JPH07123208B2 - Low frequency amplifier - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low frequency amplifier used for electronic equipment such as a radio receiver.

2. Description of the Related Art In recent years, there have been increasing demands for miniaturization and cost reduction of various electronic devices, and accordingly, demands for miniaturization and cost reduction of various components have increased. Further, due to the miniaturization of the battery used with the miniaturization, further current saving of the equipment is required.

Usually, a low-frequency amplifier used in a small electronic device is used as a class B operation from the viewpoint of current saving and high efficiency.
In this case, crossover distortion occurs because the base-emitter voltage-collector current characteristic of the output stage transistor is non-linear. For this reason, it is necessary for the transistor to set the operating point in the absence of signal so that its nonlinearity does not matter, and to allow the collector bias current to flow even in the absence of signal. This operating point setting changes depending on the state of the output load. When the output load impedance becomes low, the output current change necessary to obtain the same output voltage change as when the load impedance is high becomes large.
The operating point is set so that the base-emitter voltage-collector current characteristic becomes more linear. That is, it is necessary to set the operating point in a state where more collector bias current flows. On the other hand, when a high output load is driven in a state where the bias is set to match the low output load, the bias current may become larger than the load current, which causes a problem in saving current. Therefore, using the same low frequency amplifier,
When driving a plurality of loads having different impedances, it is desirable to be able to set an appropriate bias state for each load.

FIG. 4 shows a conventional circuit.

In FIG. 4, the transistors 15 and 24 are for current supply, and a current equal to the current I of the constant current source 13 is supplied to the load circuit as the collector current of each transistor. The transistors 17 and 18 form a differential amplifier circuit, and the transistors 19 and 20 are active loads of the differential amplifier circuit. Resistance 10,12,1
Reference numerals 6, 21, 22, and 23 are for biasing the differential amplifier circuit. The output of the differential amplifier circuit is input from the collector of the transistor 17 to the base of the signal amplifying transistor 29, and the transistor 29 drives the class B push-pull circuit composed of the transistors 30, 31, 35 and 36.

The transistor 24, the diode 33, and the resistors 26, 32, and 34 form a bias circuit of the output circuit. Here, the resistors 32 and 34 are for biasing the diode 33. In this circuit, the base-emitter voltage of the transistors 30 and 31 is given by the following equation.

_{V BE30 = (V D33 + V} BE28) - (V BE31 + V R26) ...... (1) V BE31 = (V BE30 + V R26) - (V D33 + V BE28) ...... (2) provided that V _BE28: the base of the transistor 28 emitter voltage V _BE30: voltage between the base and emitter of the transistor 30 V _BE31: base-emitter voltage V _D31 of the transistor 31 is the forward voltage drop V _R26 diode 33: voltage drop No signal get off at both ends of the resistor 26 In the operating state, the collector currents of the output transistors 35 and 36 are equal, and the transistors 30 and 31 are operating in a state of satisfying the expressions (1) and (2). In this circuit, by changing the current flowing through the resistor 26, the V _R26
The bias state is changed by changing the value of. The collector and the base of the transistor 40 are connected to the collector and the base of the transistor 24, respectively. When the switch 3 is closed, the emitter of the transistor 40 is connected to the power supply line and is in the operating state.
A current mirror circuit is formed with the transistors 14, 15 and 24, and the collector current of the transistor 40 also becomes I. Since the collectors of the transistors 24 and 40 are connected to the resistor 26, the current supplied to the resistor 26 is 21. The switch 4 that interlocks with the switch 3 is connected to the high impedance load (earphone or the like) 6 side. If you open switch 3,
Since the transistor 40 is deactivated, the current supplied to the resistor 26 becomes I, and the voltage drop across the resistor 26 decreases, so that the collector currents of the output transistors 35 and 36 increase. In this case, the switch 4 interlocking with the switch 3 is connected to the low impedance load (speaker or the like) 5 side. Output transistor 35,3 for changes of I = number + μm
The collector current of 6 changes by several mA.

Further, by inserting a resistor in series between the emitter of the transistor 40 and the power supply line, an appropriate bias state can be set by the load.

Problems to be Solved by the Invention However, in the conventional configuration shown in FIG. 4, the collector current of the amplifying transistor 29 is changed by opening / closing the switch 3, so that the gain of the transistor 29 is changed. When the switch 3 is closed, the collector current supplied to the transistor 29 increases, the voltage gain of the transistor 29 increases, and the negative feedback amount of the low frequency amplifier as a whole increases. There was a possibility of causing oscillation.

In view of the above problems, the present invention provides a low frequency amplifier capable of changing only the bias current without changing the advantage of the amplifier.

Means for Solving the Problems In order to solve the above problems, the present invention provides a bias circuit control circuit capable of changing only the bias state of a bias circuit without changing the collector current of an amplifying transistor. It is characterized by the addition.

Function The present invention makes it possible to change the collector bias current of the output transistor without changing the voltage gain of the amplifier.

Embodiment 1 FIG. 1 shows a first embodiment of the present invention. The circuit configuration shown in Fig. 1 eliminates transistor 40 and transistor 5
The circuit is the same as that of FIG. 4 except that 0, 51, 52 and 53 are added.

A bias control circuit composed of transistors 50, 51, 52 and 53 in FIG. 1 will be described below.

As described in the conventional circuit shown in FIG. 4, the base-emitter voltages of the transistors 30 and 31 are given by the equations (1) and (2), respectively. Also in this circuit, as in FIG. 4, the bias state is changed by changing the voltage drop across the resistor 26. When the switch 3 is closed, the transistors 50 and 51 having the same base as the transistor 24 operate. Assuming that the transistors 50 and 51 have the same characteristics, the collector current during operation is I ', where I'≤
I. Since the transistors 52 and 53 form a current mirror circuit, if the characteristics of the two transistors are equal, the collector current of the transistor 52 is also I '.
Therefore, the current flowing into the resistor 26 from the point e is the difference between the collector current of the transistor 24 and the collector current of the transistor 52, that is, (I-I '). As a result, the voltage drop across the resistor is reduced, and the collector bias current of the output transistors 35 and 36 is increased. On the other hand, the collector current of the transistor 50 flows into point f, which is the other end of the resistor. If the characteristics of the transistors 50 and 51 are equal to each other, this current becomes I ′, which is equal to the collector current of the transistor 52, so that the collector current of the transistor 48, that is, the collector current of the amplifying transistor 29 remains constant at I,
The voltage gain of the amplifying transistor does not change. In this state, the switch 4 interlocking with the switch 3 is connected to the low impedance load (speaker or the like) 5 side. (That is, contrary to the conventional example, the bias current becomes the minimum when the switch 3 is open.) In this example, I '= 50 μA,
The collector current of the output transistors 35 and 36 changes by about 5 mA.

FIG. 2 shows a second embodiment of the present invention. This embodiment is the same as the first embodiment shown in FIG. 1 except that the resistor 37 is added. In this embodiment, when the switch 3 is closed, a resistor 37 is inserted between the emitters of the transistors 50 and 51 and the current line 2, and the resistance value allows the bias state of the output circuit to be appropriate. The collector current of 50 and 51 can be controlled. The current value may be controlled by changing the emitter areas of the transistors 14, 24, 50, 51 when the circuit of the embodiment is integrated into an IC. Further, a resistor may be inserted between the emitter of each of the transistors 14, 24, 50 and 51 and the power supply line, and the current value may be controlled by the resistance ratio.

In addition, as in the third embodiment shown in FIG.
It is also possible to drive the separate circuit 54 by another transistor 53 which is in operation or inactive at the same time as 50 and 51.

The circuit configuration other than the bias circuit is not limited to the embodiment.

As described above, according to the present invention, by adding the bias control circuit,
It is possible to change the bias current of the output stage without changing the gain of the amplifier, and when it is integrated into an IC, the bias current becomes the minimum when the control circuit does not operate, so when used with a single load, In a small current-saving device, which originally wants to omit external parts, it does not require external parts as in the conventional case, and enables downsizing and cost reduction of the device.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a low frequency amplifier of the present invention, FIG. 2 is a circuit diagram showing a second embodiment of a low frequency amplifier of the present invention, and FIG. FIG. 4 is a circuit diagram showing a third embodiment of a frequency amplifier, and FIG. 4 is a circuit diagram of a conventional low frequency amplifier. 24 …… first transistor, 26 …… first resistor, 28 ……
Second transistor, 29 ... Third transistor, 30 ...
4th transistor, 31 5th transistor, 5
1,52,53,54 …… Bias control circuit configuration transistor.

Claims (1)

[Claims] 1. A collector of a first transistor for supplying a current to a bias circuit, a base of a second transistor, and one end of a first resistor are connected, and a collector of the second transistor and a first resistor are connected. The other end is connected, the emitter of the second transistor is connected to the collector of the third transistor for low frequency amplification, and the collector and the emitter of the second transistor have fourth and fifth outputs, respectively. The bases of the stage transistors are connected so that a bias voltage is applied and currents for driving are respectively taken out, and a constant current is applied to one end of the first resistor that is activated or deactivated by opening / closing a switch. And a bias control circuit for shunting a current equal to the supplied current from the other end, and the collector current of the third transistor is And then in a state, low-frequency amplifier, characterized in that the collector current at no signal of said fourth and fifth transistors configured to be varied by the bias control circuit.