GB2271234A - Amplifier arrangement without over-swing using feedback and limiter - Google Patents
Amplifier arrangement without over-swing using feedback and limiter Download PDFInfo
- Publication number
- GB2271234A GB2271234A GB9220819A GB9220819A GB2271234A GB 2271234 A GB2271234 A GB 2271234A GB 9220819 A GB9220819 A GB 9220819A GB 9220819 A GB9220819 A GB 9220819A GB 2271234 A GB2271234 A GB 2271234A
- Authority
- GB
- United Kingdom
- Prior art keywords
- amplifier
- input
- limiter
- coupled
- signal
- 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.)
- Withdrawn
Links
- 230000003321 amplification Effects 0.000 claims description 3
- 230000005540 biological transmission Effects 0.000 claims description 3
- 238000003199 nucleic acid amplification method Methods 0.000 claims description 3
- 230000005236 sound signal Effects 0.000 claims 1
- 238000013459 approach Methods 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
- 230000001052 transient effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03G—CONTROL OF AMPLIFICATION
- H03G3/00—Gain control in amplifiers or frequency changers
- H03G3/20—Automatic control
- H03G3/30—Automatic control in amplifiers having semiconductor devices
- H03G3/3005—Automatic control in amplifiers having semiconductor devices in amplifiers suitable for low-frequencies, e.g. audio amplifiers
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03G—CONTROL OF AMPLIFICATION
- H03G11/00—Limiting amplitude; Limiting rate of change of amplitude ; Clipping in general
Landscapes
- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Amplifiers (AREA)
- Control Of Amplification And Gain Control (AREA)
Abstract
An amplifier arrangement comprises a variable gain amplifier (10) for amplifying a signal to be transmitted, said amplifier having a control input and a feedback loop to the control input for automatic gain control. The invention is characterized by a limiter amplifier (12) coupled to the variable gain amplifier for providing an output signal which is related to the input signal and is limited to a maximum level. An error amplifier (20) is preferably also provided having an input coupled to the input of the limiter amplifier, which input is responsive to a rise in the input impedance of the limiter amplifier, for example from virtually zero ohms, and having an output coupled to the control input of the variable gain amplifier for providing automatic gain control feedback when the limiter amplifier reaches its maximum level. <IMAGE>
Description
AGC LOOP WITHOUT OVER-SWING
Field of the invention
This invention relates to an amplifier comprising a variable gain amplifier and a feedback loop for automatic gain control (AGC). The invention may be utilized, for example, in an audio amplifier.
BackgIound to the invention For signal amplifier stages with the demand for correct peak level limiting, there are typically two approaches, each having certain disadvantages.
A first typical approach used to obtain high peak level limiting requirements, such as prior to a modulator stage of a transceiver, is a simple limiter amplifier without AGC in combination with a downstream lowpass filter. However thissolution unfortunately produces an appreciable increase in the distortion factor. Furthermore, during overdrive the up-stream modulation pre-emphasis is clipped to a flat audio response. Fig. 1 illustrates a standard limiter amplifier lowpass response characteristic and an associated distortion factor measured during modulation. With a 4:1 overdrive, the distortion factor rises to over 30%.
A second approach for distortion reduction is an AGC circuit as shown in Fig. 2. This prior art arrangement provides a variable gain amplifier 10 connected to an amplifier comprising a resistor 11 and an amplifier 12 with a feedback resistor 13 providing an output Vout for passing to the next stage and to create a loop gain control voltage after rectification in rectifier circuit 14,15. With the exception of transient recovery time, which is dependent on loop time constant and transmission frequency, the use of this AGC loop generates the unwanted side effects of overswing and unstable peak level operating points (see Figs. 3 and 4).
There is a need for an improved amplifier with automatic gain control.
Summajy of the Invention
According to the present invention, an amplifier is provided comprising a variable gain amplifier for amplifying a signal to be transmitted, said amplifier having a control input and a feedback loop providing feedback of the signal amplified by the variable gain amplifier to the control input of the variable gain amplifier for automatic gain control. The invention is characterized by a limiter amplifier coupled to the variable gain amplifier for providing an output signal which is related to the input signal and is limited to a maximum level.
An error amplifier is preferably provided having an input coupled to the input of the limiter amplifier, which input is responsive to a rise in the input impedance of the limiter amplifier, for example from virtually zero ohms, and having an output coupled to the control input of the variable gain amplifier for providing automatic gain control feedback when the limiter amplifier reaches its maxim= level.
The arrangement is preferably such that when the limiter amplifier reaches its maxim= level, a signal from the output of the error amplifier causes a reduction in the gain of the variable control amplifier and returns the limiter amplifier to below its maximum level.
The input of the limiter amplifier to which the error amplifier is coupled is preferably an inverting input to provide substantially zero ohm impedance during linear operation.
The amplifier is preferably an audio amplifier of a radio transmitter modulator, for example in a mobile radio transceiver.
A preferTed embodiment of the invention will now be described, by way of example only, with reference to the drawings.
Brief descrigtion of the Drawings Fig.1 shows a measured response characteristic and distortion factor of a prior art amplifier with lowpass circuit (without AGC).
Fig. 2 shows a prior art audio amplifier of a radio transmitter.
Figs. 3 and 4 illustrate time function diagrams for the circuit of Fig. 2.
Fig. 5 shows the circuit of an amplifier in accordance with the present invention.
Fig. 6 shows compression loop response and distortion factor characteristics of the circuit of Fig. 5 and Fig. 7. illustrates a time function diagram for the circuit of Fig. 5.
Detailed Description of the Preferred Embodiment
Fig. 3 illustrates a preferred embodiment of the present invention, wherein elements already described in relation to Fig. 1 have the same reference numerals. An amplifier 20 is connected to the inverting input of the lin-dter amplifier 12 via a resistor 21. The inverting amplifier 12 is used to ensure an accurately specified peak output voltage, which is defined by a supply voltage and a Hmiter amplifier saturation voltage. An error voltage amplifier 20 senses the voltage at the inverting input of the limiter amplifier and therefore detects non-zero voltages at that input.
In a linear amplification mode, a specified amount of output signal negative feedback ensures that the sum of the currents to the input of the limiter amplifier is zero. Moreover, the a.c. voltage to the error voltage amplifier is also zero. With a rising input signal Vin, limitation and a non- linear amplification occur. On entering the non-linear region of operation of the amplifier 12, its input impedance rises substantially. A limited feedback signal causes a non-zero current sum at the input and, upon detection of a non-zero voltage generated across resistor 21, the error voltage amplifier amplifies this voltage by a high gain factor (e.g. 1000) and adjusts the variable gain amplifier by reducing the gain such that the output swing of the limiter amplifier is corrected to a maximum non-limited level.
In this way, the lin-dter amplifier is returned to its linear operating region (see Fig. 4). Thus, after automatic gain control correction, the peak output voltage is never exceeded, and transmission takes place with a substantially minimum distortion factor (refer also to Fig. 4).
In this way, the maximum response for a system, i.e. the optirnisation of dynamics of an audio system, is achieved.
Claims (7)
1. An amplifier comprising:
a variable gain amplifier for amplifying a signal to be transmitted, said amplifier having a control input and a feedback loop providing feedback of the signal amplified by the variable gain amplifier to the control input of the variable gain amplifier for automatic gain control, characterized by a limiter amplifier coupled to the variable gain amplifier for providing an output signal which is related to the input signal and is limited to a maximum level.
2. An amplifier according to claim 1, wherein said limiter amplifier has a an input impedance that rises substantially when the maximum level is reached on its output and further comprising an error amplifier having an input coupled to the input of the lin-titer amplifier, which input is responsive to a rise in the input impedance of the limiter amplifier and having an output coupled to the control input of the variable gain amplifier for providing automatic gain control feedback when the limiter amplifier reaches its maxim= level.
3. An amplifier according to claim 1 arranged such that when the limiter amplifier reaches its maximum level, a signal from the output of the error amplifier causes a reduction in the gain of the variable control amplifier and returns the limiter amplifier to below its maximum level.
4. An amplifier according to claim 1, 2 or 3 wherein the input of the limiter amplifier to which the error amplifier is coupled is an inverting input.
5. An amplifier according to any one of the preceding claims, comprising a resistive element coupled between the input of the limiter amplifier and the input of the error amplifier such that or conducting current from the input of the inverter amplifier when the output of the inverter amplifier is at its maximum and developing a voltage across the resistor for amplification by the error amplifier.
-5
6. An amplifier according to any one of the preceding claims in combination with a modulator and a radio transmitter, wherein the amplifier is coupled to the modulator for amplifying an audio signal for modulation 5 and for transmission through the transmitter.
7. An amplifier substantially as hereinbefore described with reference to Fig. 3.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9220819A GB2271234A (en) | 1992-10-03 | 1992-10-03 | Amplifier arrangement without over-swing using feedback and limiter |
| JP26947093A JPH06224668A (en) | 1992-10-03 | 1993-10-04 | Agc loop without excessive oscillation |
| DE19934333762 DE4333762A1 (en) | 1992-10-03 | 1993-10-04 | AGC loop without overshoot |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9220819A GB2271234A (en) | 1992-10-03 | 1992-10-03 | Amplifier arrangement without over-swing using feedback and limiter |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9220819D0 GB9220819D0 (en) | 1992-11-18 |
| GB2271234A true GB2271234A (en) | 1994-04-06 |
Family
ID=10722904
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9220819A Withdrawn GB2271234A (en) | 1992-10-03 | 1992-10-03 | Amplifier arrangement without over-swing using feedback and limiter |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPH06224668A (en) |
| DE (1) | DE4333762A1 (en) |
| GB (1) | GB2271234A (en) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1286258A (en) * | 1969-10-22 | 1972-08-23 | Emi Ltd | Improvements in or relating to limiter circuits |
| US4967163A (en) * | 1973-09-17 | 1990-10-30 | The United States Of America As Represented By The Secretary Of The Navy | Combined limiting/AGC IF system |
| EP0493956A2 (en) * | 1990-12-29 | 1992-07-08 | Nec Corporation | Baseband signal processing circuit for a radio communication apparatus |
-
1992
- 1992-10-03 GB GB9220819A patent/GB2271234A/en not_active Withdrawn
-
1993
- 1993-10-04 JP JP26947093A patent/JPH06224668A/en active Pending
- 1993-10-04 DE DE19934333762 patent/DE4333762A1/en not_active Withdrawn
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1286258A (en) * | 1969-10-22 | 1972-08-23 | Emi Ltd | Improvements in or relating to limiter circuits |
| US4967163A (en) * | 1973-09-17 | 1990-10-30 | The United States Of America As Represented By The Secretary Of The Navy | Combined limiting/AGC IF system |
| EP0493956A2 (en) * | 1990-12-29 | 1992-07-08 | Nec Corporation | Baseband signal processing circuit for a radio communication apparatus |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9220819D0 (en) | 1992-11-18 |
| JPH06224668A (en) | 1994-08-12 |
| DE4333762A1 (en) | 1994-04-07 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |