JPS6126845B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a shock noise prevention circuit in a transistor amplifier for preventing unpleasant shock noise from being generated from a speaker serving as a load when the power is turned on. The object of the present invention is to provide an effective shock noise prevention circuit that can be used in transistor amplifiers.

Output end connected to load via capacitor
In a SEPP (single edge push pull) amplifier circuit, when the power is turned on, the voltage at the output terminal suddenly rises from zero to a stable value (+1/2Vcc), which generates a shock noise. Therefore, various circuits have been proposed to prevent the occurrence of the shock noise. For example, in FIG. 1, a ripple filter transistor 1 is inserted into the power supply line, and the power supply voltage delayed through the ripple filter is applied to the base of the positive side driver transistor 2, thereby reducing shock noise. The aim is to prevent the occurrence of In the circuit shown in Figure 1, power switch 3
When the ripple filter transistor 1 is turned on, the base voltage of the ripple filter transistor 1 increases with a time constant determined by the resistor 4 and the capacitor 5, and becomes as shown in FIG. 2A.
Further, as shown in FIG. 2B, the emitter voltage of the transistor 1 also gradually increases to a value lower than the base voltage by V _BE (the voltage between the base and emitter of the transistor). Therefore, the voltage at the output terminal C of the amplifier gradually increases in accordance with the emitter voltage of the transistor 1 , as shown in _FIG . This prevents the occurrence of shot noise. Therefore, in the circuit shown in Figure 1, the time required for stabilization is
Since it follows the time constant determined by the resistor 4 and capacitor 5 of the ripple filter, it has the advantage that it does not malfunction and there is little variation in the way it rises. Also, since the voltage at the output terminal C of the amplifier starts to rise immediately after the power is turned on, charging of the NF capacitor 7 via the NF (negative feedback) path also starts immediately after the power is turned on, resulting in a fast starting time. Benefits can be obtained. However, the circuit shown in Figure 1 requires a special ripple filter, which increases the complexity of the circuit and the IC.
The loss voltage at maximum drive, which causes an increase in the number of external components when the power supply is switched on, increases due to the voltage drop caused by the resistor 4 and the V _BE of the transistor 1, and the power supply utilization rate decreases. In order to prevent the decrease in the power utilization rate,
If the value of the resistor 4 is made small, the ripple filter effect will be reduced and hum noise will be more likely to be generated, which is not perfect.

FIG. 3 shows another example of a conventional shock noise prevention circuit. In the circuit shown in FIG. 3, the emitter of a specially provided control transistor 8 is connected to a reference potential point D, and the base is connected to one end of a capacitor 10 of a base bias circuit of a differential amplifier circuit 9 . This is to prevent the occurrence of shock noise by rapidly charging the feedback capacitor 11 and forcibly controlling the predriver transistor 12. Therefore, the circuit shown in FIG. 3 forcibly turns on the pre-driver transistor 12 from immediately after the power is turned on until just before the amplifier reaches a stable state.
Since the positive side driver transistor 13 and the positive side power transistor 14 can be forcibly turned off, there is an advantage that the introduction of hum in a transient state can be completely prevented. However, in the circuit shown in FIG. 3, in order to suppress the shock noise, the control transistor must be kept conductive until just before the circuit stabilizes.
Therefore, there is a drawback that the starting time becomes long, and since the base voltage and emitter voltage of the control transistor 8 are close to each other in a steady state, there is a drawback that malfunction is likely to occur due to sudden fluctuations in the power supply voltage. FIG. 4 shows voltage changes at points D, E, and F in FIG. 3.

The present invention has been made in view of the various points mentioned above, and will be described below based on embodiments of the present invention with reference to the drawings.

In FIG. 5, reference numeral 15 denotes a differential amplifier circuit consisting of a pair of transistors 16 and 17 of the same conductivity type; 18;
19 is an interstage transistor, 19 is a pre-driver transistor 20, and driver transistors 21 and 2.
2. Consisting of power transistors 23 and 24
25 is a speaker serving as a load connected to the output terminal J of the SEPP amplifier circuit 19 via a capacitor 26; 27 is a shock noise prevention circuit; the shock noise prevention circuit 27 has a base connected to the differential A first transistor 29 connected to a capacitor 28 (hereinafter referred to as a bias capacitor) in a base bias circuit of one transistor 16 of the amplifier circuit 15 ;
9, a second transistor 30 whose emitter is connected to the power supply line, and the emitter of the first transistor 29 and the second transistor 30.
a first resistor 31 connected between the collector of the
a second resistor 32 connected between the emitter of the first transistor 29 and ground, and a base connected to the second resistor 32;
The third transistor 33 includes a collector of the transistor 30, an emitter connected to the base of the driver transistor 21, and a collector connected to the base of the pre-driver transistor 20.

Next, the operation will be explained. When the power switch 34 is not turned on, the bias capacitor 28, negative feedback capacitor 35, and output capacitor 26 are in a discharged state. Now, when the power switch 34 is turned on at time _t0 , the bias capacitor 28 is charged according to the time constant determined by the first bias resistor 36 and the bias capacitor 28, and the voltage at point G is 2 bias resistor 37. As the voltage at the point G increases, the collector current of the first transistor 29 increases, and the collector current of the second transistor 30 also increases. Therefore, if the base voltage of the first transistor 29 is V _G , the emitter voltage V _H is V _H = V _G - V _BE (1) (where V _BE is the base-emitter voltage of the first transistor 29). Therefore, the collector voltage of the second transistor 30 is V
_I is V _I = R ₁ + R ₂ / R ₂ V _H = R ₁ + R ₂ / R ₂ (V _G - V _BE )...(2) [However, R ₁ is the resistance value of the first resistor 31, and R ₂ is Resistance value of second resistor 32] Therefore, due to the increase in the base voltage of the first transistor 29, its emitter voltage, that is,
The voltage at point H increases as shown in Figure 6 H, and the second
The collector voltage of transistor 30, that is, point I
The voltage increases as shown in FIG. 6I.

Immediately after the power is turned on, the voltage at point I is almost zero, so the third transistor 33 is sufficiently forward biased, and the current flowing through the resistor 38 flows through the emitter-collector path of the third transistor 33. Being sidetracked. Therefore, the negative feedback capacitor 35 and the output capacitor 26 are not charged.
The voltage at the output point J of the amplifier is approximately zero. As time passes, as the base voltage of the first transistor 29, that is, the voltage at point G increases, the voltages at points H and I also increase accordingly, and the base voltage of the third transistor 33 approaches the emitter voltage. go. Therefore, the emitter current of the third transistor 33 gradually decreases, and the remaining current flowing through the resistor 38 is passed through the positive side driver transistor 21 and the power transistor 23 to the negative feedback capacitor 35 and the output capacitor 26. It is used to charge the battery. Since the remaining amount increases as the emitter current of the third transistor 33 decreases, the voltage at the output point J gradually increases in accordance with the voltage at the point I, as shown in FIG. 6J. go.

After sufficient time has passed, bias capacitor 2
8, the base voltage of the first transistor 29 (voltage at point G) reaches a predetermined value determined by the values of the first and second bias resistors 36 and 37, and accordingly, the voltage at points H and I increases. The voltage also reaches a predetermined value. For example, the first and second bias resistors 36
and 37 are made equal, and the first and second resistors 31
If the values of and 32 are set equal, the voltage at point G is
1/2V _CC (however, V _CC is the power supply voltage), and the point H
From equation (1), the voltage at point I becomes [1/2V _CC -V _BE ], and from equation (2), the voltage at point I becomes [V _CC -2V _BE ].
becomes. When the voltage at point I reaches a predetermined value, the third transistor 33 is already in a reverse bias state, and the voltage at point J is 1/2V _CC , which is in a steady state.

The amplification operation of the amplifier in a steady state is no different from that of a normal amplifier, so a description thereof will be omitted. Furthermore, in the shock noise prevention circuit, since the base voltage (voltage at point I) of the third transistor 33 is fixed to [V _CC -2V _BE ], the emitter voltage of the third transistor 33 is fixed to [V _CC -V BE ]. There is no effect on the amplifier unless the value exceeds _BE ]. By the way, in a no-signal state, the voltage at the midpoint J of the amplifier's output is
Since the voltage is 1/2V _CC , the third transistor 33
The emitter voltage is [1/2V _CC +2V _BE ], so even if a considerably large input signal is applied, the third transistor 33 will not be forward biased.

In the embodiment shown in FIG. 5, the voltage obtained at the collector of the first transistor 29 acts on the base of the second transistor 30 to form a feedback circuit from the collector to the emitter of the first transistor 29. Therefore, point I has the same hum removal function as point H. Therefore, hum from the time the power is turned on until the steady state is reached is reliably eliminated.

As described above, the shock noise prevention circuit according to the present invention utilizes the gradually increasing voltage of the bias circuit of the amplifier and boosts the voltage several times to drive the control transistor, so it is particularly effective. It has the advantage that no circuit is required to obtain a gradually increasing voltage. In addition, there is no need to provide a separate capacitor, and the voltage for driving the control transistor can be set only by the resistance ratio, which provides a great advantage when integrated into an IC. Further, since charging of the feedback capacitor and the output capacitor starts immediately after the power is turned on, there is an advantage that the starting time is short. Furthermore, in a steady state, the control transistor is sufficiently reverse biased, so malfunctions caused by large signals can almost certainly be prevented, and a circuit with good ham-rejection can be obtained without using a ripple filter. It is an excellent product with advantages.

[Brief explanation of the drawing]

1 and 3 are circuit diagrams showing a conventional shock noise prevention circuit, FIGS. 2 and 4 are characteristic diagrams showing voltage changes at each part, and FIG. 5 is a circuit showing an embodiment of the present invention. 6 are characteristic diagrams showing voltage changes at each part. Explanation of main drawing numbers, 19 ...SEPP amplifier circuit, 2
7... Shock noise prevention circuit, 29, 30, 33...
...transistor, 31, 32, 36, 37...resistance.

Claims (1)

[Claims] 1 This is a circuit to prevent the shock noise that occurs due to an unbalanced state of the amplifier when the power is turned on.It is a circuit that prevents the shock noise that occurs due to the unbalanced state of the amplifier when the power is turned on. A detection circuit for detecting, a circuit for boosting the output voltage of the detection circuit to a predetermined times, and an output of the boosting circuit whose emitter is connected to the base of an amplification transistor constituting an amplifier, and the output of the boosting circuit is applied to the base. The voltage at the output point of the amplifier is increased by increasing the emitter voltage of the control transistor and increasing the conductivity of the amplification transistor as time passes after the power is turned on. A shock noise prevention circuit characterized in that the output voltage of the boosting circuit is set to a predetermined value and the control transistor is placed in a reverse bias state when a predetermined time has elapsed from power-on.