JPH0421939B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a recording amplifier with ALC for a tape recorder.

Generally, in a tape recorder, if an excessive signal is input from a microphone or the like during recording, the recorded signal will be distorted, so an ALC circuit is provided to apply ALC to the recorded signal.

The present invention includes a low frequency amplification circuit having a variable impedance element directly connected in a direct current manner, and a comparison circuit that compares the output voltage of the low frequency amplification circuit with a reference voltage. Now controls the impedance element
In the recording amplifier circuit with ALC, a power supply with low output impedance is provided, and the output terminal of this power supply is connected to the input terminal of the comparison circuit through a resistor, and the output terminal of the low frequency amplifier circuit is connected through a capacitor to the input terminal of the comparison circuit. By connecting to the input terminal of the comparator circuit, the frequency characteristics during ALC operation can be made variable, and the purpose of this is to obtain a wide variety of recording characteristics.

Hereinafter, one embodiment based on the present invention will be described in detail using the drawings.

FIG. 1 is a circuit diagram when the present invention is applied to an integrated circuit of a recording amplifier with ALC for a tape recorder.

The recording amplifier with ALC shown in FIG.
a comparator circuit C that detects the output voltage of the comparator circuit C, a reference voltage source B of the comparator circuit C, and a low output impedance power supply G that is an input side bias voltage source of the comparator circuit C.
, a drive circuit D that amplifies the output signal of the comparison circuit C and drives the variable impedance element E, and a transient sound reduction circuit F when the power is turned on.

The comparison circuit C is constituted by differential amplifiers Q ₃₅ and Q ₃₆ each having at least one or more diodes D ₃ and D ₄ inserted in series at their emitters.

In FIG. 1, 3 is a power supply terminal, 1 is a signal input terminal, and a signal is input from a signal source Sig such as a microphone having a signal source resistance Rg via a coupling capacitor _C1 . 5 is the signal output terminal, R _L
is a load resistance with a value equivalent to the input impedance of the next stage, and the output signal is taken out via the coupling capacitor _C4 . 2 is the AC negative feedback terminal of the low frequency amplifier circuit A, C ₂ is the capacitor for DC separation, R _NF is the resistor R ₁₁ in the low frequency amplifier circuit A.
It is also a resistor for determining the amount of AC negative feedback, and determines the AC voltage gain of the low frequency amplifier circuit A.

Reference numeral 4 denotes a ripple filter terminal, which is provided to connect a capacitor C ₃ and remove ripple components in the power supply voltage applied to the power supply terminal 3 together with a resistor R ₁₂ .

8 is a terminal provided in the drive circuit D for amplifying the ALC signal and driving the variable impedance element E, _C8 and R _T smooth the ALC signal, and
These are capacitors and resistors that determine the time constants of ALC attack and recovery. 6 is a ground terminal, 7 is an input terminal of the comparison circuit C, and a capacitor C ₆ is connected between it and the signal output terminal 5.
The output signal of the low frequency amplifier circuit A is inputted through the low frequency amplifier circuit A.

The connections and electrical operations of the circuit elements will be explained below.

_R1 is variable impedance element E during ALC operation
It is a resistor to divide the input signal together with the transistors Q ₁ and Q ₂ that make up the transistor.
The collectors and emitters of Q ₁ and Q ₂ are reversely connected to reduce input signal distortion during ALC operation.

_R50 is a resistor for providing a base bias of the transistor _Q3 , and together with the resistor _R1 , it is set to a high resistance value (about 50KΩ) to reduce attenuation.

Transistors Q ₃ and Q ₄ constitute a complementary differential input stage, the collector of transistor Q ₃ is grounded, the base is connected to input terminal 1 via resistor R ₁ , and is biased by resistor R ₅₀ . and an input signal is applied. Its emitter is connected to the emitter of transistor _Q4 . The base of transistor Q ₄ is the feedback resistor R ₁₁
It is connected to the AC feedback determining terminal 2 and to the collector of the output DC bias setting transistor _Q6 .

_R11 is a feedback resistor, and is an AC negative feedback resistor.
Together with _RNF , it determines the AC voltage gain of the low frequency amplifier circuit A, and also provides DC feedback to the base of the transistor _Q4 using the potential drop generated by the sink current of the transistor _Q6 .

The diode-connected transistor Q ₁₀ and the transistor Q ₅ whose emitter is connected to the resistor R ₂ form a current mirror circuit, and the mirror ratio is the resistor.
Determined by R ₂ . The collector current of transistor Q ₅ determined by resistor R ₂ is supplied to the complementary differential amplifier circuit composed of transistors Q ₄ and Q ₃ , and
The collector of _Q5 serves as an active load for the complementary differential amplifier circuit. _C7 is a capacitor connected between the base and collector of transistor _Q4 .

Transistors Q ₆ , Q ₇ , Q ₈ , Q ₉ , diodes
D ₁ , resistors R ₃ , R ₄ , and R ₅ are connected so that the DC potential V ₀ of the output terminal 5 of the low frequency amplifier circuit A is at the midpoint potential regardless of the power supply voltage V _CC , that is, 1/2·V _CC A current source is configured to set the potential to be .
In order to prevent the ripple component of the power supply voltage from entering the current source, the base of the transistor _Q9 is biased through the resistor _R12 after removing the ripple component with the resistor _R12 and capacitor _C3 . ing.

In the collector of the transistor Q ₇ there is a diode D ₁
and a resistor _R4 are connected in series to determine the current of the current source. A resistor _R13 is connected between the base of the transistor _Q9 and the cathode of the diode _D1 . The bases of transistors Q ₆ and Q ₇ are connected to the emitter of transistor Q ₈ , and each emitter is grounded via resistors R ₃ and R ₅ of the same value. The collector of transistor Q ₆ is connected to the base of transistor Q ₄ and also to feedback resistor R ₁₁ .

On the other hand, the collector of transistor _Q7 is connected to the base of transistor _Q8 , and also to one end of resistor _R4 .

Now, in the above configuration, assuming that the voltage V _BE between the base and emitter of each transistor is constant and the current amplification factor β of each transistor is very large,
Neglecting their respective base currents, the current flowing through resistor _R4 is from the supply voltage V _CC to transistor _Q9's current.
V _BE and the forward voltage of diode D ₁ (V _BE if D ₁ is a diode-connected transistor)
and the voltage obtained by subtracting the V _BE of transistors Q ₈ and Q ₇ divided by the resistor R ₄ . Here, resistance R ₅
Assuming that the voltage generated at is negligible compared to V _CC (V _BE ), the bias current flowing through resistor R ₄ is (V _CC −4V _BE )/R ₄ . Based on this bias current, the DC potential V ₀ of the output terminal 5 is expressed by the following equation.

V ₀ = {(V _CC −4V _BE )/R ₄ } ×R ₁₁ +2V _BE ……(1) Here, if we set R ₄ ≒2R ₁₁ , then V ₀ ≒1/2V _CC −2V _BE ≒ 1/2V _CC ...(2) Therefore, the output DC potential V ₀ is always the midpoint voltage of the power supply voltage, regardless of the power supply voltage V _CC .

Next, the resistor R ₆ connected to the emitter of the transistor Q ₈ determines its collector current.
Since the voltage across resistor R ₆ 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 ₁₀ , Q ₅ and resistor R ₂ , so transistor Q ₄ , _Q3
The current supplied to the complementary differential amplifier circuit consisting of transistor Q ₃ is also constant.
The base voltage resulting from the base current and resistance R ₅₀ is also
It remains constant regardless of the power supply voltage.

The input signal applied to terminal 1 is the transistor
Amplified by a complementary differential amplifier circuit consisting of Q ₃ and Q ₄ with transistor Q ₅ as the load,
Applied to the base of transistor _Q11 . The collector of transistor Q ₁₁ is connected to power supply terminal 3,
The emitter is connected to the collector of a constant current source transistor _Q12 which serves as its current source. The emitter of transistor _Q12 is grounded via resistor _R7 . The signal is output to the emitter of transistor _Q11 and applied to the base of transistor _Q13 .
Transistor Q ₁₃ , diode D ₂ , and resistors R ₈ and R ₉ constitute an inverting amplifier. The emitter current is determined by the resistor _R9 connected to the collector of the transistor _Q13 , and the gain is determined by the resistors _R8 and _R9 .

The output signal is applied to the base of transistor Q ₁₄ and output to its collector with transistor Q ₁₇ and diodes Q ₁₆ and Q ₁₅ as loads. Transistor Q ₁₇ constitutes a current mirror circuit together with transistor Q ₁₀ , and biases diodes Q ₁₅ and Q ₁₆ and transistor Q ₁₄ . The collector of transistor _Q17 becomes the active load of transistor _Q14 . _C5 is a capacitor connected between the base and collector of transistor _Q17 . The collector of transistor Q ₁₄ is connected to PNP transistor Q ₁₈
It is connected to the base of one transistor _Q18 of a composite transistor in which an NPN transistor _Q20 is combined as shown. The collector of the transistor Q ₁₇ is connected to the base of the transistor Q ₁₉ , to which a signal amplified by the transistor Q ₁₄ is applied, and from the connection point between the emitter of the transistor Q ₁₉ , the emitter of the transistor Q ₁₈ , and the collector of the transistor Q ₂₀ . It is output to the output terminal 5 via the resistor _R51 , and taken out as an AC output signal to the load resistor R _{L. C10} is a phase compensation capacitor connected between the base and collector of transistor _Q14 .

The above output signal is output from the transistors of the differential amplifiers Q ₃₆ and Q ₃₅ configuring the comparison circuit C of the ALC circuit.
Input via capacitor _C6 to the base of _Q35 . The emitters of each of transistors Q ₃₅ and Q ₃₆ are
Each of them is connected to the collector of a transistor Q ₃₃ , which is a current source, through diodes D ₃ and D ₄ with their cathodes in common. Transistor Q ₃₃ ,
Q ₃₂ and resistors R ₁₈ and R ₁₇ constitute a current mirror circuit, which is supplied from the collector of transistor Q ₃₀ to diode-connected transistor Q ₃₂ . The base of transistor Q ₃₅ is connected to reference voltage source B
is connected to the emitter of the reference voltage output transistor _Q31 . The collector of transistor Q ₃₆ is
It is connected to power supply terminal 3. On the other hand, the collector of transistor _Q35 is connected to power supply terminal 3 via load resistor _R19 .

Now, add a diode to each emitter as shown above.
If the base voltages of differential amplifiers Q ₃₅ and Q ₃₆ in which D ₃ and D ₄ are inserted in series are V ₁ and V ₂ respectively, and the collector current of constant current transistor Q ₃₃ is Io, then the collector current of transistor Q ₃₅ is Ic is Ic=Io/{1+expq/2KT(V ₁ −V ₂ )}...(3). On the other hand, if we assume that there are no diodes D ₄ and D ₃ at the emitters of the differential amplifier, and that they are directly connected in common and connected to the collector of the transistor Q ₃₃ , then Ic=Io/{1+expq/KT(V ₁ −V ₂ )} ...(4).

Therefore, the change in Ic with respect to (V ₁ -V ₂ ), that is, the change in the collector current of transistor Q ₃₅ , is made gentler by inserting the diodes D ₃ and D ₄ .

The reference voltage source B is basically a transistor Q ₂₁ ,
It consists of Q ₂₃ , Q ₂₂ , Q ₂₁ and resistors R ₁₃ and R ₁₄ .

The base of transistor Q ₃₁ is connected to the above resistors R ₁₃ and R ₁₄
The emitter is connected to the collector of the constant current source transistor _Q29 , and supplies the emitter current of the transistor _Q31 . The emitter potential of transistor _Q31 is extracted as a reference voltage and applied to the base of transistor _Q36 .

Transistors Q ₂₂ , Q ₂₃ , Q ₂₆ , Q ₂₇ , Q ₂₅ , Q ₂₄ ,
Q ₂₈ , resistors R ₁₅ and R ₁₆ constitute a constant current source,
The current value is determined by resistors R ₁₅ and R ₁₆ . resistance
The current determined by R ₁₅ is current mirrored by transistors Q ₂₄ and Q ₂₅ , and is supplied from the collector of transistor Q ₂₅ to the collector of transistor Q ₂₆ and the base of transistor _{Q 27 .}
The base-emitter voltage and the current determined by the resistor R ₁₆ are current mirrored by the transistors Q ₂₈ , Q ₂₉ , and Q ₃₀ .

The reason for cascade-connecting the current sources in two stages as described above is to minimize the dependence of the constant current value on the power supply voltage. This stabilizes the current supplied to the transistor _Q36 constituting the comparator circuit C, and also reduces fluctuations in Ic, which is a reference for ALC operation, due to fluctuations in the power supply voltage.

Now, when the base potential of transistor Q ₃₅ becomes higher than the base potential of transistor Q ₃₆ , the collector current of transistor Q ₃₅ increases and the load resistance
An ALC control signal is taken out at _R19 . Then, when the base voltage of transistor _Q35 reaches a specified level, the transistor constituting drive circuit D
If the value of resistor _R19 is set so that a bias is applied between the base and emitter of _Q39 , current flows through transistor _Q39 that constitutes drive circuit D for driving variable impedance element E, and capacitor C ₈ and provides the base current of the transistor _Q40 that constitutes the drive circuit D. When the transistor Q ₄₀ is turned on, the transistor forming the variable impedance element E via the resistor R ₅₂
Drives the bases of Q ₁ and Q ₂ to change their saturation resistances and divide the input signal together with resistor R ₁ .
Note that _R29 constituting the drive circuit D is a resistor connected to the emitter of the transistor _Q39 .

The input side bias voltage source G of the comparator circuit C includes transistors Q ₅₀ , Q ₅₁ , Q ₅₂ , Q ₅₃ , Q ₅₄ , Q ₅₅ , Q ₅₆ ,
It is composed of Q ₅₇ and resistors R ₅₃ , R ₅₄ , R ₅₅ , and R ₅₆ , forming a low output impedance power supply whose output terminal is the common connection point of each emitter of transistors Q ₅₆ and Q ₅₇ . The output terminal is connected to the input terminal 7 of the comparator circuit C via a resistor _R58 .

The base of transistor Qs ₄ , which constitutes the transient noise reduction circuit F connected to improve the transient noise at power-on, is connected exclusively to terminal 4, the collector is connected to power supply terminal 3, and the emitter is connected through resistor R ₂₅ . It is connected to the collector and base of a diode-connected transistor Qs ₅ , and also to the base of a transistor Qs ₆ . Transistor Qs ₅ ,
The emitter of Qs ₆ is grounded. transistor
The collector of Qs ₆ is connected to the power supply terminal 3 via a resistor R ₂₄ and also to the base of the transistor Qs ₇ . The emitter of transistor Qs ₇ is grounded and the collector of transistor Qs ₇ is resistor
It is connected to the bases of transistors Qs ₁ and Qs ₂ via R ₂₃ . The emitter of transistor Qs ₁ is connected directly, and the emitter of transistor Qs ₂ is connected to a resistor.
It is connected to the power supply terminal 3 via _R10 . The collector of the transistor Qs ₁ is connected to the base of the transistor Qs ₁₀ , the emitter of the transistor Qs ₁₀ is connected to the terminal 2, and its collector is connected to the power supply terminal 3 via the resistor R ₅₇ . transistor
The collector of Qs ₂ is connected to the base of transistor Qs ₃ , the emitter of transistor Qs ₃ is connected to ground,
The collectors are also connected to the bases of the output transistors _Q19 , respectively.

Now, if we consider the case where the power is turned on without this transient noise reduction circuit F, negative feedback resistor R ₁₁ and capacitor C ₂ are required to make terminal 2 a normal bias 2V _BE .
The time constant becomes , and the output bias remains close to the power supply voltage for several seconds, and does not become a normal output bias.
However, when this circuit F is connected, the base current is supplied to the transistor Qs ₇ for approximately 2V _BE (diode forward voltage of Qs ₄ and Qs ₅ ) after the power is turned on, and the base current is supplied to the transistors Qs ₁ and Qs _10. charges terminal 2 to the 2-terminal voltage of 2V _BE regardless of the negative feedback resistor R ₁₁ , and the transistors Qs ₂ and Qs ₃
Absorb the base current of the output transistor _Q19 to prevent abnormal noise from occurring in the output, and set the output bias to approximately the ground potential.

When terminal 4 reaches a voltage that makes transistors Qs ₄ and Qs ₅ conductive, transistor Qs ₆ becomes conductive and no longer supplies current to the base of transistor Qs ₇ , so charging of terminal 2 and base shorting operation of output transistor Q ₁₉ are canceled. and normal amplification operation will occur. Note that this charging time is controlled by the capacitor _C3 connected to the terminal 4.

In the above configuration, the low frequency amplifier circuit A
output DC potential, that is, the DC potential of output terminal 5
When Vo is set so that R ₄ =2R ₁₁ , it becomes 1/2V _CC as shown in equation (2) above.

On the other hand, the output DC voltage V _B of the reference voltage source B is expressed by the following equation.

V _B = (V _CC −3V _BE ) × R ₁₄ / R ₁₃ + R ₁₄ + 3V _BE ……(5) Here, when setting R ₁₃ = R ₁₄ , V _B = 1/2V _CC + 3/2V _BE ……(6) becomes.

Further, the output current voltage V _G of the low output impedance power supply G is expressed by the following equation.

V _G = (V _CC −2V _BE ) × R ₅₄ / R ₅₃ + R ₅₄ + V _BE ……(7) Here, if we set R ₅₃ = R ₅₄ , V _G = 1/2V _CC ……( 8) becomes.

Therefore, the relationship between Vo, V _B and V _G is
It always remains constant even if V _CC fluctuates.

ALC operation start voltage V _S when output terminal 5 and input terminal 7 are directly connected under these conditions
That is, the threshold value of comparison circuit C is expressed by the following equation.

V _S = V _B − V _G /√2=3/2√2V _BE ……(9) The base voltage of the transistor _Q35 that constitutes the comparator circuit C is always 2/3V _BE higher than the reference voltage _VB . is low and no current flows to the collector of transistor _Q35 unless it exceeds the threshold value in equation (9) above, so transistor _Q39 is not driven through load resistor _R19 , so drive circuit D does not operate. . Therefore, the variable impedance element E is in a cut-off state, and the impedance between the collectors and emitters of the transistors Q ₁ and Q ₂ becomes very large, so the input signal is not divided into voltages with the resistor R ₁ .
A signal amplified by the gain of the low frequency amplifier circuit A is taken out to the load resistor R _L.

On the other hand, the output signal voltage of low frequency amplifier circuit A is
When the threshold value determined by equation (9) is exceeded, the transistor
Current flows through the collector of Q ₃₅ . This current causes a voltage drop across resistor R ₁₉ and transistor Q ₃₉ ,
Q ₄₀ becomes conductive, lowering the saturation resistance between the collectors and emitters of transistors Q ₁ and Q ₂ , which are variable impedance elements E, and dividing the input signal with resistor R ₁ , thereby increasing the signal level output to terminal 5. lowers and enters ALC operation.

When the output terminal 5 and the input terminal 7 are directly connected in this way, the ALC operation is performed depending on the magnitude of the output signal voltage of the low frequency amplifier circuit A.

In this case, the frequency characteristic during ALC operation will be a flat characteristic as shown in A of FIG. 2 unless the low frequency amplifier circuit A has a frequency characteristic.

However, in the case of the embodiment shown in FIG.
Do not connect input terminal 7 directly to capacitor C ₆ .
Since it is connected via a , it is possible to change the frequency characteristics during ALC operation and give flexibility to the recording characteristics.

That is, since the capacitor _C6 is connected, the signal input to the comparator circuit C is voltage-divided between it and the resistor _R58 .

This shows that the signal voltage input to the comparator circuit C changes depending on the frequency, and in the case of this example, the lower the frequency, the smaller the signal that exceeds the threshold of the comparator circuit C. As a result, the start of operation of the ALC is delayed, and the output voltage of the low frequency amplifier circuit A becomes large, so that a characteristic as shown in FIG. 2 (b) is obtained.

The frequency inflection point fω in this case is expressed by the following equation.

fω=1/2πC ₆ R ₅ ...(10) Therefore, the resistor R ₅₈ connected between the output terminal and input terminal 7 of the low output impedance power supply G, and the resistor R 58 connected between the output terminal 5 and input terminal 7 By changing the size of capacitor _C6 , the frequency characteristics during ALC operation can be changed freely.

In addition, if you want to slow down the ALC operation in high frequencies,
Connect a resistor between output terminal 5 and input terminal 7,
It is also possible to obtain desired characteristics by connecting a capacitor between the input terminal 7 and ground.

Moreover, when used for a stereo tape recorder, the low frequency amplifier circuit A, the comparator circuit C, the drive circuit D, and the variable impedance element E may each be configured with two circuits. Even in this case, if the semiconductor integrated circuit is used, variations between the two circuits can be sufficiently reduced.

Although the above description has been given as a recording amplifier with ALC, it is also possible to provide a switch S at the terminal 8 as shown in FIG. 3 and use it as a recording/playback switch. That is, when the switch S is open, it can be used as a recording amplifier as described above, and when the switch S is closed, it can be used as a reproducing amplifier.

During playback, that is, when the switch S is closed, the base of the transistor _Q40 is grounded, so the operation of the drive circuit D is stopped and the variable impedance element E is not driven. Therefore, any output signal level,
That is, at any input signal level, the emitters and collectors of transistors Q ₁ and Q ₂ are open, the input signal is not voltage divided, and the output signal is amplified by the gain of low frequency amplifier circuit A and sent to terminal 5. It is output from

During recording, that is, when switch S is open, ALC operation is possible as described above.

As described above, the recording amplification circuit of the present invention
A low frequency amplifier circuit having a variable impedance element directly connected in a direct current manner, and a comparison circuit that compares the output voltage of the low frequency amplifier circuit with a reference voltage, and controlling the variable impedance element by the output signal of the comparison circuit. In the recording amplification circuit which controls the output level of the low frequency amplification circuit, a low output impedance power supply is provided, and the output terminal of this power supply is connected to the input terminal of the comparison circuit through a resistor. A capacitor was installed, one electrode of which was connected to the output terminal of the above-mentioned low-frequency amplifier circuit, and the other electrode connected to the input terminal of the above-mentioned comparison circuit, so that the frequency characteristics during ALC operation could be changed to record. Characteristics can be freely selected, and in particular, when the signal input to the low frequency amplifier circuit has a low frequency, the output voltage of the low frequency amplifier circuit can be increased, and an output that emphasizes the low frequency can be obtained. . In addition, especially when constructed using a semiconductor integrated circuit, desired frequency characteristics can be obtained with a single external capacitor, which is extremely advantageous.

[Brief explanation of drawings]

Fig. 1 is an electrical wiring diagram showing one embodiment of the present invention, Fig. 2 is a characteristic diagram showing frequency characteristics during ALC operation, and Fig. 3 is an electrical wiring diagram of main parts showing another embodiment of the invention. It is a diagram. In the figure, A is a low frequency amplifier circuit, B is a reference voltage source, C is a comparison circuit, D is a drive circuit, E is a variable impedance element, G is a low output impedance power supply, _C6 is a capacitor, and _R58 is a resistor. .

Claims (1)

[Claims] 1. A low-frequency amplification circuit with a variable impedance element directly connected in a direct current manner, a comparison circuit that compares and detects the output voltage of this low-frequency amplification circuit with a reference voltage, and outputs a reference voltage to be applied to this comparison circuit. a drive circuit that controls the output level of the low frequency amplifier circuit by controlling the variable impedance element using a reference voltage source and an output signal of the comparison circuit;
a capacitor having one electrode connected to the output terminal of the low frequency amplifier circuit and the other electrode connected to the input terminal of the comparison circuit; A recording amplifier circuit characterized in that a resistor is connected between an input terminal and an output terminal of the low output impedance power supply.