JPH0766639A - Amplifier - Google Patents

Abstract

PURPOSE:To enable a low-voltage and low-current operation while compensating the degradation of a characteristic by constituting an output amplification stage by two transistor circuits, making one transistor circuit perform an A-class amplification operation when a signal is a low frequency and making the both circuits operate as A-class push-pull circuits when the signal is a high frequency. CONSTITUTION:When a signal is a low frequency, a C7 can be neglected, an output amplification stage operates as a current mirror circuit where a transistor Q7 is on an output side and a Q8 is on an input side, constant current IO flows in the Q7, the Q7 operates as constant current, and a transistor Q6 operates by an A class and supplies an amplification signal to a load 4. When the signal is a high frequency, a R7 is neglected, the equivalent circuits of the Q6 and Q7 operate as complementary A class push-pulls, complements the degradation of the characteristics of the Q6 and the Q7 and excellently keeps an overall characteristic. Thus, a stable operation is enabled, current consumption is reduced by reducing the collector current of the Q6, the number of element is also reduced and an amplifier which is suitable for making into IC can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier suitable for low voltage and IC.

[0002]

2. Description of the Related Art As an operational amplifier, for example, a circuit as shown in FIG. 4 is considered. That is, 1 is the input stage, 2
Is a drive stage, 3 is an output stage, and 4 is a load.

The input stage 1 has an in-phase input terminal and an anti-phase input terminal. Further, the drive stage 2 includes a differential amplifier 21.
And a current mirror circuit 22. The differential amplifier 21 is composed of transistors Q1 to Q3, and the current mirror circuit 22 is composed of a transistor Q1.
It is composed of 4 and Q5. The current mirror circuit 22 is connected so as to be a load on the differential amplifier 21.

Further, in the output stage 3, Q6 is an output transistor, the emitter of which is connected to the power supply terminal T1 and the collector of which is connected to the constant current source transistor Q7 to be grounded. Has been done. The base is connected to the drive stage 2 and the load 4 is connected to the collector.

In addition, R1 and C1 are resistors and capacitors for phase correction, R2 is a resistor for negative feedback, and T2 is a ground terminal.

Therefore, the transistor Q6 operates in class A with the collector current of the transistor Q7, that is, the magnitude of the constant current as the center value (DC level), and the load 4 uses the transistor Q6 to amplify the current. Signal is supplied.

According to this amplifier, the transistor Q6 is the collector output, that is, the transistor Q6 is at the upper peak portion of the output signal supplied to the load 4.
Even if the collector-emitter voltage of 6 becomes about 0.2V,
The transistor Q6 operates normally. Similarly, in the lower peak portion of the output signal supplied to the load 4, the transistor Q7 operates normally even if the collector-emitter voltage of the transistor Q7 becomes about 0.2V.

Therefore, this amplifier can take out an output signal having a peak-to-peak value close to the power supply voltage VCC of the terminal T1, and a device such as a headphone stereo which operates at a low voltage, for example, 1.5V. This is advantageous when used in a device that uses a battery as a power source.

However, in this amplifier, when the collector current of the transistor Q6 becomes small, the high frequency characteristic of the transistor Q6 deteriorates, so that the phase rotation caused by this deteriorates and oscillation easily occurs. Therefore, it is necessary to give the collector current of the transistor Q6 a sufficient margin with respect to the size required for the transistor Q6 to drive the load 4, resulting in a large current consumption. . Then, such an increase in current consumption is a problem when a battery is used as a power source.

In particular, when an active filter is constructed, 10 or more amplifiers may be required depending on the frequency characteristic, so that the increase in current consumption is even more problematic.

Alternatively, in order to prevent oscillation due to phase rotation while the current consumption is suppressed while the collector current of the transistor Q6 is kept small, it is necessary to increase the value of the capacitor C1 for phase correction. However, increasing the value of the capacitor C1 is disadvantageous in forming an IC and has a limit.

Therefore, as shown in FIG. 5, an amplifier using an output transistor Q6 as an emitter follower is also considered. The transistor Q7 also operates as a constant current source, and the transistor Q6 operates in class A.

In this amplifier, since the transistor Q6 is an NPN transistor and the high frequency characteristic of the transistor itself is excellent, and the transistor Q6 is an emitter follower and the high frequency characteristic is also excellent in operation. , The stability of the transistor Q6 is excellent.

However, in this amplifier, since the transistor Q6 operates as an emitter follower,
The waveform of the signal voltage of the base is equal to the waveform of the signal voltage of the emitter, but when the lower peak part of the signal voltage of the base becomes lower than 0.7V which is the base-emitter voltage, the transistor Q6 becomes Cut off. In addition, the collector-emitter voltage of about 0.2 V is required for the transistor Q7 to operate normally.

Therefore, in this amplifier, the peak-to-peak value of the signal voltage that can be taken out is
Since it is limited to the voltage (Vcc- (0.7V + 0.2V)), there is a problem in using it when the power supply voltage Vcc is low, for example, in the case of a battery of 1.5V.

[0016]

As described above, the amplifier of FIG. 4 can be used even at a low voltage, but it consumes a large amount of current.
It becomes unsuitable for C conversion. Further, the amplifier of FIG. 5 has excellent stability and is suitable for use as an IC, but a large output cannot be obtained when operated at a low voltage.

The present invention is intended to solve such a problem.

[0018]

Therefore, in the present invention, when the reference numerals of the respective parts correspond to the embodiments described later, the emitter of the first transistor Q6 is connected to one output terminal T1 of the power supply, The collector of the first transistor Q6 is connected to the collector of a second transistor Q7 having a polarity opposite to that of the first transistor Q6, and the emitter of the second transistor Q7 is connected to the other output terminal T2 of the power supply. Then, the base of the first transistor Q6 is connected to the base of the second transistor Q7 through a capacitor C7 having a predetermined value, and a bias current is supplied to the base of the second transistor Q7 through an element R7 having a predetermined impedance. Supplying an input signal to the base of the first transistor Q6 and extracting an output signal from the emitter of the first transistor Q6 A.

[0019]

When the signal frequency is low, the transistor Q
Although 7 operates as a constant current source, when the signal frequency is high, the transistors Q7 and Q8 operate as complementary class A push-pull amplifiers to compensate for the deterioration of the characteristics of the transistor Q7.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the example shown in FIG.
It is composed of a differential amplifier 21 and a current mirror circuit 22.

A PNP transistor Q6 is provided, the base of which is connected to the output end of the drive stage 2, that is, the collectors of the transistors Q2 and Q5, the emitter of the transistor Q6 is connected to the power supply terminal T1, and the collector thereof is a load. 4 is connected to the collector of an NPN transistor Q7, and the emitter of the transistor Q7 is connected to the ground terminal T2.

Further, a series circuit of a resistor R1 for phase correction and a capacitor C1 is connected between the base and collector of the transistor Q6, the base of the transistor Q6 is connected to the base of the transistor Q7 through the capacitor C7, and the transistor Q6 is connected. A resistor R2 for negative feedback is connected between the collector and the input stage 1.

Further, N having the same characteristics as the transistor Q7
A PN transistor Q8 is provided, its emitter is connected to the terminal T2, its collector is connected to the constant current source Q0, and resistors R7, R8 (R7
= R8) and connected to the bases of the transistors Q7 and Q8. Thus, the base bias circuit 5 of the transistor Q7 is constructed.

According to such a configuration, when the frequency of the signal is low, the impedance of the capacitor C7 seen from the signal is large, which is equivalent to not connecting the capacitor C7.

The transistor Q7 is supplied with a base current having a value equal to that of the transistor Q8 by the transistor Q8.
7, Q8, transistor Q8 input side, transistor Q7
Operates as a current mirror circuit whose output side is. The constant current source Q0 supplies a constant current I0 of a predetermined magnitude to the transistor Q8. Therefore, the constant current I0 also flows through the collector of the transistor Q7, and the transistor Q7 operates as a constant current source.

Therefore, when the frequency of the signal is low, the transistor Q6 operates in the class A in which the DC level of the collector current is the value I0, and the signal amplified by the transistor Q6 is applied to the load 4 as in the amplifier of FIG. Is supplied to.

When the frequency of the signal increases, the characteristic of the transistor Q7 deteriorates, but at this time, the impedance of the capacitor C7 seen from the signal decreases. Further, the resistor R7 has a relatively large value, that is,
The size is such that it can be ignored from the viewpoint of signals.

Therefore, when the signal frequency is high, the AC equivalent circuit for the transistors Q6 and Q7 is as shown in FIG. At this time, the transistors Q6 and Q7 are operating in class A. Therefore, transistors Q6 and Q7
Operates as a complementary class A push-pull amplifier for high frequency signals.

As a result, even if the collector current of the transistor Q6 decreases and its characteristics deteriorate at the upper peak portion of the signal supplied to the load 4, the collector current of the transistor Q7 increases at that time. Since the characteristics are kept in a good state, it can be regarded as a good amplifier with a small phase rotation in consideration of the total characteristics of the transistors Q6 and Q7.

Therefore, the collector current of the transistor Q6 can be reduced to reduce the current consumption, and sufficient phase correction can be performed even if the value of the capacitor C1 is small, and stable amplification operation can be performed. .

By the way, in the amplifier of FIG. 4, even if the value of the capacitor C1 is increased to 30 pF, if the collector current of the transistor Q6 is decreased, oscillation occurs. However, according to the amplifier of FIG. 1, when C1 = 3 to 5 pF C7 = 2 pF R7 = 100 kΩ, the collector currents of the transistors Q6 and Q7 are 50%.
It was sufficiently stable even at -100 μA.

Further, the resistor R7 is for preventing a high frequency signal from the capacitor C7 from being bypassed to the transistor Q8 side, and the resistor R8 is connected to the resistor R7. And transistors Q7 and Q8
This is for balancing the direct current of. Therefore,
The frequency at which the transistors Q6 and Q7 can be regarded as a push-pull operation is determined by the time constant of the capacitor C7 and the resistor R7. Therefore, by increasing the value of the resistor R7,
The value of the capacitor C7 can be reduced.

Moreover, like the amplifier of FIG. 4, the power supply voltage V
An output signal with a peak-to-peak value close to CC can be taken out, which is advantageous when used in equipment that operates at low voltage, such as headphone stereo, for example, equipment that uses a 1.5V battery as its power source. Is.

FIG. 3 shows that, for example, in an active filter, a plurality of N sets of amplifiers A1 to AN are used and
An example of the case where the amplifiers A1 to AN each have the output stage 3 of FIG. 1 is shown.

That is, each of the output stages of the amplifiers A1 to AN is similar to the output stage 3 of the amplifier of FIG.
Transistors Q6, Q7, resistors R1, R7, capacitor C
1 and C7. Further, the constant current source Q0, the transistor Q8, and the resistor R8 are connected in the same manner as in the case of FIG. 1, and the voltage obtained at the collector of the transistor Q8 is supplied to the voltage follower circuit 6 for the buffer and the bias circuit. 5 are configured. The output voltage of the voltage follower circuit 6 is supplied as base bias voltages to the bases of the transistors Q7 to Q7 of the amplifiers A1 to AN through the resistors R7 to R7, respectively.

Therefore, the output stages 3 to 3 of the amplifiers A1 to AN
3, the same operation as in the case of FIG. 1 is performed, and the amplifiers A1 to AN operate stably with a small operating current.

In this case, the constant currents I0 to I0 can be supplied to the transistors Q7 to Q7 of the amplifiers A1 to AN by simply supplying the constant current I0 to the transistor Q8, so that the amplifiers A1 to AN are biased. Compared with the case where the circuit 5 is provided, the total current consumption can be reduced and the circuit scale can be reduced.

Although the circuit 1 is used as the input stage in the above description, the drive stage 2 may be used as the input stage. In addition, the base of transistor Q7 and transistor Q8
By making the ratio of the junction area between the emitters different, the magnitude of the constant current of the transistor Q7 can also be made different from the constant current I0 provided by the constant current source Q0.

[0039]

According to the present invention, since the transistors Q6 and Q7 operate as a complementary class A push-pull amplifier for a high frequency signal, the collector current of the transistor Q6 decreases at the peak portion of the signal. Even if the characteristic is deteriorated, considering the total characteristic of the transistors Q6 and Q7, it operates as a good amplifier with little phase rotation.

Therefore, the collector current of the transistor Q6 can be reduced to reduce the current consumption, and sufficient phase correction can be performed even if the value of the capacitor C1 is small, and stable amplification operation can be performed. .

Moreover, the number of elements required for that purpose is small, and this is also advantageous for IC implementation.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an example of the present invention.

FIG. 2 is an AC equivalent circuit diagram of the circuit of FIG. 1 at a high frequency.

FIG. 3 is a connection diagram showing another example of the present invention.

FIG. 4 is a connection diagram showing a conventional example.

FIG. 5 is a connection diagram showing another conventional example.

[Explanation of symbols]

 1 input stage 2 drive stage 3 output stage 4 load 5 bias circuit A1 to AN operational amplifier Q0 constant current source Q6 output transistor Q7 transistor

Claims (3)

[Claims] 1. An emitter of a first transistor is connected to one output terminal of a power supply, and a collector of the first transistor is connected to a collector of a second transistor having a polarity opposite to that of the first transistor. The emitter of the second transistor is connected to the other output terminal of the power supply, the base of the first transistor is connected to the base of the second transistor through a capacitor having a predetermined value, An amplifier configured to supply a bias current to the base of the transistor through a device having a predetermined impedance, supply an input signal to the base of the first transistor, and extract an output signal from the emitter of the first transistor. 2. The amplifier according to claim 1, wherein when the frequency of the input signal is high, the value of the capacitor and the element having the impedance is supplied to the base of the second transistor through the capacitor. Amplifier selected to the value that is supplied. 3. The amplifier according to claim 1, wherein a plurality of N sets of the amplifier are provided by a plurality of N sets of the first transistor, the second transistor, the capacitor, and the element having the impedance. And a base bias is supplied from a common bias circuit to each of the second transistors of the plurality of N sets of amplifiers.