JP2000124752A - Audio amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an IC for an audio amplifier capable of volume adjustment and muting. SOLUTION: In this audio amplifier, a resistance/current conversion circuit 16 for converting the resistance value of a variable resistor R11 to the size of a current, a variable gain amplifier 13 for amplifying audio signals SAU and a shunt circuit 17 are made into a 1-chip IC. The output current of the resistance/current conversion circuit 16 is supplied to the variable gain amplifier 13 as a control current and the gain of the variable gain amplifier 13 is changed corresponding to the resistance value of the variable resistor R11. The shunt circuit 17 is controlled by muting signals and the magnitude of the control current supplied to the variable gain amplifier 13 is controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an audio amplifier.

[0002]

2. Description of the Related Art For example, when listening to FM broadcasts on a commuter train, the received electric field level changes as the train runs, but when the received electric field level becomes low, the noise level rises and the noise is heard by the ear. It will come on. Also, when the tuning frequency shifts, the noise level rises and noise becomes noticeable.

[0003] Therefore, in a portable receiver called a commuter radio, when the received electric field level becomes low, the gain of the audio signal system is reduced by a predetermined amount so that noise is not heard.

FIG. 5 shows an example of such an FM receiver, and a portion 10 surrounded by a chain line is formed into an IC.
Symbols T11 to T19 denote external terminal pins, and pin T17
Is a ground terminal pin, and pin T19 is a power terminal pin.

[0005] Then, the FM broadcast wave signal is received by the antenna 11, and the tuner circuit 12 is received through the terminal pin T11.
Supplied to The tuner circuit 12 includes an antenna input circuit to an FM demodulation circuit, and is configured in a synthesizer system. An FM broadcast is selected according to a user's selection operation, and the FM broadcast is selected. F
An audio signal SAU of M broadcast is output.

The audio signal SAU from the tuner circuit 12 is supplied to the terminal pin T13 → DC cut capacitor C11 → terminal pin T14 → preamplifier 13 → output amplifier 14 → terminal pin T18 → DC cut capacitor C12.
Speaker (or earphone) through the signal line
Supplied to SP. In this case, the preamplifier 13 is a variable gain amplifier, the gain of which changes according to the control voltage VC supplied thereto.

Further, a constant voltage V10 of, for example, 1.25 V is formed inside the IC 10, and this voltage V10 is supplied to each circuit of the IC 10 as a reference voltage thereof and taken out to a terminal pin T15. And this terminal pin T15
Is divided by a variable resistor (potentiometer) R11, and the divided voltage is applied to a terminal pin T16.
Is supplied to the preamplifier 13 as a control voltage VC of the gain.

Therefore, when the variable resistor R11 is adjusted, the voltage VC changes in accordance with the adjustment.
Since the gain of the preamplifier 13 changes according to C, the level of the audio signal SAU supplied from the preamplifier 13 to the output amplifier 14 changes. Therefore, the volume of the reproduced sound output from the speaker SP can be adjusted by adjusting the variable resistor R11.

Further, the IC 10 is provided with a transistor Q15 having a common emitter, the collector of which is connected to a terminal pin T.
12 and from the tuner circuit 12
By rectifying the M intermediate frequency signal, a reception level detection current I12 whose level changes with the characteristic shown in FIG. 6, for example, corresponding to the reception electric field level of the broadcast wave signal is taken out, and this current I12 is supplied to the transistor Q15. Supplied to the base. Incidentally, originally, the transistor Q15 is for tuning display, and an LED for tuning display is connected to the terminal pin T12.

The terminal pin T12 is connected to the base of the transistor Q11 having a common emitter, and the base is connected to the terminal pin T15 through the resistor R12.
The collector of transistor Q11 is connected through resistor R13 to terminal pin T14.

Therefore, when the reception electric field level of the broadcast wave signal is high, the transistor Q15 is turned on by the reception level detection current I12, whereby the transistor Q11
Is off, the audio signal SAU output from the tuner circuit 12 is directly transmitted to the preamplifier 13 as described above.
, So that you can listen to the broadcast normally.

On the other hand, when the reception electric field level of the broadcast wave signal decreases, the reproduced sound output from the speaker SP increases in noise. However, at this time, since the reception electric field level is small, the reception level detection current I12 is also small, and the transistor Q15 is turned off. As a result, the transistor Q11 is turned on. Shunted by the switch R13 and the transistor Q11. Therefore, the level of the audio signal SAU supplied to the preamplifier 13 decreases,
The volume of the reproduced sound output from the speaker SP also decreases.

Thus, according to the FM receiver shown in FIG. 5, when the reception electric field level is low or when the tuning is deviated,
Even if the noise level increases, the level of the reproduced sound is reduced, so that the noise is not heard.

[0014]

However, the FM shown in FIG.
In the receiver, since noise suppression is realized by the transistor Q11, external components to the IC 10 increase.

The present invention is intended to solve such a problem.

[0016]

Therefore, according to the present invention, an external terminal pin to which a variable resistor is externally connected and a resistor for converting a resistance value of the variable resistor into a current value are provided.
While amplifying the audio signal with the current conversion circuit,
A variable gain amplifier whose gain changes according to the control current and a shunt circuit that shunts the control current are formed into a one-chip IC, and the output current of the resistance / current conversion circuit is supplied to the variable gain amplifier as the control current. The gain of the variable gain amplifier is changed according to the resistance value of the variable resistor, and the magnitude of the control current supplied to the variable gain amplifier is controlled by controlling the shunt circuit with a muting signal. The audio amplifier is configured as described above. Therefore, in the variable gain amplifier, control of the signal level for volume adjustment and control of the signal level for muting are executed.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a portion 10 surrounded by a chain line is formed as an IC. Symbols T11 to T19 are external terminal pins, pin T17 is a ground terminal pin, and pin T19 is a power terminal pin.

Then, the FM broadcast wave signal is received by the antenna 11, and the tuner circuit 12 is transmitted through the terminal pin T11.
Supplied to The tuner circuit 12 includes an antenna input circuit to an FM demodulation circuit, and is configured in a synthesizer system. An FM broadcast is selected according to a user's selection operation, and the FM broadcast is selected. F
An audio signal SAU of M broadcast is output.

Then, the audio signal SAU from the tuner circuit 12 is supplied to the preamplifier 13 through a signal line from the terminal pin T13 → the capacitor C11 for cutting direct current → the terminal pin T14. 1, the preamplifier 13 includes an input stage 131 and a subsequent differential amplifier 132. The differential amplifier 132 is configured such that the emitters of the transistors Q31 and Q32 are connected to the collector of a transistor Q33 for a constant current source having the terminal pin T19 as a reference potential point. Also, the transistor Q
The collector of Q31 and Q32 has a transistor Q3
5, Q36 is connected and the transistor Q3
5, Q36 constitutes a current mirror circuit 133 using the terminal pin T17 as a reference potential point.

Then, the transistor Q
The audio signal SAU and the bias voltage V
13 and the bias voltage V13 is supplied to the base of the transistor Q32. Therefore, the audio signal SAU is extracted from the collectors of the transistors Q32 and Q36.

Then, the extracted audio signal SAU is supplied to a speaker (or an earphone) SP through a signal line of an output amplifier 14 → terminal pin T18 → DC cut capacitor C12.

Further, an operational amplifier 15 is provided. The operational amplifier 15 constitutes a resistance / current conversion circuit 16 together with the constant current circuit 20 and the transistor Q41. The non-inverting input terminal of the operational amplifier 15 is connected to the terminal pin T16. Current circuit 20
Is connected. Further, the output terminal of the operational amplifier 15 is connected to the base of the transistor Q41.
The emitter of the transistor 41 is connected to the terminal pin T17 through the resistor R41, so that the transistor Q41 functions as an emitter follower.
The emitter of the transistor Q41 is connected to the operational amplifier 15
, And 100% negative feedback is applied to the operational amplifier 15.

Further, the collector of the transistor Q41 is connected to the emitters of the transistors Q42 and Q43. The transistors Q42 and Q43 constitute the shunt circuit 17, and a bias voltage V40 is supplied to the base of the transistor Q42.
It is supplied to the base of the transistor Q43 through 42. Further, the collector of the transistor Q42 is connected to the transistor Q44.
And the collector of the transistor Q43 is connected to the terminal pin T19.

Further, an FM intermediate frequency signal is extracted from a limiter amplifier of an intermediate frequency circuit of the tuner circuit 12, and this signal is supplied to a level detection circuit 18 where it is rectified. A reception level detection current I18 whose level changes with a characteristic as shown in FIG. 6 corresponding to the level is extracted. Then, this detection current I18 is supplied to the base of the transistor Q45 having a common emitter, and its collector is connected to the base of the transistor Q43.

Further, the transistor Q44 and the transistor Q33 form the current mirror circuit 19 with the terminal pin T19 serving as a reference potential point and the transistor Q44 serving as the input side. Also, the terminal pin T16 and the terminal pin T
A variable resistor R11 for controlling the gain of the preamplifier 13 is externally connected to the variable resistor R17.

Although not shown, the tuner circuit 12
Rectifies the FM intermediate frequency signal to extract a reception level detection current whose level changes with characteristics as shown in FIG. 6, for example, corresponding to the reception electric field level of the broadcast wave signal. It is taken out to the terminal pin T12 through the transistor of the collector.

According to such a configuration, the audio signal SAU supplied to the terminal pin T14 is amplified by the differential amplifier 132 and the current mirror circuit 133, further amplified by the output amplifier 14, and supplied to the speaker SP. You.

In this case, the operational amplifier 15 has
Since 100% negative feedback is applied through the transistor Q41, Vp: DC voltage at the non-inverting input terminal of the operational amplifier 15 Vm: DC voltage at the inverting input terminal of the operational amplifier 15, Vp = Vm 1) Then, since the voltage Vm is also the terminal voltage of the resistor R41, if I41 is a DC current flowing through the resistor R41, then I41 = Vm / R41 (2)

Further, if I20 is the output current of the constant current circuit 20, this current I20 flows through the variable resistor R11, and the terminal voltage of the variable resistor R11 is the input voltage Vp of the operational amplifier 21. , Vp = I20.R11 (3)

Therefore, from the equations (1) to (3), I41 = Vm / R41 = Vp / R41 = I20.R11 / R41 = (I20 / R41) .times.R11. That is, the circuit 16 sets the resistance value R11 to the current value I
It becomes a resistance / current conversion circuit that converts to 41.

When the reception electric field level is high,
Since the transistor Q45 is turned on by the reception level detection current I18, the transistor Q43 is turned off and the transistor Q42 is turned on. Therefore, the current I41 flowing through the resistor R41 flows through the collector of the transistor Q41, and further flows through the transistor Q42 to the transistor Q44. Since the transistors Q44 and Q33 form the current mirror circuit 19, if I33 is the collector current of the transistor Q33, then I33 = I41 = (I20 / R41) × R11 (4) . That is, a collector current I33 having a magnitude proportional to the value of the variable resistor R11 flows through the transistor Q33.

The gain of the differential amplifier 132 is determined by its mutual conductance gm, and this mutual conductance gm is determined by the collector currents of the transistors Q31 and Q32, that is, the collector current I33 of the transistor Q33.

Therefore, if the variable resistor R11 is adjusted, the current I33 changes from the equation (4), and the gain of the differential amplifier 132 changes. As a result, the current is supplied from the differential amplifier 132 to the output amplifier 14. The volume of the audio signal SAU is adjusted by changing the level of the audio signal SAU.

On the other hand, when the reception electric field level is low, the transistor Q45 is turned off by the reception level detection current I18, so that the base bias voltages of the transistors Q42 and Q43 become substantially equal. Therefore, the current I41 flowing through the resistor R41 is applied to the transistor Q41 through the transistor Q41.
The current flows to the transistor Q43 as well as to the transistor Q43. That is, the current I41 is equal to the transistor Q42 and the transistor Q42.
43, and the magnitude of the current I41 flowing through the transistor Q42 is の of that when the reception electric field level is high.

Therefore, the magnitude of the current I33 flowing through the transistor Q33 is １ ／ of that when the reception electric field level is high.
Therefore, the gain of the differential amplifier 132 is １ ／ (−6 dB) when the reception electric field level is high. Therefore,
When the reception electric field level is low, that is, when there is much noise, the level of the reproduced sound output from the speaker SP decreases.

As described above, according to this receiver, when the received electric field level is low, or when the tuning is deviated, or when the noise level is increased, the level of the reproduced sound is reduced. Nothing. In this case, in particular, according to this receiver, it is not necessary to externally attach a transistor or the like for noise suppression to the IC 10, as is apparent from FIG.

In the shunt circuit 17, the base-emitter junction area of the transistor Q43 is
If the current I41 is shunted to the transistors Q42 and Q43, the shunt current flowing to the transistor Q43 is twice the shunt current flowing to the transistor Q42, so that the current flowing to the transistor Q42 is twice as large as that of the transistor Q42. When the transistor Q43 is off (when the reception electric field level is high), the noise level can be reduced to 1/3 (about -10 dB) when the reception electric field level is low.

That is, by setting the ratio between the base-emitter junction area of the transistor Q42 and that of the transistor Q43, it is possible to arbitrarily set the level to be reduced when the reception electric field level is low. In addition, when the level is lowered, it is possible to prevent a sense of incongruity in hearing.

FIG. 2 shows an example of measuring the input / output characteristics of the above-mentioned FM receiver. Curve S + N: Level characteristic of an audio signal (including a noise component) output from the amplifier 14 Curve N: Amplifier 14 Curve TUNE: Level characteristic of tuning display current output from terminal pin T12. The scale for the curves S + N and N is the left vertical axis, and the scale for the curve TUNE is the right vertical axis.

According to this measurement result, as the reception electric field level becomes lower, the reception electric field level becomes 15 dB.
The noise level (characteristic N) sharply increases from around μ, but the audio signal level (characteristic S + N) gradually decreases due to the above-described control operation, and when the reception electric field level approaches 0 dBμ, the audio level becomes lower. The signal level is-
This is a 6 dB decrease. This decrease in the audio signal level is due to the shunting operation of the shunt circuit 17, which prevents noise from being heard when the reception electric field level is low, and allows the broadcast to be received comfortably.

FIGS. 3 and 4 show the resistance / current conversion circuit 1.
6 and another specific example of the preamplifier 13, in which the right side of FIG. 3 is connected to the left side of FIG. However, the connection part is partially shown.

Then, a 1.25 V DC voltage is extracted from the DC voltage source V51, and this DC voltage is converted into a current by the transistor Q61 and its emitter resistor R61, and this current is supplied from the collector of the transistor Q61 to the transistor Q62. You. The transistor Q62, together with the transistors Q63 and Q64, forms a current mirror circuit using the terminal pin T19 as a reference potential point, and the transistors Q63 and Q64 are used as constant current sources.

Further, the emitters of the transistors Q51 and Q52 are connected to the collector of a transistor Q64 for a constant current source to form a differential amplifier 51. The transistors Q51 and Q52
Is connected to the transistors Q55 and Q56 through the emitter and collector of the transistors Q53 and Q54.
In this case, the transistors Q53 and Q54
A base bias voltage is supplied by 65 and the base is grounded. The transistors Q55 and Q56 form a current mirror circuit with the terminal pin T19 as a reference potential point. Thus, the operational amplifier 15 is constituted by the transistors Q51 to Q56.

The base of the transistor Q51 is connected to the collector of the transistor Q63 used as the constant current circuit 20, and is connected to the variable resistor R11 through the terminal pin T16. The transistors Q54 and Q54
The collector of 56 is connected to the base of transistor Q41,
A resistor R41 is connected to the emitter and the transistor Q
An emitter follower 41 has its emitter connected to the base of the transistor Q52. Thus,
The resistance / current conversion circuit 16 is formed, and the transistor Q41
A current I41 having a magnitude corresponding to the resistance value of the variable resistor R11 is taken out of the collector of the variable resistor R11.

The DC voltage from DC voltage source V51 is converted into a current by transistor Q91 and its emitter resistor R91, and this current is supplied from the collector of transistor Q91 to transistor Q92. The transistor Q92 is connected to the terminal pin T together with the transistors Q93 and Q94.
A current mirror circuit is configured with 19 as a reference potential point, and transistors Q93 and Q94 are used as constant current sources.

Further, the transistor Q71 is made an emitter follower by the transistor Q93, and the audio signal S
AU is supplied from the terminal pin T14 to the bases of the transistors Q72 and Q74 through the transistor Q71. in this case,
The emitters of the transistors Q72 and Q73 are connected to the collector of the transistor Q76 for a constant current source to form a differential amplifier 134 having the terminal pin T19 as a reference potential point. The emitters of the transistors Q74 and Q75 are transistors for a constant current source. A differential amplifier 135 connected to the collector of Q77 and having the terminal pin T19 as a reference potential point is configured. The collector current of the transistor Q94 is supplied to the transistor Q95 to form a constant voltage.
The bias voltage is supplied to the base of Q75.

The collectors of the transistors Q72 and Q74 are connected to the transistor Q81, and the transistor Q82 is connected to the transistor Q81.
A current mirror circuit having 17 as a reference potential point is configured. The collectors of the transistors Q73 and Q75 are connected to the transistor Q83, and
Is connected to a transistor Q84 to form a current mirror circuit having the terminal pin T17 as a reference potential point.

The collector output of the transistor Q84 is connected to the transistors Q85,
The result is added to the collector output of the transistor Q82 through Q86, and the addition result is supplied to the output amplifier 14. Thus, the preamplifier 13 is configured.

Further, a transistor Q44 is connected to the transistors Q76 and Q77 to form a current mirror circuit 19 having the terminal pin T19 as a reference potential point. The collector of the transistor Q41 is connected to the emitters of the transistors Q42 and Q43, and the base of the transistor Q42 is supplied with the bias voltage V40.
Is supplied to the base of the transistor Q43 through the resistor R42. Further, the collector of the transistor Q42 is connected to the collector of the transistor Q44, and the collector of the transistor Q43 is connected to the terminal pin T19. Thus, the shunt circuit 17 is configured.

Then, a reception level detection current whose level changes with the characteristic shown in FIG. 6, for example, corresponding to the reception electric field level of the broadcast wave signal is taken out from the muting signal source 21, and this detection current is supplied to the transistor Q43. Supplied to the base.

Therefore, as described in the receiver of FIG. 1, by operating the variable resistor R11, the gain of the differential amplifiers 134 and 135 is changed, and the volume is adjusted. When the received electric field level is low, the current I41 is shunted by the shunt circuit 17, and as a result, the differential amplifier 1
Muting is performed because the gains of 34 and 135 decrease.

[0052]

According to the present invention, when the received electric field level is low, or when the tuning is deviated, or when the noise level rises, the level of the reproduced sound is lowered, so that the noise is noticeable. There is no. In that case, there is no need to externally attach a transistor or the like for noise suppression to the IC.

Further, by setting the base-emitter junction area of the transistor, it is possible to arbitrarily set the level to be reduced when the reception electric field level is low. In addition, when the level is lowered, it is possible to prevent a sense of incongruity in hearing.

[Brief description of the drawings]

FIG. 1 is a connection diagram illustrating one embodiment of the present invention.

FIG. 2 is a characteristic diagram illustrating measurement results of characteristics of one embodiment of the present invention.

FIG. 3 is a connection diagram showing another embodiment of the present invention.

FIG. 4 is a connection diagram showing another embodiment of the present invention.

FIG. 5 is a system diagram for explaining the present invention.

FIG. 6 is a characteristic diagram for explaining the circuit of FIG. 5;

[Explanation of symbols]

10 IC, 11 antenna, 12 tuner circuit, 1
3: Preamplifier, 14: Output amplifier, 15: Operational amplifier, 16: Resistance / current conversion circuit, 17: Shunt circuit, 18
... Level detection circuit, 19 ... Current mirror circuit, 20 ...
Constant current circuit, R11: variable resistor, SP: speaker

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 1/16 H04B 1/16 Z F term (Reference) 5J100 AA05 AA15 BA01 BB01 BB21 BB22 BC07 CA00 CA01 CA05 CA22 DA10 EA02 FA02 JA01 KA05 LA00 QA01 QA03 SA02 5K052 AA01 BB04 DD02 EE13 EE32 FF12 GG04 GG13 GG16 GG55 5K061 AA10 BB04 CC42 JJ01 JJ09

Claims (2)

[Claims] 1. An external terminal pin to which a variable resistor is externally attached, a resistor / current conversion circuit for converting a resistance value of the variable resistor into a current value, an amplifier for amplifying an audio signal and gain by a control current And a shunt circuit that shunts the control current are integrated into a single-chip IC. The variable gain amplifier supplies the output current of the resistance / current conversion circuit to the variable gain amplifier as the control current. The audio of which changes the gain of the variable resistor in accordance with the resistance value of the variable resistor and controls the magnitude of the control current supplied to the variable gain amplifier by controlling the shunt circuit by a muting signal. Amplifier. 2. The audio amplifier according to claim 1, wherein said resistance / current conversion circuit comprises: a constant current circuit for supplying a constant current to said variable resistor through said external terminal pin; An audio amplifier configured to convert a voltage obtained at the external terminal pin when a constant current is supplied to a corresponding output current.