JP2000174569A - Amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the change of frequency characteristic caused by dispersion in hfe of transistors(TRs) constituting an amplifier. SOLUTION: A collector of a transistor(TR) Q3 being a component of a current mirror circuit is connected to emitters of TRs Q1, Q2 being components of a 1st stage differential amplifier circuit. A TR Q20 is connected to a base of a TR Q4 configuring the current mirror circuit via a resistor 20 with the TR Q3. Furthermore, a collector of the TR Q20 is connected to a collector of the TR Q4 and an emitter of the TR Q20 is connected to ground. A current outputted from a constant current source J2 is distributed to the TRs Q4, Q2, but the collector current of the TR Q20 depends on its hfe. Thus, the current flowing to the TR Q4 depends on the hfe and similarly, the current flowing to the TR Q3 has dependence on its hfe. As a result, change in the frequency characteristic of the amplifier circuit depending on the hfe can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an amplifier, and more particularly, to an amplifier including a differential amplifier circuit.

[0002]

2. Description of the Related Art For example, an amplifying device such as an operational amplifier is very important because it is a basic unit constituting an electronic device.

In recent years, with the advance of circuit technology, many high-performance amplifiers have been distributed to the market. FIG. 5 is a circuit diagram of an application circuit using a conventional amplifying device (hereinafter, appropriately referred to as an operational amplifier).

Before describing the details of this figure, an outline of this circuit will be described with reference to FIG. As shown in FIG. 6, the circuit shown in FIG. 5 forms an adding circuit. In this figure, R1 to R4 are input resistors to which input voltages Vin1 to Vin4 are applied, respectively. Resistance R5
Are resistors for voltage division, and input voltages Vin1 to Vin4
Is divided between the resistors R1 to R4. here,
It is assumed that R1 = R2 = R3 = R4 = 1/2 × R5.
In that case, the voltage applied to the resistor R5 is equal to the input voltage 2
/ 3 times.

[0005] The resistors R6 and R7 are feedback resistors.
= 1/8 x R7. As a result, the gain of the operational amplifier OP is (1 + R7 / R
6) = 9.

Accordingly, the following relationship is established between the input voltages Vin1 to Vin4 and the output voltage Vout.

[0007]

Vout = 6 (Vin1 + Vin2 + Vin3 + Vin4) (1) Next, returning to FIG. 5, a configuration example of a conventional operational amplifier will be described. In this figure, parts corresponding to those in FIG. 6 are denoted by the same reference numerals, and the description thereof will be omitted.

In FIG. 1, transistors Q1, Q2
Constitutes a differential amplifier circuit at the input stage. The transistors Q3 to Q6 constitute a Wilson-type current mirror circuit, and the sum of the currents flowing through the emitters of the transistors Q1 and Q2 is always constant.

The constant current source J2 includes transistors Q1 to Q6
Is a main current circuit for determining a current flowing through a current mirror circuit configured by the current mirror circuit. Transistor Q7,
Q8 forms a constant current circuit. Transistor Q
9, Q10, transistors Q15 to Q17, and constant current source J1 constitute a Wilson-type current mirror circuit, as in the case described above.

The transistors Q11 and Q12 form a current mirror circuit. The transistor Q13 is a constant current circuit, and the transistor Q14 is a common emitter circuit using the transistor Q13 as a load. These two transistors make up the gain stage of this circuit.

The transistor Q18 is a constant current circuit.
The transistor Q19 is an emitter follower circuit having the transistor Q18 as a load, and constitutes an output stage of this circuit.

Next, the operation of the above conventional example will be described. The voltage V is applied to the bases of the transistors Q1 and Q2.
+ And V− are applied (change voltage ΔVi
n is input), the emitters of these transistors are connected to a current mirror circuit, which is a constant current source.
Corresponding change currents Δi and −Δi flow, respectively.

At this time, the change voltage ΔVin and the change current Δ
When the relationship of i is represented by an equation, it can be expressed as follows. Here, rb indicates the base resistance of the transistors Q1 and Q2, hfe indicates the current amplification factor, Vt indicates the thermal voltage, and IC indicates the collector current of the transistor Q3.

[0014]

Δi = ΔVin / (4 × (rb / hfe + 2 × Vt / IC)) (2) Since the transistors Q7 and Q8 are constant current circuits, Δi and − Δi will be deducted. As a result, transistors Q9, Q
The change currents -Δi and Δi flow through the ten emitters. At this time, since the base current flows through the transistors Q9 and Q10, the collector current is reduced by that amount, but the current value is small and is ignored. As a result, similarly, the change currents -Δi and Δi flow through the collectors of the transistors Q9 and Q10, respectively.

Since transistors Q11 and Q12 form a current mirror circuit, transistors Q9 and Q12
The difference between the currents output from the ten collectors is output to transistor Q14. As a result, the change current −2Δi flows into the base of the transistor Q14.

The changing current is applied to the base of the transistor Q14.
When 2Δi flows, a change current −2 × hfe × Δi multiplied by hfe flows through the collector. At this time, the change voltage ΔVo generated at the collector of the transistor Q14 can be expressed by the following equation, where r is the output resistance of the transistor Q14.

[0017]

ΔVo = r × 2 × hfe × Δi (3) where r is the collector resistance ro14 of the transistor Q14.
And a parallel resistance of the collector resistance ro13 of the transistor Q13, which is represented by ro (= ro14 × ro13 / (ro14 + ro1).
Assuming 3)), equation (3) can be expressed as follows.

[0018]

ΔVo = ro × 2 × hfe × Δi (4) The transistor Q19 is an emitter follower circuit,
Since the gain is “1”, the output change voltage ΔVou
t can be represented by the following equation.

[0019]

ΔVout = ro × 2 × hfe × Δi (5) Accordingly, the operational amplifier portion (the resistors R1 to R1) of the circuit shown in FIG.
R7) can be represented by a block diagram as shown in FIG.

That is, when the input voltages V + and V- are applied to the bases of the transistors Q1 and Q2, the difference ΔVin is calculated, and the difference voltage ΔVin is applied to the collectors of the transistors Q1 and Q2. On the other hand, a difference current Δi obtained by multiplying the value of the block CM1 is generated.

The difference current Δi generated in the transistors Q1 and Q2 is doubled by the transistors Q7 to Q12 and flows into the base of the transistor Q14. In the transistor Q14, as shown in the block CM2, the input change current 2Δi is multiplied by hfe and further multiplied by the collector resistance ro of the transistor Q19 and output. Also, since the capacitors C1 and R10 for phase compensation are added to the collector of the transistor Q14, the output is multiplied by SC / (1 + SCR) and fed back to the input. Here, C is the capacitance value of the capacitor C1, and R is the resistance R
10, and S = jω (j is an imaginary number, ω =
2πf).

The change voltage generated at the collector of transistor Q14 is applied to the base of transistor Q19.
Transistor Q19 is an emitter follower circuit, and its gain is "1". Therefore, the change voltage generated at the collector of transistor Q14 is multiplied by "1" and output as output change voltage ΔVout.

FIG. 8 is a block diagram corresponding to the conventional circuit (circuit including a feedback circuit) shown in FIG. In this block diagram, a portion corresponding to the operational amplifier shown in FIG. 7 is combined into a block CM3, and a block CM4 corresponding to the resistors R6 and R7 is added. Further, an addition circuit for adding the input voltages Vin1 to Vin4 and a block for multiplying the input voltage by "2/3" are added to the input portion.

[0024]

When the above circuit is formed on a semiconductor substrate, the relative accuracy between elements formed on the same semiconductor substrate can be kept relatively high. However, the relative accuracy between elements formed on different substrates (that is, the absolute accuracy of the elements) cannot be made sufficiently high.

Therefore, the characteristics of the individual operational amplifiers may vary due to the variations of the elements constituting the operational amplifiers as described above. For example, the current amplification factor hfe of the transistor is an important parameter that determines the characteristics of the operational amplifier. If the current amplification factor hfe varies, the frequency characteristics also differ for each operational amplifier.

FIG. 9 is a graph showing the open gain of the operational amplifier shown in FIG. In this figure, curves (a),
(B) and (c) correspond to the case where hfe is large, medium, and small, respectively.
fe is shown. As shown in this figure,
The gain changes according to the value of hfe.

FIG. 10 shows the frequency characteristics of the entire circuit shown in FIG. Also in this figure, curves (a),
(B) and (c) correspond to the case where hfe is large, medium, and small, respectively. The frequency characteristic of the addition circuit shown in FIG. 5 changes according to the value of hfe. That is, the larger the hfe, the wider the band.

A major cause of such a change in the frequency characteristic due to hfe is a change in the hfe of the transistors Q1 and Q2 shown in the block CM1 shown in FIG. A change in hfe of these transistors Q1 and Q2 and a change in gain of the block CM1 are as shown by a curve (a) in FIG. As shown in this figure, it can be seen that the gain of the block CM12 increases according to the value of hfe. Also, it can be seen that there is a difference of about 1.5 dB between the case where hfe is 50 and the vicinity of 190.

As described above, since the characteristics of the operational amplifier vary depending on the hfe of the transistor, it is necessary to set R10, C1 and the like for phase compensation at the time of design, assuming the worst case. As a result, an unnecessarily large phase compensating element is added, and the characteristics of the circuit may be unnecessarily suppressed.

The present invention has been made in view of such a point, and even when hfe has a variation,
It is an object of the present invention to provide an amplifying device having little influence on the overall characteristics.

[0031]

According to the present invention, there is provided an amplifying apparatus including a differential amplifying circuit.
It has two emitter-coupled transistors constituting the differential amplifier circuit, and a constant current circuit connected to the emitter side of the two transistors, and the constant current circuit corresponds to the current amplification factor hfe. Thus, there is provided an amplifying device characterized in that the output current changes.

Here, the constant current circuit connected to the emitter side of the two emitter-coupled transistors constituting the differential amplifier circuit included in the amplifier has a current amplification factor h.
The output current changes according to fe.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram illustrating a configuration example of an embodiment of the present invention. In this figure, parts corresponding to those in FIG. 5 are denoted by the same reference numerals, and a description thereof will be omitted as appropriate.

In this embodiment, a transistor Q20 and a resistor R20 are added as compared with the case of FIG. Other configurations are the same as those in FIG. The resistor R20 has one end connected to the bases of the transistors Q3 to Q5 and the other end connected to the transistor Q20.
Connected to the base.

The transistor Q20 has a base connected to the resistor R20 and a collector connected to the transistor Q4.
And the emitter is grounded. Next, the operation of the above embodiment will be described.

Now, assuming that the collector current of the transistor Q20 is IC20, the base current of the transistor Q20 is represented by IC20 / hfe. Therefore, the base potential of the transistor Q20 has a value obtained by subtracting IC20 × R20 / hfe from the base potential of the transistor Q4.

When the base potential of transistor Q20 is hfe
, The collector current of the transistor Q20 increases or decreases in proportion to hfe. That is, hf
When e is large, the potential is high, and when it is small, it is low.

As a result, the collector current of the transistor Q20 increases or decreases in proportion to hfe. Here, the current output from the constant current source J2 is the collector of the transistor Q4, the base of the transistor Q6, and
Although the current is distributed to the collector of the transistor Q20, since the base current of the transistor Q6 is very small, ignoring this, the current output from the constant current source J2 is distributed to the transistor Q4 and the transistor Q20. Become.

Therefore, when the collector current of the transistor Q20 increases, the collector current of the transistor Q4 decreases. Since the transistor Q4 and the transistor Q3 form a current mirror circuit, when the collector current of the transistor Q4 decreases, the collector current of the transistor Q3 also decreases.

On the other hand, when the collector current of the transistor Q20 decreases, the collector current of the transistor Q4 increases, and as a result, the collector current of the transistor Q3 increases.

In summary, the hfe of transistor Q20
And the collector current of the transistor Q3 have an inversely proportional relationship. Therefore, the transistor Q2
When the hfe of 0 is large, the collector current of the transistor Q3 decreases, and the hfe of the transistor Q20 also decreases.
Is smaller, the collector current of transistor Q3 increases. When this relationship is represented by an equation, the following equation (6) is obtained. Here, n indicates the ratio of the multiplier of Q4 and Q20, and I2 is the current value output from the constant current source J2.

[0042]

R20 × (I2-IC) / (Vt × hfe) = ln (n × IC / (I2-IC)) (6) FIG. 2 is a diagram showing the relationship between hfe and IC. It is. From this figure, it can be seen that there is an inversely proportional relationship between the two.

As described above, if there is an inversely proportional relationship between the collector current IC of the transistor Q3 and hfe, the proportional constant can be appropriately set to obtain the relationship shown in FIG.
, The denominator of the block CM1 can be kept constant.

Note that hf appearing in the block shown in FIG.
e is that of the transistors Q1 and Q2. As described above, since the relative accuracy of the elements formed on the semiconductor substrate is high, the transistors Q20 and Q1,
The hfe of Q2 can be considered substantially equal (or can be set to a predetermined ratio).

A curve (b) shown in FIG. 11 indicates a block CM1 shown in FIG. 7 in the case where the above embodiment is used.
Shows the relationship between hfe and gain. As shown in this figure, even when hfe changes (when hfe varies), the variation in gain is suppressed to a maximum of about 0.5 dB. In this example, R20 = 10 kΩ
Is set to

FIG. 3 is a graph showing the frequency characteristics of the open gain of the operational amplifier. As shown in this figure, even when the current amplification factor hfe changes, the variation is smaller than in the case of FIG. 9 showing the characteristics of the conventional circuit.

FIG. 4 is a diagram showing frequency characteristics of the entire circuit shown in FIG. As shown in this figure, even when the current amplification factor hfe changes, the variation is smaller than in the case of FIG. 10 showing the characteristics of the conventional circuit.

As described above, according to the present embodiment, the output current of the constant current source of the differential amplifier circuit constituting the first stage of the operational amplifier is changed according to the value of hfe. It is possible to suppress a change in the frequency characteristic of the circuit due to the variation in the frequency.

Although hfe changes depending on the temperature of the element, according to the above-described embodiment, it is possible to prevent the characteristic of the circuit from changing due to the temperature change of the element. .

In the above embodiment, the constant current circuit in the first stage of the operational amplifier has been described as an example. However, the present invention is not limited to the differential amplifier circuit constituting the other stages in addition to the first stage. Can also be implemented.

In the above embodiment, the transistor Q2 constituting the constant current circuit is replaced by the transistor Q2.
Although 0 and the resistor R20 are added, the present invention is not limited to only such a case. In short, hf
What is necessary is just to make the current flowing through the constant current source constituting the differential amplifier circuit vary according to the value of e.

[0052]

As described above, according to the present invention, in the amplifying device including the differential amplifier circuit, the current flows to the constant current circuit connected to the emitter side of the two emitter-coupled transistors constituting the differential amplifier circuit. The current is increased by the current amplification factor h.
Since the frequency characteristic is changed according to fe, it is possible to prevent the frequency characteristic of the amplifier from changing due to the variation of hfe.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration example of an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between hfe of a transistor Q20 and a collector current of a transistor Q3 in the embodiment shown in FIG.

FIG. 3 is a diagram showing a frequency characteristic of an open gain of the operational amplifier constituting the embodiment shown in FIG. 1;

FIG. 4 is a diagram showing frequency characteristics of the embodiment shown in FIG. 1;

FIG. 5 is a circuit diagram showing a configuration example of an application circuit using a conventional amplification device.

FIG. 6 is a diagram showing a circuit equivalent to the circuit shown in FIG. 5;

FIG. 7 is a block diagram corresponding to a portion excluding a feedback circuit of the circuit shown in FIG. 5;

FIG. 8 is a block diagram corresponding to the circuit shown in FIG. 5;

9 is a frequency characteristic of a portion of the circuit shown in FIG. 5 excluding a feedback circuit.

FIG. 10 is a frequency characteristic of the circuit shown in FIG. 5;

FIG. 11 is a diagram illustrating a relationship between hfe and a gain of a differential amplifier circuit at a first stage of the circuits illustrated in FIGS. 1 and 5;

[Explanation of symbols]

R1 to R16: resistor, Q1 to Q19: transistor, J1, J2: constant current source, Vref: reference voltage

Claims (3)

[Claims] 1. An amplifying device including a differential amplifier circuit, comprising: two emitter-coupled transistors constituting the differential amplifier circuit; and a constant current circuit connected to the emitters of the two transistors. An amplifying device, wherein the constant current circuit changes an output current according to a current amplification factor hfe. 2. The constant current circuit according to claim 1, wherein the current amplification factor hf
2. The amplifying device according to claim 1, wherein the output current changes in inverse proportion to e. 3. The constant current circuit is connected via a resistor to a current mirror circuit, a main current circuit for determining a current flowing through the current mirror circuit, and a base of a transistor constituting the current mirror circuit. 2. The amplifying device according to claim 1, further comprising: a transistor of the same type as a transistor constituting the differential amplifier circuit, the collector of which is connected to the main current circuit.