC signal components than for signal components in the audio frequency range.
- 5. An amplifier circuit substantially as hereinbefore described with reference to Figure 1 or Figures 2 and 3 of the accompanying drawings.5. An amplifier circuit as claimed in any one of the preceding claims, further including a second feedback circuit comprising means for attenuating the output signal to produce an attenuated signal and means for supplying said attenuated signal to the input of the main amplifier as a negative feedback signal.6. An amplifier circuit substantially as hereinbefore described with reference to Figure 1 or Figures 2 and 3 of the accompanying drawings.Amendments to the claims have been filed as follows 1. An amplifier circuit comprising a main amplifier section having an input for receiving an input signal and an output for delivering an output signal, and a feedback circuit comprising means for attenuating the output signal to the level of the input signal to produce an attenuated signal, means for obtaining the difference between the input signal and the attenuated signal to produce a difference signal, means for amplifying the difference signal to produce a feedback signal, and means for supplying the feedback signal to the input of the main amplifier section as a negative feedback signal.2. An amplifier circuit as claimed in Claim 1, in which the feedback circuit includes means for correcting the phase of the input signal prior to supplying the input signal to the difference obtaining means.3. An amplifier circuit as claimed in Claim 1 or Claim 2, in which, in the feedback circuit, the gain of the amplifying means expressed as the ratio of output voltage to the input voltage is greater for DC signal components than for signal components in the audio frequency range.4. An amplifier circuit as claimed in any one of the preceding claims, further including a second feedback circuit comprising means for attenuating the output signal to produce a second attenuated signal and means for supplying said second attenuated signal to the input of the main amplifier section as a negative feedback signal.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9203043A GB2261785B (en) | 1992-02-13 | 1992-02-13 | Amplifier circuit |
| EP93904188A EP0626111A1 (en) | 1992-02-13 | 1993-02-11 | Amplifier circuit |
| PCT/GB1993/000280 WO1993016524A1 (en) | 1992-02-13 | 1993-02-11 | Amplifier circuit |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9203043A GB2261785B (en) | 1992-02-13 | 1992-02-13 | Amplifier circuit |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB9203043D0 GB9203043D0 (en) | 1992-03-25 |
| GB2261785A true GB2261785A (en) | 1993-05-26 |
| GB2261785B GB2261785B (en) | 1995-07-05 |
Family
ID=10710321
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9203043A Expired - Fee Related GB2261785B (en) | 1992-02-13 | 1992-02-13 | Amplifier circuit |
Country Status (3)
| Country | Link |
|---|---|
| EP (1) | EP0626111A1 (en) |
| GB (1) | GB2261785B (en) |
| WO (1) | WO1993016524A1 (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1996002974A1 (en) * | 1994-07-13 | 1996-02-01 | Bhc Consulting Pty. Ltd. | An improvement in amplifiers |
| AU696882B2 (en) * | 1994-07-13 | 1998-09-24 | Bhc Consulting Pty Ltd | An improvement in amplifiers |
| GB2390946A (en) * | 2002-07-10 | 2004-01-21 | Aubrey Max Sandman | Improved secondary feedback |
| GB2400995A (en) * | 2003-03-28 | 2004-10-27 | Gary Miller | Linear amplifiers with distortion correction by current comparison |
| WO2008110444A2 (en) * | 2007-03-09 | 2008-09-18 | Austriamicrosystems Ag | Amplifier arrangement and method for amplifying a signal |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6489847B1 (en) | 2000-01-28 | 2002-12-03 | Telefonaktiebolaget Lm Ericsson | Low distoration driving amplifier for integrated filters |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3825854A (en) * | 1970-12-10 | 1974-07-23 | Honeywell Inf Systems | Amplifier with substantially zero distortion products |
| US4476442A (en) * | 1981-04-03 | 1984-10-09 | Nippon Gakki Seizo Kabushiki Kaisha | Amplifier with distortion cancellation |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS60127805A (en) * | 1983-12-14 | 1985-07-08 | Nippon Gakki Seizo Kk | Amplifier circuit cancelling distortion |
-
1992
- 1992-02-13 GB GB9203043A patent/GB2261785B/en not_active Expired - Fee Related
-
1993
- 1993-02-11 WO PCT/GB1993/000280 patent/WO1993016524A1/en not_active Application Discontinuation
- 1993-02-11 EP EP93904188A patent/EP0626111A1/en not_active Withdrawn
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3825854A (en) * | 1970-12-10 | 1974-07-23 | Honeywell Inf Systems | Amplifier with substantially zero distortion products |
| US4476442A (en) * | 1981-04-03 | 1984-10-09 | Nippon Gakki Seizo Kabushiki Kaisha | Amplifier with distortion cancellation |
Cited By (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2305039A (en) * | 1994-07-13 | 1997-03-26 | Bhc Consulting Pty Ltd | An improvement to amplifiers |
| AU696882B2 (en) * | 1994-07-13 | 1998-09-24 | Bhc Consulting Pty Ltd | An improvement in amplifiers |
| GB2305039B (en) * | 1994-07-13 | 1998-12-30 | Bhc Consulting Pty Ltd | An improvement to amplifiers |
| US5892398A (en) * | 1994-07-13 | 1999-04-06 | Bhc Consulting Pty Ltd | Amplifier having ultra-low distortion |
| WO1996002974A1 (en) * | 1994-07-13 | 1996-02-01 | Bhc Consulting Pty. Ltd. | An improvement in amplifiers |
| GB2390946B (en) * | 2002-07-10 | 2006-07-12 | Aubrey Max Sandman | Electronic amplifier circuit |
| GB2390946A (en) * | 2002-07-10 | 2004-01-21 | Aubrey Max Sandman | Improved secondary feedback |
| GB2400995A (en) * | 2003-03-28 | 2004-10-27 | Gary Miller | Linear amplifiers with distortion correction by current comparison |
| GB2400995B (en) * | 2003-03-28 | 2006-03-15 | Gary Miller | Linear amplifiers |
| WO2008110444A2 (en) * | 2007-03-09 | 2008-09-18 | Austriamicrosystems Ag | Amplifier arrangement and method for amplifying a signal |
| WO2008110444A3 (en) * | 2007-03-09 | 2008-12-18 | Austriamicrosystems Ag | Amplifier arrangement and method for amplifying a signal |
| US8063700B2 (en) | 2007-03-09 | 2011-11-22 | Austriamicrosystems Ag | Amplifier arrangement and method for amplifying a signal |
| DE102007011715B4 (en) * | 2007-03-09 | 2012-08-30 | Austriamicrosystems Ag | Amplifier arrangement for amplifying a signal |
Also Published As
| Publication number | Publication date |
|---|---|
| WO1993016524A1 (en) | 1993-08-19 |
| EP0626111A1 (en) | 1994-11-30 |
| GB9203043D0 (en) | 1992-03-25 |
| GB2261785B (en) | 1995-07-05 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20020213 |