JPS61145908A - Low frequency amplifier - Google Patents

Abstract

PURPOSE:To switch a bias current only through the addition of one terminal in integrating the titled amplifier circuit by providing the 2nd transistor (TR) whose collector is connected to that of the 1st TR applying a current to a bias circuit and operated or non-operated by the switching of a switch. CONSTITUTION:In closing a switch 3, an emitter of a TR40 is connected to a power line, the TR40 is operated, TRs 14, 15, 24, 40 constitute a current mirror circuit and the collector current of the TR40 is a current I of a constant current source 13. Since collectors of the TRs 24, 40 are connected to a resistor 26, the current applies to the resistor 26 is 2I. The switch 4 is connected to an earphone load 6. In opening the switch 3, the TR40 is inoperative, the current applies to the resistor 26 is I and the voltage drop across the resistor 26 is small, then the collector current of output TRs 35, 36 is increased. In this case, the switch 4 is connected to a speaker load 5.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a low frequency amplifier used in electronic equipment such as radio receivers.

BACKGROUND OF THE INVENTION In recent years, there has been an increasing demand for miniaturization and cost reduction of various electronic devices, and along with this, demand for miniaturization and cost reduction of various parts has also increased. Furthermore, as the batteries used become smaller due to miniaturization, reducing the current consumption of devices has become a major issue.

Low-frequency amplifiers used in small electronic devices usually save current;
Class B operation is used for high efficiency. In this case, crossover distortion occurs because the base-emitter voltage vs. collector current characteristic of the output stage transistor is nonlinear. For this reason, it is necessary to set the operating point of the transistor when no signal is present so that its nonlinearity does not become a problem, and to keep the collector bias current flowing even when there is no signal. This operating point setting changes depending on the state of the output load. When a speaker is used as a load and the impedance of the output load is low, the change in output current required to obtain the same output voltage change is larger than when the load impedance is high, so the base-emitter voltage vs. collector current characteristic All operating points are set so that it becomes closer to linearity. In other words, it is necessary to set the operating point to a state where a larger amount of collector bias current flows. Negative in this state: ir
High impedance, for example 300Ω
When an earphone is used, the bias current of the L output transistor may become larger than the L output load current, which poses a problem in terms of current saving. Therefore, it is necessary to individually set appropriate bias conditions depending on the load impedance.

FIG. 4 shows a conventional circuit. In FIG. 4, transistors 15 and 24 are for current supply;
A current mirror circuit is formed, and a current equal to the current of the constant current source 13 is supplied to the load circuit as the collector current of each transistor.

Transistors 17 and 18 constitute the entire differential amplifier circuit, and transistors 19 and 20 are active loads of the differential amplifier circuit (resistors 10, 12, 16, 21, 22, and 23 are for bias of the differential amplifier circuit). ).

The output of this differential amplifier circuit is inputted from the collector of the transistor 17 to the base of the signal amplifying transistor 29,
By this transistor 29, transistors 3Q, 31
．． Drives a class B Phnom Yuble output circuit consisting of 35.36. Transistor 24, diode 33, resistor 2
6, 32, and 34 constitute a bias circuit of the output circuit.

Resistors 32 and 34 are for biasing the diode 33.

In this circuit, the base-emitter voltage of transistors 30 and 31 is given by the following equation.

val130- (VD33 +VBI+28) (
'/+n+s++VR26) -(')vsz3+=(
vaxso+vn26) (Vos3+”ax2a)
-(2) However, vBII2[1: Base-emitter voltage V□3o of transistor 28: Base-emitter voltage V□3. : Base-emitter voltage VD33 of transistor 31 : Forward voltage drop vR2 of diode 33
6: Voltage drop across resistor 26 In a no-signal operating state, the collector currents of output transistors 36 and 36 are equal, and transistors 30 and 31 operate in a state that satisfies equations (1) and (2) above. ing. In order to change the entire collector bias current of the output transistor 36°360, it is sufficient to change the voltage between the base and emitter of the transistors 30 and 31 when there is no signal. Conventionally, this has been dealt with by allowing the resistance value of the resistor 26 to change completely, and as shown in Figure 4, switches 3 and 4 are linked, and when switch 3 is open, switch 4 is connected to the earphone load 6 side. has been done. (Resistor 26 is set to be in an appropriate bias state in this state.Next, switch 3
When closed, the switch 4 is connected to the speaker load 6 side. By closing switch 3! :47L27
is connected in parallel with the resistor 26, so the voltage drop across this parallel resistor becomes small. Therefore, equation (1) and (2
The value of vR26 in the equation ) becomes smaller, the base-emitter voltage of the transistors 30 and 31 becomes larger, and the collector currents of the transistors 30 and 31 become larger. Since this current becomes the base current of the output transistors 35 and 36, the collector bias current of the output transistors also increases. The value of the resistor 27 is set to a value that provides an appropriate bias state when driving the speaker when the resistor 27 is fully connected in parallel with the resistor 26. By opening and closing the switch 3 as described above, it becomes possible to select an appropriate bias state depending on the load.

Problems to be Solved by the Invention However, in the conventional configuration shown in FIG. When integrated into an IC, the two terminals a and b forming the switch 3 must be provided as external terminals for bias switching, which increases the number of pins of the IC package. Miniaturization as mentioned above.

As demand for lower costs increases, I
The increasing size and cost of C packages has become a problem.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention connects the collectors of the first transistor and the first transistor that supplies the entire current to the bias circuit, and creates a bias circuit that is activated or deactivated by opening and closing a switch. The main feature is that a second transistor is provided for supplying current to the circuit.

Function The present invention allows a bias current switching switch to be configured between the power supply line of the low frequency amplifier, so 1. When integrated into an IC, it is possible to switch the bias current by adding one terminal, allowing for miniaturization of the IC package. Embodiment FIG. 1 shows a first embodiment of the present invention.

The circuit configuration of FIG. 1 is the same as the circuit of FIG. 4 except for the addition of transistor 40 and the elimination of resistor 27.

The bias switching circuit shown in FIG. 1 will be described below. Fourth
As mentioned in the circuit description of the figure, transistor 30.31
The base Φ emitter voltage of is expressed by equation (1) and (
2) Given by Eq. In the circuit of Figure 4, transistor 3
In order to change the bias state by changing the base-emitter voltage of 0.31, the voltage drop across the resistor 26 is changed by changing the value of the resistor 26. On the other hand, in FIG. 1, the value of the resistor 26 is constant, and the resistor 2
By varying the current supplied to 6 it is also possible to vary the voltage drop across it.

In FIG. 1, transistors 14, 15, 24°40
The bases of the transistors 14, 15, and 24 are connected to each other, and the emitters of the transistors 14, 15, and 24 are all connected to the power supply line. Therefore, these three transistors form a current mirror circuit f114, and a constant current source 13
A current equal to the current factor flows as the collector current of each transistor. When the switch 3 is closed, the emitter 7 of the transistor 40 is connected to the power supply line and becomes operational, the transistors 14115, 24, and 40 form a current mirror circuit, and the collector current of the transistor 40 also becomes a current. The collector of transistor 24.40 is resistor 26
Since the current supplied to the resistor 26 is connected to
Since the transistor 40 is inactive, the current that can be supplied to the resistor 26 is reduced, and the voltage drop across the resistor 26 becomes smaller, so that the collector currents of the output transistors 35 and 36 increase. In this case, the switch 4 which is interlocked with the switch 3 is completely connected to the speaker load 5 side. In this example, for I = 501tA, the output transistor 3
The collector current of 5.36 changes by about 5 mA. When this embodiment is implemented as an IC, bias current switching can be performed by adding one terminal, a terminal of the switch 3.

This embodiment is the same as the first embodiment shown in FIG. 1 except for the addition of a resistor 41. In this embodiment, when the switch 3 is closed, a resistor 41 is inserted between the emitter of the transistor 40 and the power supply line, and the resistance value of the resistor 41 causes the output circuit to be biased appropriately.
collector current can be controlled. The current value is controlled by the transistor 24.40 when the entire circuit of the embodiment is implemented as an IC.
Row 1 may be formed by changing the emitter area of the . A resistor may be inserted between the emitters of the transistors 14, 24, and 40 and the power supply line, respectively, and the current value may be controlled by the resistance ratio.

Further, as in the third embodiment shown in FIG. 3, another transistor 42 which operates or does not operate at the same time as the transistor 40 allows a separate path 60>! It is also possible to move l;l<.

Note that the formation of circuits other than the bias circuit is not limited to the embodiment.

Effects of the Invention As described above, the present invention makes it possible to save current by configuring an open/close switch between the power supply line and setting the collector bias current of the output transistor according to the output load by opening and closing the switch. In addition, when converting to IC, 1
Since the circuit can be realized by adding pin terminals, it is also possible to reduce the size and cost of the IC by reducing the size of the package.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the first embodiment of the present invention, Fig. 2 is a circuit diagram of the second embodiment, Fig. 3 is a circuit diagram of the third embodiment, and Fig. 4 is a conventional circuit diagram. It is. , 1... Input terminal, 2... Power line, 3, 4... Switch, 6... Speaker (low impedance load), 6... ... Earphones (high impedance load), 7.8, 11.38・River・
・Capacitor, 10゜12.16, 21.22, 23
,26,27゜32.34,36,37.41...
・p (pile, 14°15.17,18,19,20,24
,28°29.30,31. 35, 38, 40.42
...Transistor, 13... Constant current source, 33... Diode, 6o... Another circuit supplied with current by the transistor 42.

Claims (1)

[Claims] The collector of the first transistor for supplying current to the bias circuit and the collector of the second transistor are connected, and the transistor is provided between the collectors of the first and second transistors and the collector of the fourth transistor for signal amplification. The circuit is configured to take out signal currents for driving the fifth and sixth transistors of the complementary-connected output stage from the bias circuit, respectively, and change the collector currents of the fifth and sixth transistors when there is no signal. , characterized in that, in order to change the bias current when there is no signal, the second transistor is activated or deactivated by opening and closing a switch, thereby changing the current supplied to the bias circuit. Low frequency amplifier.