JPH0226808B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ALC circuit used in a recording amplifier of a tape recorder or the like.

Generally, in a tape recorder, when an excessive signal is input from a microphone or the like during recording, the recorded signal may be greatly distorted. To avoid this, ALC is applied to the recorded signal.

The present invention provides at least one low frequency amplification circuit having a variable impedance element directly connected to it in terms of direct current, a reference voltage source with which the output voltage of the low frequency amplification circuit can be compared, and a reference voltage source for the reference voltage source. ALC comprising a comparison circuit that compares the voltage with the output voltage of the low frequency amplification circuit, and controlling the variable impedance element using the output signal of the comparison circuit.
Regarding the circuit, the above comparator circuit is composed of a differential amplifier with at least one diode connected in series to each emitter, and this makes the ALC operation extremely smooth as the input level increases. It is characterized in that it starts operating immediately. In particular, there is a problem with the input level at which ALC begins to operate, that is, if an even slightly larger input level is applied at the input level where ALC begins to operate, the ALC circuit will suddenly operate, causing the output level of the recording signal to exceed the specified value. suitable for integrated circuits to prevent problems such as becoming too small.
The purpose is to provide an ALC circuit.

An embodiment based on the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a specific electrical wiring diagram when the ALC circuit of the present invention is applied to an integrated circuit of an ALC-equipped recording amplifier for a stereo tape recorder.

The recording amplifier with ALC shown in FIG.
, a reference voltage source B that can be compared with the output voltage of the low frequency amplification circuit A, and a comparison circuit C that compares the reference voltage generated by the reference voltage source B with the output voltage of the low frequency amplification circuit A. and a drive circuit D for amplifying the output signal of this comparison circuit C and driving the variable impedance element E. The comparator circuit C includes differential amplifiers Q ₂₃ , Q ₂₄ , Q ₂₆ , Q in which at least one diode D ₅ , D ₆ , D ₇ , D ₈ is inserted in series in each emitter. It is composed of ₂₇ people.

The recording amplifier with ALC shown in Fig. 1 is applied as a recording amplifier with ALC for a stereo tape recorder, and the low frequency amplifier A and the variable impedance element E are one each for each left and right channel. It is provided.

In Fig. 1, 3 is a power supply terminal, 1 and 9 are signal input terminals for the left channel and right channel, respectively, and a signal is sent from a signal source Sig having a signal source resistance Rg such as a microphone via coupling capacitors C ₁ and C ₇ . is input. 5 and 7 are signal output terminals for the left channel and right channel, respectively, and R _L is a load resistance with a value equivalent to the input impedance of the next stage, and the output signal is output via coupling capacitors C ₄ and C ₅ , respectively. taken out. 2 and 8 are the AC negative feedback terminals of the low frequency amplification circuit A for the left channel and the right channel, respectively, C ₂ and C ₆ are capacitors for DC separation, and R _NF is the terminal in the low frequency amplification circuit A, respectively. This is a resistor for determining the amount of AC negative feedback together with the resistor _R12 , and determines the AC gain of the low frequency amplification circuit A.

4 is a terminal for connecting a capacitor _C3 for a ripple filter, and is provided to remove ripple components in the power supply voltage applied to the power supply terminal 3 together with a resistor _R11 .

10 is a terminal provided in the drive circuit D for amplifying the ALC signal and driving the variable impedance element E, and _C8 and R _T are terminals for smoothing the ALC signal and controlling each of the attack and recovery of the ALC. This is for determining the time constant.

6 is a ground terminal.

The connections and electrical operations of the circuit elements will be explained below. We will mainly explain only the left channel, but the right channel operates in the same way.

_R1 is variable impedance element E during ALC operation
It is a resistor for dividing the input signal together with the transistors Q ₁ and Q ₂ , and its value is about several KΩ. The collectors and emitters of Q ₁ and Q ₂ are reversely connected to reduce input signal distortion during ALC operation.

_R2 is a resistor for providing base bias to the transistor _Q5 , and together with _R1, it is set to a high resistance value (approximately 50KΩ) to reduce attenuation.
Transistors Q ₄ and Q ₅ constitute a complementary differential input stage, the collector of transistor Q ₅ is grounded, the base is connected to input terminal 1 via resistor R ₁ and biased by resistor R ₂ . and an input signal is applied. and,
The emitter is connected to the emitter of transistor _Q4 . The base of transistor _Q4 is a feedback resistor
It is fed back via _R12 and is connected to the AC feedback determining terminal 2, and is also connected to the collector of the feedback current determining transistor _Q6 . _R12 is a feedback resistor, and together with the AC feedback resistor _RNF , determines the AC gain of the low frequency amplification circuit A. The potential drop generated by the feedback current provides DC feedback to the base of transistor _Q4 . Diode connected transistor _Q8
A transistor Q ₃ whose emitter is connected to a resistor R ₃ constitutes a current mirror circuit, and the mirror ratio is determined by the resistor R ₃ . The collector current of transistor Q ₃ determined by resistor R ₃ is supplied to the complementary differential amplification circuit composed of transistors Q ₄ and Q ₅ , and the collector of transistor Q ₃ is connected to the complementary differential amplification circuit described above. It has become an active load.

Transistors Q ₆ , Q ₁₀ , Q ₁₁ , Q ₇ , diodes
D ₁ , resistors R ₄ , R ₅ , and R ₆ are arranged so that the DC potential V _O of the output terminal 5 of the low frequency amplification circuit A becomes the midpoint potential, that is, the potential of Vcc/2, regardless of the power supply voltage Vcc. Configure and configure the current source for setting.

In order to prevent the ripple component of the power supply voltage from the power supply terminal 3 from entering the current source, the base of the transistor _Q7 is connected to the current source through the resistor _R11 after removing the ripple component with the resistor _R11 and the capacitor _C3 . They are base biased. and transistor
A diode D ₁ and a resistor R ₅ are each connected in series to the emitter of Q ₇ to determine the current of the current source. The bases of each of transistors Q ₆ and Q ₁₀ are connected to the emitter of transistor Q ₁₁ ,
Each emitter is grounded via resistors R ₄ and R ₆ of the same value. The collector of transistor Q ₆ is connected to the base of transistor Q ₄ and
Connected to feedback resistor _R12 . On the other hand, the collector of transistor _Q10 is connected to the base of transistor _Q11 and to one end of resistor _R5 .

Now, in the above configuration, if the voltage V _BE between the base and emitter of each transistor is constant, β of each transistor is very large, and the base current of each transistor is ignored, the current i flowing through the resistor R ₅ is from the power supply voltage Vcc. V _BE of transistor Q ₇ and forward voltage of diode D ₁ (V _BE if diode D ₁ consists of a diode-connected transistor), V _BE of transistor Q ₁₁ , V _BE of transistor Q ₁₀ , divided by _R5 . Assuming that the voltage generated across the resistor _R6 can be ignored compared to the power supply voltage Vcc, then i=(Vcc- _4VBE )/ _R5 .

Since the bases of transistors Q ₁₀ and Q ₆ are connected in common and the resistors R ₄ and R ₆ have the same value, the collector current of transistor Q ₆ is also i,
Flows through resistance R ₁₂ . Therefore, the DC voltage at output terminal 5
V _O is the voltage drop across resistor R ₁₂ and the transistor
It is the sum of the V _BE of Q ₄ and the V _BE of transistor Q ₅ .

That is, V _O = {(Vcc−4V _BE )/R ₅ }×R ₁₂ +2V _BEHere , if we set R ₅ ≒2R ₁₂ , then V _O ≒1/2Vcc−2V _BE +2V _BE ≒1/2Vcc. , the DC voltage V _O at the output terminal 5 can always be set to the midpoint voltage of the power supply voltage, regardless of the power supply voltage Vcc.

The collector current of the transistor _Q11 is then determined by the resistor _R7 connected to the emitter of the transistor Q11.
Since the voltage across resistor R ₇ is maintained at approximately V _BE regardless of the power supply voltage, its collector current is also constant, and the current is mirrored by transistors Q ₈ , Q ₃ and resistor R ₃ , so transistors Q ₄ , Q ₅ The current supplied to the complementary differential amplification circuit consisting of is also constant, and the base current of transistor Q ₅ and the base voltage generated by resistor R ₂ are also constant regardless of the power supply voltage.

The input signal applied to terminal 1 is the transistor
A complementary differential circuit consisting of Q ₅ and Q ₄ amplifies the signal using transistor Q ₃ as a load and applies it to the base of transistor Q ₁₂ . The collector of transistor Q ₁₂ is connected to power supply terminal 3, and the emitter is a constant current transistor Q ₃₉ that serves as its current source.
connected to the collector. The signal is output to the emitter of transistor Q ₁₂ and is output to the emitter of transistor Q ₁₃
applied to the base of Transistor Q ₁₃ , resistor
R ₉ and R ₁₀ constitute an inverting amplifier. transistor
The resistor _R10 connected to the collector of _Q13 determines its emitter current, and the resistor _R9 connected to the emitter and the resistor _R10 determine its gain. The output signal is taken out across the resistor _R10 , applied to the base of the transistor _Q15 , and output to its collector with the transistor _Q14 and diodes _D2 and _D3 as loads. The base of transistor Q ₁₄ is diode-connected transistor Q ₈
Together, they form a current mirror circuit, biasing the diodes D ₂ and D ₃ and the collector of the transistor Q ₁₅ . The collector of transistor _Q14 becomes the active load of transistor _Q15 .

The collector of transistor _Q15 is connected to the base of transistor _Q17 , one of the composite transistors in which PNP type transistor _Q17 and NPN type transistor _Q18 are combined as shown in the figure. The collector of transistor _Q14 is connected to the base of transistor _Q16 , and a signal amplified by transistor _Q15 is applied to the emitter of transistor _Q16 and the emitter of transistor _Q17 , respectively.
It is output to the connection point of the collector of Q ₁₈ , that is, the output terminal 5, and taken out as an AC output signal to the load resistor R _L.

C is a phase compensation capacitor.

The output signal is connected to be input to the base of transistor Q 27 of differential amplifiers Q ₂₆ and _{Q 27} that constitute comparator circuit C of the ALC circuit.

The emitters of transistors Q ₂₆ and Q ₂₇ have their cathodes in common via diodes D ₇ and D ₈ , respectively, and are connected to the collector of transistor Q ₂₈ , which is a current source. Transistors Q ₂₈ , Q ₃₀ ,
Resistors R ₁₆ and R ₁₇ constitute a current mirror circuit, which is supplied from the collector of transistor Q ₂₉ to diode-connected transistor Q ₃₀ . The base of transistor _Q26 is connected to the emitter of reference voltage output transistor _Q32 of reference voltage source B. The collector of transistor Q ₂₆ is connected directly to the power supply. On the other hand, the collector of transistor _Q27 is connected to the power supply via load resistor _R14 .

Now let the base voltages of the differential amplifiers Q ₂₇ and Q ₂₆ with diodes inserted in series in their emitters as described above be V ₁ and V ₂ respectively, and the constant current transistor Q ₂₈
When the collector current I _O of the transistor Q ₂₇ is I c , the collector current I _c is I _c I _O /{1+expq/2KT (V ₁ −V ₂ )} (1).

On the other hand, a diode is now placed on the emitter of the differential amplifier.
Assuming that D ₇ and D ₈ are not present and are directly connected in common and connected to the collector of transistor Q ₂₈ , I _c =I _O /{1+expq/KT (V ₁ −V ₂ )} (2).

A comparison of (V ₁ −V ₂ ) versus I _c in the above equations (1) and (2) is shown in FIGS. 2A and 2B, respectively.

As is clear from Fig. 2B, the diode D ₇ ,
By adding D ₈ and configuring _the comparator circuit C as shown in FIG _{. 1 ,} I The change in _c , that is, the change in the collector current of transistor _Q27 becomes slower.

The reference voltage source B is basically a transistor Q ₃₈ ,
It consists of Q ₃₆ , Q ₃₇ , Q ₃₂ and resistors R ₂₀ and R ₂₁ .

The base of the transistor _Q38 is biased through the resistor _R11 after removing the ripple component with the resistor _R11 and capacitor _C3 to prevent the ripple component of the power supply from being mixed into the output reference voltage. There is.

Resistors R ₂₀ and R ₂₁ should have the same value, and V _BE (approximately
0.7V) is sufficiently small compared to the power supply voltage,
The connection point between the resistors R ₂₀ and R ₂₁ becomes approximately 1/2 Vcc of the midpoint voltage of the power supply voltage, regardless of the power supply voltage.

The base of transistor Q ₃₂ is connected to the above resistor R ₂₀ ,
It is connected to the connection point of _R21 , and its emitter is connected to the collector of constant current power supply transistor _Q31 , and supplies the emitter current of transistor _Q32 . Therefore, the emitter potential of transistor _Q32 becomes Vcc/2+0.7V almost regardless of the power supply voltage, which is taken out as the reference voltage, and the transistor
Applied to the base of Q ₂₆ .

Transistors Q ₃₃ , Q ₃₄ , Q ₃₅ , Q ₃₆ , Q ₃₇ diodes D ₁₀ , D ₉ constitute a constant current source, and the resistor
The current value is determined by R ₁₉ and R ₁₈ . The current determined by the resistor R ₁₉ is the diode D ₁₀ , the transistor
Current mirrored transistor through _Q35
From the collector of Q ₃₅ is supplied to transistor Q ₃₄ ,
The current determined by resistor R ₁₈ is then passed through diode D ₉
The current is mirrored by transistors Q ₃₁ and Q ₂₉ .

As described above, the reason why the current sources are connected in two stages is to minimize the dependence of the constant current value on the power supply voltage. This also stabilizes the current supplied to the transistors Q ₂₆ and Q ₂₇ that constitute the comparator circuit C,
This contributes to eliminating fluctuations in the ALC signal I _c due to the power supply voltage.

Now, as will be described later, when the base potential of transistor _Q27 becomes higher than the base potential of transistor _Q26 , the collector current of transistor _Q27 increases as shown in Figure 2, and the ALC control signal is sent to load resistor _R14 . is taken out. If the value of resistor _R14 is set so that a bias is applied between the base and emitter of transistor _Q22 when the base voltage of transistor _Q27 reaches a specified level, variable impedance element E is driven. Drive circuit D for
A current flows through the transistor Q ₂₂ composing the transistor Q 22 , which is smoothed by the capacitor C ₈ and provides the base current of the transistor Q ₂₁ . When a current flows through the base-biased transistor Q ₂₁ , it is mirrored by a current mirror circuit consisting of a diode D ₄ and a transistor Q ₁₉ , and the collector current of the transistor Q ₁₉ flows through the transistors Q ₁ and Q, which constitute a variable impedance element E. ₂ changes its saturation resistance, and together with resistor R ₁ divides the input signal.

Now, in the above configuration, when an input signal is applied to the input terminal 1, it is amplified by the low frequency amplification circuit A by a gain approximately determined by the resistors R ₁₂ and R _NF , and is output to the output terminal 5. The DC voltage at this point is the midpoint voltage of the power supply voltage Vcc, so the output signal is
It becomes an AC signal that swings around 1/2Vcc. Therefore, the base input signal of transistor _Q27 is 1/1 of the supply voltage.
On the other hand, the base of transistor _Q26 is always approximately (1/2Vcc+ _VBE ).

The base input signal of transistor _Q27 is shown in Figure 3A.
As shown in the figure, when the base potential of transistor Q ₂₆ is lower than approximately (1/2 Vcc + V _BE ), that is, when the input signal level is small, no current flows to the collector of transistor Q ₂₇ , so the current flows through load resistor R ₁₄ . Since transistor _Q22 is not biased by
The drive circuit D does not operate, the variable impedance element E is cut off, and the transistors Q ₁ ,
The impedance between the collector and emitter of Q ₂ becomes very large, and the input signal is not voltage divided with the resistor R ₁ , and the signal amplified by the gain of the low frequency amplification circuit A is sent to the load resistor R _L. taken out.

Next, the input signal level increases, and as shown in FIG.
As shown in , if there is a part where the base input signal of transistor Q ₂₇ becomes higher than the base potential (Vcc/2+V _BE ) of transistor Q ₂₆ (only when it is higher than Vcc/2+V _{BE )} , the current I corresponding to the excessive amount increases. _c flows into the transistor Q _27. This current causes a voltage drop across the resistor R ₁₄ and also applies a base bias to the transistor Q ₂₂ , making the transistor Q ₂₂ conductive and causing a pulsating current to flow through the capacitor C _8.
The base bias of transistor _Q21 is also applied. A current determined by a resistor _R13 flows through the collector of the transistor _Q21 , and is passed through the variable impedance element E through a current mirror circuit consisting of a diode _D4 and a transistor _Q19 .
By lowering the saturation resistance between the collector and emitter of the transistors Q ₁ and Q ₂ and dividing the voltage together with the resistor R ₁ , the signal level output to the terminal 5 is lowered and ALC operation is started.

Above, we have described an example in which one diode was inserted into each emitter of the differential amplifier.
Insert multiple diodes and increase (V ₁ −V ₂ )
It is also possible to make the I _c characteristics less strict.

Although the operation for only one channel has been described, in FIG. 1, the same operation is performed for the other channels as well. The reference voltage source B is shared by both channels, and in the embodiment, a comparison circuit for the left channel,
and the transistor of the comparison circuit for the right channel.
The collectors of Q ₂₃ and Q ₂₇ are connected to each other and connected to a common load resistor R ₁₄ , and are configured to thereafter drive the variable impedance element E of each channel via a common drive circuit D.

As mentioned above, by sharing the reference voltage source B and the drive circuit D, the level at which ALC starts to be applied,
In addition, the variation of the ALC effect in both the left and right channels in the ALC state is reduced, and when the circuit is further integrated, the transistors Q ₁ , Q ₂ , Q ₃₉ ,
Since it is also possible to make the characteristics of Q ₄₀ , Q ₁₉ , and Q ₂₀ the same, it is expected that the above-mentioned variation between the left and right channels will be further reduced.

Although the above description has been given as a recording amplifier with ALC, it is also possible to provide a switch at the terminal 10 and use it as a recording/playback switch, as shown in FIG. That is, when the switch S is open, it can be used as a recording amplifier as explained above, and when the switch S is closed, it can be used as a reproduction amplifier.

In other words, when switch S is closed, the transistor
Since the base of Q ₂₁ is grounded, the operation of the driving circuit D is stopped and the variable impedance element E is not driven. Therefore, at any output signal level, that is, at any input signal level, the transistors Q ₁ ,
The emitter and collector of Q ₂ are open,
The input signal is transferred to transistor _Q5 without being divided.
The output signal is amplified by the gain of the low frequency amplification circuit A and output from terminals 5 and 7. Therefore, it can be used as a reproduction amplifier in this state.

As is clear from the examples above, the present invention
The ALC circuit uses a differential amplifier with a diode connected in series to each emitter as a comparison circuit that compares the output voltage of the low frequency amplifier circuit and the reference voltage from the reference voltage source. The strength of ALC operation in response to level changes can be changed relatively smoothly, allowing input signal levels to
The advantage is that it is possible to completely prevent the inconvenience that the output voltage of the low frequency amplification circuit suddenly decreases and reaches below the specified level due to a sudden large ALC action when the ALC operation start level is reached. has.

[Brief explanation of drawings]

Fig. 1 is a specific electrical wiring diagram showing an embodiment of a recording amplifier with ALC for a tape recorder that employs the ALC circuit of the present invention, and Fig. 2 is a characteristic diagram of a comparison circuit constituting the same embodiment. , FIG. 3 is an explanatory diagram of the operation of the same embodiment, and FIG. 4 is an electrical connection diagram of main parts of another embodiment. A...Low frequency amplification circuit, B...Reference voltage source, C
...Comparison circuit, D...Drive circuit, E...Variable impedance element.

Claims (1)

[Claims] 1. an input terminal, a low frequency amplification circuit connected to this input terminal via a variable impedance element consisting of a first resistor and a transistor, and an output terminal connected to this low frequency amplification circuit;
A reference voltage source that can be compared with the output voltage of the low frequency amplifier circuit, and a voltage of the reference voltage source applied to the base of the first transistor and a voltage of the output terminal applied to the base of the second transistor. and a comparison circuit consisting of a DC differential amplifier in which at least one or more diodes are connected in series to the emitters of the first transistor and the second transistor, respectively, and the voltage difference generated by the comparison circuit is An ALC circuit comprising a drive circuit that changes the saturation resistance of the variable impedance element accordingly and divides the voltage at the input terminal by the first resistance and the saturation resistance.