JP3309878B2 - Amplifier - Google Patents

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 For example, a circuit as shown in FIG. 3 is considered as an operational amplifier. That is, 1 indicates an input stage, 2 indicates a drive stage, 3 indicates an output stage, and 4 indicates a load.

[0003] The input stage 1 has an in-phase input terminal and an anti-phase input terminal. The drive stage 2 includes a differential amplifier 21
And a current mirror circuit 22. The differential amplifier 21 includes transistors Q1 to Q3, and the current mirror circuit 22 includes a transistor Q1.
4, Q5. The current mirror circuit 22 is connected so as to be a load of the differential amplifier 21.

Further, in the output stage 3, Q6 is an output transistor. The transistor Q6 has an emitter connected to the power supply terminal T1, a collector connected to the transistor Q7 for the constant current source, and a grounded emitter. Have been. The base is connected to the drive stage 2 and the collector is connected to the load 4.

R1 and C1 are a resistor and a capacitor 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 magnitude of 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 is subjected to current amplification by the transistor Q6. The supplied signal is supplied.

According to this amplifier, since the transistor Q6 has a collector output, that is, the transistor Q6 has an upper peak portion of the output signal supplied to the load 4,
Until the collector-emitter voltage of 6 becomes about 0.2V,
Transistor Q6 operates normally. Similarly, the transistor Q7 operates normally until the collector-emitter voltage of the transistor Q7 becomes about 0.2 V at the lower peak portion of the output signal supplied to the load 4.

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

However, in this amplifier, when the collector current of the transistor Q6 decreases, the high-frequency characteristics of the transistor Q6 deteriorate, so that phase rotation occurs due to this and oscillation easily occurs. For this reason, it is necessary to provide a sufficient margin to the collector current of the transistor Q6 with respect to the size required for the transistor Q6 to drive the load 4, resulting in an increase in current consumption. . Such an increase in current consumption becomes a problem when a battery is used as a power supply.

In particular, when an active filter is formed, ten or more amplifiers may be required, depending on the frequency characteristics, so that an increase in current consumption becomes even more problematic.

Alternatively, in order to prevent oscillation due to phase rotation while keeping the current consumption low while keeping the collector current of the transistor Q6 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 for IC integration and has a limit.

Therefore, as shown in FIG. 4, an amplifier using an output transistor Q6 as an emitter follower can be 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 has excellent high-frequency characteristics, and the transistor Q6 is an emitter follower and has excellent high-frequency characteristics in operation. The stability of the transistor Q6 is excellent.

However, in this amplifier, since the transistor Q6 operates as an emitter follower,
Although the waveform of the signal voltage of the base is equal to the waveform of the signal voltage of the emitter, when the lower peak portion of the signal voltage of the base becomes lower than the base-emitter voltage of 0.7 V, the transistor Q6 is turned on. Cut off. For the transistor Q7 to operate normally, a voltage between the collector and the emitter of about 0.2 V is required.

Therefore, in this amplifier, the peak-to-peak value of the signal voltage that can be extracted is:
Since the voltage is limited to (VCC- (0.7V + 0.2V)), there is a problem in use when the power supply voltage VCC is low, for example, a 1.5V battery.

[0016]

As described above, the amplifier shown in FIG. 3 can be used even at a low voltage, but consumes a large amount of current.
It is not suitable for C conversion. The amplifier shown in FIG. 4 has excellent stability and is suitable for use in an integrated circuit. However, when operated at a low voltage, a large output cannot be obtained.

The present invention is 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 the first potential point T1, A collector of the first transistor Q6 is connected to a collector of a second transistor Q7 having a polarity opposite to that of the first transistor Q6, and an emitter of the second transistor Q7 is connected to a second potential point T2; The emitter of a third transistor Q8 having the same polarity as the first transistor Q6 is connected to the base of the first transistor Q6, the collector of the third transistor Q8 is connected to the base of the second transistor Q7, The base of the third transistor Q8 is connected to the third potential point V8, and the first transistor Q6
An input signal is supplied to a connection point between the base of the third transistor Q8 and the emitter of the third transistor Q8, and an output signal is taken out from a connection point between the collector of the first transistor Q6 and the collector of the second transistor Q7. Things.

[0019]

Since the transistor Q8 operates with the base grounded, the transistors Q6 and Q7 operate as SEPP amplifiers, and the characteristics required for the amplifier are secured.

[0020]

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

A PNP transistor Q6 is provided, its base is connected to the output terminal of the drive stage 2, ie, the collectors of the transistors Q2 and Q5, the emitter of the transistor Q6 is connected to the power supply terminal T1, and its collector is connected to the load. 4 and connected to the collector of an NPN transistor Q7, the emitter of which is connected to the ground terminal T2. It is assumed that the transistor Q6 and the transistor Q7 have the same current amplification factor.

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

A transistor Q8 having the same polarity as that of the transistor Q6 is provided, and its emitter is connected to the transistor Q6.
Of the transistor Q7
, And a bias voltage V8 is supplied to the base. Further, a capacitor C2 is connected in parallel between the emitter and the collector of the transistor Q8.

According to such a configuration, the voltage V8 is supplied to the series circuit between the emitter and the base of the transistors Q6 and Q8.
Q8 each operate in the active area. At this time,
The transistor Q8 operates with the base grounded.

When there is no signal, the transistor Q5
Is equal to the collector current of the transistor Q2, the output current i2 of the drive stage 2 becomes zero. When i2 = 0, the base current ib (shown by a broken line) of the transistor Q6 is directly applied to the transistor Q8.
Flows to the base of the transistor Q7 through the emitter and collector of the transistor Q7.

Therefore, the base current ib of the same magnitude flows through the transistors Q6 and Q7, so that the collector currents of the transistors Q6 and Q8 become equal to each other, and the load 4
No current flows through.

However, during a half cycle period in which the collector current of the transistor Q5 is larger than the collector current of the transistor Q2, i2> 0. Then, this current i2
Flows into the base of the transistor Q6 and the emitter of the transistor Q8, so that the base current of the transistor Q6 decreases and the base current of the transistor Q8 increases.

Therefore, the collector current of the transistor Q6 decreases and the collector current of the transistor Q7 increases, so that the current i4 flows through the load 4 with the polarity shown in FIG.

On the other hand, during a half cycle period in which the collector current of the transistor Q5 is smaller than the collector current of the transistor Q2, i2 <0. Then, this current i2 becomes
Increase the base current of transistor Q6 and decrease the base current of transistor Q8. Therefore, the collector current of the transistor Q6 increases and the collector current of the transistor Q7 decreases, so that the current i4 flows through the load 4 in a polarity opposite to that of FIG.

Therefore, transistors Q6 and Q7 operate as a single-ended push-pull amplifier. The operating points of the transistors Q6 and Q7 at this time are determined by the bias voltage V8, but can be class A or class B.

If the transistors Q6 and Q7 operate as push-pull amplifiers, even if the collector current of the transistor Q6 decreases at the upper peak portion of the signal supplied to the load 4 and its characteristics deteriorate, this characteristic can be reduced. At this time, since the collector current of the transistor Q7 increases and its characteristics are maintained in a good state, it can be regarded as a good amplifier having a small phase rotation in consideration of the overall characteristics of the transistors Q6 and Q7.

At this time, since the current ib flowing through the transistor Q8 is the base current of the transistors Q6 and Q7 and is very small, the high-frequency characteristics of the transistor Q8 deteriorate, but the high frequency compensation is performed by the capacitor C2. . That is, as the frequency of the signal increases, the impedance of the capacitor C2 decreases, and the signal current i2 from the drive stage 2 is supplied to the transistor Q7 also through the capacitor C2, so that the deterioration of the high frequency characteristics of the transistor Q8 is compensated. Is done.

Accordingly, the current consumption can be reduced by reducing the collector current of the transistor Q6, and even if the value of the capacitor C1 is small, sufficient phase correction can be performed, and a stable amplification operation can be performed. .

Further, as compared with the amplifier of FIG. 3 or FIG.
Only the addition of the elements Q8 and C2, rather the value of the capacitor C1 can be reduced, which is advantageous for the implementation of an IC. As an example, C1 = 2 pF C2 = 2 pF R1 = 2 kΩ.

Further, similarly to the amplifier shown in FIG.
The output signal of the peak-to-peak value close to CC can be extracted, and even when used in a device operating at a low voltage, such as a headphone stereo, for example, a device using a 1.5V battery as a power supply, Large output can be obtained.

FIG. 2 shows an example of a bias circuit when a plurality of N sets of amplifiers A1 to AN are used in an active filter, for example, when the amplifiers A1 to AN each have the output stage 3 of FIG. Show.

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 to Q8, resistor R1, capacitors C1, C
It is composed of two.

The emitters and the bases of the transistors Q81 and Q82 are connected in series, and a constant current is supplied to the series circuit by a constant current source Q80.
And a bias voltage V8 is formed. And
This bias voltage V8 is applied to the transistors Q8 to Q8 of the amplifiers A1 to AN through the buffer voltage follower 81.
The base of Q8 is supplied as a base bias voltage.

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

Further, since the bias circuit 8 is commonly used for the amplifiers A1 to AN, the total current consumption can be reduced as compared with the case where a bias circuit is provided for each of the amplifiers A1 to AN. The circuit scale can be reduced.

In the above description, the circuit 1 is used as an input stage, but the drive stage 2 can also serve as an input stage.

[0042]

According to the present invention, the transistor Q6,
Since Q7 operates as a single-ended push-pull amplifier, the transistor Q6
Even if the collector current of the transistor Q6 decreases and its characteristics deteriorate, the amplifier operates as a good amplifier with a small phase rotation, considering the overall characteristics of the transistors Q6 and Q7.

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

In addition, the number of elements required for this is small, which is also advantageous for IC.

[Brief description of the drawings]

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

FIG. 2 is a connection diagram showing another example of the present invention.

FIG. 3 is a connection diagram showing a conventional example.

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

[Explanation of symbols]

 1 input stage 2 drive stage 3 output stage 4 load 8 bias circuit Q6 output transistor Q7 output transistor Q8 grounded base transistor

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03F 3/30 H03F 1/02

Claims (4)

(57) [Claims] An emitter of the first transistor is connected to a first potential point; 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; An emitter of the second transistor is connected to a second potential point; an emitter of a third transistor having the same polarity as the first transistor is connected to a base of the first transistor; A collector of the transistor is connected to a base of the second transistor, a base of the third transistor is connected to a third potential point, a base of the first transistor, and an emitter of the third transistor; An input signal is supplied to a connection point of the first transistor, and an output signal is taken out from a connection point between the collector of the first transistor and the collector of the second transistor. Unishi was amplifier. 2. The amplifier according to claim 1, wherein the amplifier constitutes an output stage. 3. The amplifier according to claim 1, wherein a high-frequency compensation capacitor is connected in parallel between the emitter and the collector of the third transistor. 4. The amplifier according to claim 1, wherein said third potential point is shared by a plurality of said amplifiers.