JPS6347161B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier bias circuit suitable for use in a class B push-pull amplifier or the like.

Conventionally, as a bias circuit for this type of amplifier, the one shown in FIG. 1 is known. The bias circuit shown in this figure consists of transistor 1
Darlington connected first drive transistor 2 driven by (PNP transistor)
a, second drive transistor 2b, first output transistor 3a (both NPN transistors)
A Darlington-connected third drive transistor 2c, fourth drive transistor 2d, and second output transistor 3b (all of which have a complementary configuration with these transistors 2a, 2b, and 3a)
In the class B single-ended push-pull amplifier which has a PNP transistor) and sends out an output current through the output terminal 4, the transistor 5
(NPN transistor), a resistor 6 inserted between the collector and base of this transistor 5, and a resistor 7 inserted between the base and emitter of this transistor 5. voltage circuit), the drive transistors 2a, 2
The transistor 5 in the bias voltage source 8 and the output transistors 3a, 3b are thermally coupled to each other, and the drive transistors 2a to 2d and the output transistors are connected in accordance with the voltage generated by the bias voltage source 8. Transistors 3a, 3b
The idling current is supplied to the In this case, drive transistor 2
For a to 2d, drive transistor 2
A resistor 9 inserted between the emitters of the drive transistors 2b and 2c and a resistor 10 inserted between the emitters of the drive transistors 2b and 2d supply sufficient idling current for them to perform class A operation. It is becoming more and more like this.

In the amplifier having the conventional bias circuit as described above, the voltage is supplied to the connection point between the base of the drive transistor 2a and the bias voltage source 8 via the transistor 1, and the constant current source 11 connects the bias voltage source 8 and the drive transistor. The voltage generated between the bases of the drive transistors 2a, 2c by the current i absorbed from the connection point with the base of the drive transistors 2a, 2c is V ₁ , and the voltage generated between the bases of the drive transistors 2a, 2c is V 1 .
The base-emitter voltages of output transistors 2b, 2c, and 2d are V _BE1 , V _BE2 , V _BE3 , and V _BE4 respectively, and the base-emitter voltages of output transistors 3a and 3b are respectively
When V _BE5 and V _BE6 , output transistor 3a,
The voltage V ₂ generated between both emitters of 3b is V ₂ =V ₁ −(V _BE1 +V _BE2 +V _BE3 V _BE4 +V _BE5 +V _BE6 ) (1). Therefore, the output transistors 3a, 3
The idling current Iid flowing through b is Iid=V ₂ /Re ₁ +Re ₂ (2), where the resistance values of the emitter resistors 12 and 13 of the output transistors 3a and 3b are Re ₁ and Re ₂ , respectively. It can be seen that if the voltage V ₂ is made constant, it becomes a constant value. Here, the voltage V ₂ is the voltage between the transistor 5 corresponding to the voltage V ₁ and the voltage
This can be made constant only when the drive transistors 2a to 2d and output transistors 3a and 3b corresponding to V _BE1 to V _BE6 , respectively, are completely thermally coupled, but the drive transistors 2a, 2c, Drive transistor 2b, 2
d. The output transistors 3a and 3b each have different heat generation amounts or heat dissipation characteristics, and if the total amount of heat generated from these transistors 5 is controlled all at once, it goes without saying that the drive transistor and the output transistor individually It is difficult to say that appropriate compensation is always achieved for the operation of the system. That is, it is impossible to eliminate the temperature difference between these element bodies by normal thermal coupling, and normally only the output transistors 3a, 3b and transistor 5, which have the largest temperature change, are thermally coupled as described above. The reality is that it is happening. Therefore, when using a conventional amplifier bias circuit as _shown in FIG. As a result, especially in a class B single-ended push-pull amplifier, the resistance values Re ₁ ,
When Re ₂ is small, there is a problem that the idling current Iid becomes unstable. Furthermore, when using such a conventional bias circuit, when the output of the amplifier becomes large, the drive transistor 2a
The voltage between the base of A and the output terminal 4, or the voltage between the output terminal 4 and the base of the drive transistor 2c, exceeds the voltage _V1 , so that A
Drive transistors 2a and 2b that should perform class operation
Another problem was that the drive transistors 2c and 2d became non-conductive.

The present invention has been made in view of the above circumstances, and includes a drive transistor and an output transistor connected in a Darlington manner. The purpose of the present invention is to provide a bias circuit for a push-pull amplifier, which can obtain a stable idling current against temperature changes with an extremely simple configuration, and which prevents the drive transistor from becoming non-conductive even at high output. The bias control circuit inserted between both bases of the drive transistor is composed of complementary transistors connected in series, and the idling current of the drive transistor is electrically stabilized by the complementary transistor, and the idling current of the output transistor is It is characterized by simultaneously achieving thermal stabilization of the idling current.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 is a circuit diagram showing a configuration when an embodiment of the present invention is applied to a class B single-ended push-pull amplifier. In this figure, parts corresponding to those in FIG. Reference numerals are given and explanations thereof are omitted. In FIG. 2, resistors 14 (value R ₁ ), 15 (value R ₂ ),
16 (value R ₃ ) are inserted in series one after another. A transistor 17 (NPN transistor) and a transistor 18 (PNP transistor) are successively inserted in series between the bases of the drive transistors 2a and 2c with their current directions matching.
Also, the connection point between the resistors 14 and 15 is the transistor 1
The connection point between the resistors 15 and 16 is connected to the base of the transistor 18.
In addition, transistors 3a and 3b and transistor 1
7 and 18 are thermally coupled. Here, resistance 1
4 to 16 constitute a resistor circuit 19 for detecting the voltage between the bases of the output transistors 3a and 3b, and transistors 17 and 18 detect the voltage between the bases of the drive transistors 2a and 2c.
This constitutes a bias control circuit 20 that determines _V1 .

Next, the circuit operation of this embodiment with the above configuration will be considered. First, sufficient idling current flows through the drive transistors 2a, 2c and the drive transistors 2b, 2d through the resistor 9 and the resistors 14 to 16 to enable them to perform Class A operation. Here, if I ₁ flows through the resistors 14, 15, and 16 sequentially, and the base-emitter voltages of the transistors 17 and 18 are V _BE7 and V _BE8 , respectively, then the voltage R ₂・I ₁ is controlled by a negative feedback loop formed via transistors 17 and 18 so that R ₂ ·I ₁ =V _BE7 +V _BE6 (3). That is, in this case, the bias control circuit 20 controls the voltage V ₁ between the bases of the drive transistors 2a and 2c so that the current I ₁ satisfies the equation (3) here. In this case, the voltage between the bases of output transistors 3a and 3b
V ₃ is as follows: V ₃ = (R ₁ + R ₂ + R ₃ )・I ₁ = (1 + R ₁ + R ₃ /R ₂ ) (V _BE7 + V _BE6 )...(4) Therefore, both output transistors 3a and 3 The voltage V ₂ between the emitters is V ₂ = V ₃ − (V _BE5 + V _BE6 ) = (1+R ₁ +R ₃ /R ₂ ) (V _BE7 + V _BE6 ) (V _BE5 + V _BE6 ) (5). Also, the idling current Iid is the same as the above equation (2), Iid = V ₂ /Re ₁ + Re ₂ ... (6), so in order to keep this idling current Iid constant, the voltage V shown by the equation (5) must be adjusted. ₂ should be kept constant. In this case, as is clear from equation (5), the voltage V ₂ is the base-emitter voltages V BE5 , V _BE6 of the output transistors 3a and 3b and the base-emitter voltages V _BE7 , V _BE7 of the transistors 17 and 18.
Using only V _BE8 as a temperature variable, the base-emitter voltages V _BE1 to V _BE4 of drive transistors 2a to 2d
Since these transistors 3
If only a, 3b, 17, and 18 are thermally coupled, it can be made constant. Also, in this case, equation (5),
As is clear from equation (6), by changing the resistance values R ₁ , R ₂ , and R ₃ , the voltage V ₂ can be changed, thereby making it possible to set the idling current Iid to an arbitrary value.

Next, in this embodiment, the conduction state of drive transistor 2b or drive transistor 2d when the output of the amplifier is large will be considered. As is clear from the above equation (3), the current I ₁ is generated by the transistors 17 and 18 so that no matter what the output state of this amplifier is, I ₁ = V _BE7 + V _BE8 /R ₂ ...(7) controlled as follows. Then, the collector currents of drive transistors 2b and 2d are respectively I ₂ and
When I ₃ and the base currents of the output transistors 3a and 3b are respectively I ₄ and I ₅ , I ₂ = I ₁ + I ₄ I ₃ = I ₁ + I ₅ ... (8) holds, so the drive transistors 2 b, 2
d does not become non-conductive no matter how large the output power of this amplifier is, and therefore there is no need to apply a thermal burden to the drive transistors 2b and 2d by passing an idling current larger than necessary.

As explained above, the bias circuit of the amplifier according to the present invention includes a resistor circuit inserted between the bases of the output transistors and a voltage between the bases of the output transistors detected by this resistor circuit so that the voltage between the bases of the output transistors becomes constant. Since it is configured with a bias control circuit that controls the voltage between the bases of the drive transistor, electrical feedback is appropriately applied to the drive transistor, and thermal feedback is appropriately applied to the output transistor.
It is possible to supply an extremely stable idling current regardless of temperature variables such as each base-emitter voltage, and since both of these feedbacks are realized by complementary transistors that make up the bias control circuit, the configuration Furthermore, the value of the idling current can be set to any value by the constant of the resistor circuit inserted at the base of the output transistor, and even when the output of the amplifier is large, the drive transistor It has an excellent effect of not being in a non-conductive state.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of a bias circuit of a conventional amplifier, and FIG. 2 is a circuit diagram showing the configuration when an embodiment of the present invention is applied to a class B single-ended push-pull amplifier. 2a to 2d...Drive transistor, 3a, 3
b... Output transistor, 19... Resistance circuit, 20...
Bias control circuit.

Claims (1)

[Claims] 1. In an amplifier bias circuit for supplying a hair idling current to the drive transistor and the output transistor in an amplifier comprising two sets of Darlington-connected drive transistors and output transistors connected in a push-pull manner, between the bases of both drive transistors. The bias control circuit includes a bias control circuit consisting of complementary transistors inserted in series with their current directions matching, and a resistor circuit inserted between the bases of both the output transistors, and the bias control circuit is complementary to the bias control circuit. 1. A bias circuit for an amplifier, characterized in that an output across a resistor of the resistor circuit is applied to each base of a transistor, and the complementary transistor is thermally coupled to each of the output transistors.