JPS6313405A - Low frequency amplifier - Google Patents

Abstract

PURPOSE:To vary a collector bias current of an output transistor (TR) without changing a voltage gain of an amplifier by constituting a specific bias circuit control circuit operated or inoperated by switching. CONSTITUTION:A bias circuit is provided between the 1st TR 24 and the 2nd TR 29 for low frequency amplification and a current to drive the 3rd and 4th output TRs 35, 36 being complimentary connected is extracted from the bias circuit. Further, a bias circuit control circuit not changing the collector current of amplifier TRs 50-53 but changing only the bia state of the bias circuit is added. Thus, the bias current of the output stage is changed without changing the gain of the amplifier and the miniaturization and low cost of the device is attained in case of circuit integration.

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.

2. Description of the Related Art 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, there is a demand for even greater current savings in devices.

Generally, low frequency amplifiers used in small electronic devices are used for class B operation in order to save current and increase efficiency. In this case, crossover distortion occurs because the base-emitter voltage versus 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 the impedance of the output load is lowered, the change in output current required to achieve the same change in output voltage is greater than when the load impedance is high, so the base-emitter voltage vs. collector current characteristic becomes more linear. Set the operating point to . In other words, it is necessary to set the operating point to a state where a larger amount of collector bias current flows.

On the other hand, when a high-output load is driven with a bias setting that matches a low-output load, the bias current may become larger than the load current, which poses a problem in terms of current savings. Therefore, when driving a plurality of loads having different impedances using the same low frequency amplifier, it is desirable to be able to set an appropriate bias state for each load.

FIG. 4 shows a conventional circuit.

In FIG. 4, transistors 15 and 24 are for current supply, and a current equal to the current 2 of the constant current source 13 is supplied to the load circuit as the collector current of each transistor. Transistors 17 and 18 constitute a differential amplifier circuit,
Transistors 19 and 20 are active loads of the differential amplifier circuit. Resistors 10, 12, 16, 21, 22, 23 are for biasing the differential amplifier circuit. The output of this differential amplifier circuit is input from the collector of transistor 17 to the base of signal amplifying transistor 290, and transistor 29 drives a class B push-pull circuit composed of transistors 30, 31, 35, and 36.

Transistor 24, diode 33, resistor 26°32.
34 constitutes a bias circuit of the output circuit.

Here, the resistors 32 and 34 are for biasing the diode 33. In this circuit, the pace-emitter voltage of transistors 30 and 31 is given by the following equation.

vBE30=(vD33+vBE2B)−(vBE
31 +vR26)・・・・・・・・・ (1) vB E s 1= (vBE30+vR26) −(
"D33+vBE2B)...... (2) However, vBE28: Base-emitter voltage of transistor 28 vBE3°: Pace-emitter voltage of transistor 30 vBE31: Pace-emitter voltage of transistor 31 ■D31: Forward voltage drop of diode 33 vR26
: Voltage drop across the resistor 26 In a no-signal operating state, the collector currents of the output transistors 35 and 36 are equal, and the transistors 30 and 31 operate in a state that satisfies equations (1) and (2) above. are doing. In this circuit, the current flowing through the resistor 26 is changed to change the value of vR26, thereby changing the state of resistance. The collector and base of the transistor 4o are connected to the collector and base of the transistor 24, respectively. When switch 3 is closed, the emitter of transistor 4° is connected to the power supply line and becomes operational, and transistor 4o
constitutes a current mirror circuit together with transistors 14 and 15°24, and the collector current of transistor 40 also becomes summer. The collector of transistor 24.40 is resistor 26
Since the current supplied to the resistor 26 is 2
! becomes. A switch 4 interlocked with the switch 3 is connected to a high impedance load (earphones, etc.) 6 side. When the switch 3 is opened, the transistor 4° becomes inactive, so the current supplied to the resistor 26 becomes I, and the current supplied to the resistor 26 becomes I.
Since the voltage drop across 6 becomes smaller, the collector current of output transistor 35, 36 increases. A switch 4 interlocked with this increase switch 3 is connected to a low impedance load (speaker, etc.) 5 side. ■=Several units μm
In response to the change, the collector current of the output transistor 36.36 changes by several mA.

However, by inserting a resistor in series between the emitter of the transistor 4o and the power supply line, an appropriate bias state can be set depending on 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 changes depending on whether the switch 3 is opened or closed, and therefore the gain in the transistor 29 changes. When the switch 3 is closed, the collector current supplied to the transistor 29 increases, so
Since the voltage gain of the transistor 29 increases and the amount of negative feedback of the low frequency amplifier as a whole increases, in the worst case, abnormal oscillation may occur.

In view of the above-mentioned interlayer points, the present invention provides a low frequency amplifier in which only the bias current can be changed without changing the advantages of the amplifier.

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

Operation The present invention changes the voltage gain of the amplifier, thereby making it possible to change the collector bias current of the output transistor.

Example 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 omission of transistor 40 and the addition of transistors E50, 51, 52, and 63.

In FIG. 1, transistors 50.51 and 52.53
The bias control circuit consisting of the following will be described below.

As described for the conventional circuit shown in FIG. 4, the base-emitter voltages of transistors 30 and 31 are given by equations (1) and (2), respectively. In this circuit as well, the bias state is changed by changing the voltage drop across the resistor 26, as in FIG. When the switch 3 is closed, the transistors 50 and 51, which share the same pace as the transistor 24, operate. transistor 50
．． 51 have the same characteristics, and the collector current during operation is l
/ , provided that I′≦■. Transistor 52.
53 constitutes a current mirror circuit, so if the characteristics of the two transistors are equal, the transistor 52
The collector current of is also . Therefore, the current flowing into the resistor 26 from eight points is the difference between the collector current of the transistor 24 and the collector current of the transistor 62, that is, (■-
). As a result, since the voltage drop across the resistor is small, the collector bias current of the output transistors 35 and 36 increases. On the other hand, the collector current of the transistor 60 flows into point f, which is the other end point of the resistor.

If the characteristics of the transistors 60 and 51 are the same, this current will be 1', which is equal to the collector current of the transistor 52. Therefore, the collector current of the transistor 48, that is, the collector current of the amplifying transistor 29 will remain constant at I. , the voltage gain of the amplification transistor does not change. In this state, the switch 4 which is interlocked with the switch 3 is connected to the low impedance load (speaker etc.) 5 side. (That is, contrary to the conventional example, the bias current is minimum when switch 3 is open.) In this example, when I' = 50 μA, the collector current of the output transistor 35.36 is approximately 5 m
A. Change.

FIG. 2 shows a second embodiment of the invention.

This embodiment is the same as the first embodiment shown in FIG. 1 except for the addition of a resistor 37. 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 this resistor value sets the bias state of the output circuit to an appropriate state. A collector current of 50°61 can be controlled. The current value may be controlled by changing the emitter areas of the transistors 14, 24, 50, and 51 when the circuit of the embodiment is integrated. Also, the transistor 14°24.60
．． A resistor may be inserted between each emitter of 51 and the power supply line, and the current value may be controlled by the resistance ratio.

Further, as in the third embodiment shown in FIG. 3, it is also possible to drive a separate circuit 64 by another transistor 53 which is activated or deactivated at the same time as the transistors 60 and 51.

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

Effects of the Invention As described above, the present invention has the following advantages by adding a bias control circuit.
It makes it possible to change the output stage bias current without changing the amplifier gain, and when integrated into an IC, the bias current is minimized when the control circuit is not operating, so when used with a single load, In small-sized current-saving devices that originally want to omit external components, unlike conventional devices, external components are not required, making it possible to downsize the device and reduce costs.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the low frequency amplifier of the present invention, FIG. 2 is a circuit diagram showing a second embodiment of the low frequency amplifier of the present invention, and FIG. A circuit diagram showing a third embodiment of the frequency amplifier, and FIG. 4 is a circuit diagram of a conventional low frequency amplifier. 24...First transistor, 29...
・Second transistor, 36...Third transistor, 36...Fourth transistor, 61,
62,53゜54... Bias control circuit configuration transistor. Name of agent: Patent attorney Toshio Nakao and one other person
TX

Claims (1)

[Claims] A bias circuit is provided between a first transistor for supplying current to the bias circuit and a second transistor for low frequency amplification, and third and fourth output stage transistors are complementary connected from the bias circuit. It is configured to take out the current for driving each of the transistors, and has a bias circuit control circuit that is activated or deactivated depending on the opening and closing of the switch, and the collector current of the second transistor is kept constant. A low frequency amplifier characterized in that the collector current of the third and fourth transistors when there is no signal is changed by the bias circuit control circuit, so that the bias current when there is no signal can be changed.