JPH09232884A - Amplifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the common mode rejection ratio(CMRR) with a simple configuration by detecting a common emitter current of a 1st stage differential amplifier circuit and controlling an active load of a 2nd stage with its detection output. SOLUTION: The circuit is provided with a differential amplifier input stage 1 consisting of transistors(TRs) Q1, Q2. A common emitter circuit configures a current detection means 2 consisting of a series circuit of resistors RE1, RE2 and a voltage across the resistor RE2 is given to an active load 4 of a 2nd stage amplifier section. Then the 2nd stage amplifier element 3 consisting of a TR Q3 is driven by one side output (in this case, the Tr Q1) of the differential amplifier input stage 1. Moreover, an active load 4 consisting of a TR Q4 is connected to the TR Q3 and a voltage across the resistor RE2 of the current detection means 2 is given to the active load 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier amplifier circuit in the field of electronic equipment, and more particularly to an amplifier circuit capable of increasing a common mode component removal ratio of a low frequency amplifier circuit having a differential input section.

[0002]

2. Description of the Related Art Conventionally, in a low frequency signal amplifying circuit, a differential amplifying circuit composed of a transistor Q1 and a transistor Q2 shown in FIG. 3 is widely used as an input circuit because of high DC stability and ease of applying negative feedback. It is used. FIG. 4 shows the simplest case of a DC amplifier circuit using this differential amplifier circuit as an input circuit. In this circuit example, the common emitter current source of the differential amplifier circuit composed of the transistor Q1 and the transistor Q2 is constituted by a resistor RE, and only one transistor Q1 of the two differential outputs takes out the output and the second stage It is used as the input of the amplification element transistor Q3. The active load in the second stage is a constant current circuit formed by the transistor Q4.

Since the differential amplifier circuit operates so that a voltage is generated between two outputs in proportion to the difference between two input voltages, a large common-mode component is produced when the difference voltage between both outputs is taken out as an output. Although a removal ratio (hereinafter abbreviated as CMRR) can be obtained, the original in-phase component removal effect cannot be expected because the circuit of FIG. 4 uses only one side. To increase the CMRR, it is necessary to increase the resistance RE of the common emitter circuit, but it is not realistic to increase the resistance RE because it determines the operating current of the first stage. Therefore, in order to secure a high CMRR, as shown in FIG. 5, the common emitter circuit of the first stage differential amplifier circuit should be a constant current source using the transistor Q7, or as shown in FIG.
Differential using transistor Q5 with two outputs
A means such as a stage amplifier circuit is adopted. The transistors Q4 and Q6 are active loads.

[0004]

Among the above-mentioned conventional examples, the circuit shown in FIG. 4 has a simple structure, but C
The MRR has a low defect of about 40 db, and the CMRR is improved in the circuits shown in FIGS. 5 and 6, but in both cases, there is a problem to be solved that the circuit becomes complicated and the number of parts increases.

Therefore, the present invention detects a common emitter current of a first stage differential amplifier circuit in a low frequency amplifier circuit using a differential amplifier circuit in an input section and controls an active load of the second stage by the detected output. By doing so, it is an object to provide an amplifier circuit having an improved CMRR with a simple structure.

[0006]

An amplifier circuit according to the present invention for achieving the above object is shown in each drawing and the conventional example with reference to FIG. 1 showing an embodiment and FIG. 2 showing another modification. Describing with reference to the reference numerals used for the common parts, the first aspect of the present invention is directed to a differential amplification input stage 1, a current detection means 2 connected to a common emitter circuit of the differential amplification input stage 1, and The differential amplification input stage 1 comprises a second stage amplifying element 3 for extracting and amplifying only one of the two amplified outputs, and a second stage active load 4 controlled by the detection output of the current detecting means 2. Is an amplifier circuit.

A second invention is characterized in that, in the first invention, the current detecting means 2 is composed of a series circuit of a resistor RE1 and a resistor RE2, and a voltage across the resistor RE2 is used as a detection output. It is an amplifier circuit.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of an amplifier circuit of the present invention will be described with reference to FIG. The configuration of FIG. 1 is a first example which is the basis of the embodiment of the present invention. Reference numeral 1 is a differential amplification input stage including a transistor Q1 and a transistor Q2.

The common emitter circuit of the differential amplification input stage 1 constitutes a current detecting means 2 which is composed of a series circuit of a resistor RE1 and a resistor RE2, and a voltage across the resistor RE2 is amplified by a second stage which will be described later. It is used as the input of the active load 4 of the section.

Reference numeral 3 denotes a second-stage amplifying element composed of a transistor Q3, which is driven by the output of one of the differential amplification input stages 1 (transistor Q1 side in the figure). An active load 4 composed of a transistor Q4 is connected to the collector of the transistor Q3, and the voltage across the resistor RE2 of the current detecting means 2 is input to the active load 4 as described above.

The operation of the first embodiment will be briefly described with reference to FIG. 1. Referring to FIG. 1 and FIG. 4 showing the above-mentioned conventional example, two differential transistors Q1 and Q2 are provided.
Let I1 and I2 be the currents flowing in
It can be expressed as follows using Ic, in-phase component ΔIc, and differential component ΔId.

I1 = Ic + ΔIc + ΔId (1) I2 = Ic + ΔIc−ΔId (2) Further, the second stage current I3 is I3 = (RLI1-VBE1) / R3 = RL (Ic + ΔIc + ΔId) / R3-VBE1 / R3 ... Therefore, assuming that the current flowing through the active load is I4, the output current Io is Io = RL (Ic + ΔIc) / R3 + RLΔId / R3-VBE1 / R3-I4 (4). Here, the current flowing in the first stage emitter circuit is
Since I1 + I2 = 2 (Ic + ΔIc), this is detected and I4
By controlling and setting I4 = RL (Ic + ΔIc) / R3 (5), the in-phase component appearing in the output can be canceled.

Now, returning to the above equation (3), the resistance values of the resistors R3, R4, RL, RE2 in FIG. 1 are set to R3 = R4, RL = 2RE2 (6). If I3 = (RLI1−VBE1) / R3 = {RL (Ic + ΔIc + ΔId) −VBE1} / R3 ... (7) I4 = {2RE2 (Ic + ΔIc) −VBE2} / R4 ... (8), Io = I3-I4 = (RL / R3-2RE2 / R4) (Ic + ΔIc) + RLΔId / R3-VBE1 / R3 + VBE2 / R4 (9) where, VBE1≈VBE2 (10), the resistance of the above formula (6) From the condition of the value, Io≈RLΔId / R3 (11), and the output is only the differential component and the in-phase component does not appear.

[0014]

The basic configuration and operation of the amplifier circuit of the present invention have been described in detail as the first embodiment in the section of the above-mentioned embodiment, but the circuit shown in FIG. 2 is a modification of the first embodiment. In this case, FETQ1 'and FETQ2' are used in the first stage, and each constant is determined based on the conditions described in the first embodiment. As a result of actually measuring the CMRR in this case, about 70 dB is obtained in the band up to 20 KHz, which is improved by about 30 dB compared with the circuit of the conventional example shown in FIG. Further, in the circuit of FIG. 2, it has been confirmed by experiments that the CMRR can be improved to about 100 db by finely trimming the resistance value. For reference, if the actual measurement value of CMRR is described for the conventional example shown in FIGS. 5 and 6 in which the number of parts is larger and the configuration is more complicated than that of the present embodiment, the conventional example of FIG. In the conventional example, the value is about 108 db.

Although the embodiments which are considered to be representative of the present invention have been described above, the present invention is not necessarily limited to only the structures of the embodiments, but has the above-mentioned constitutional requirements according to the present invention, and The present invention achieves the object of the present invention and can be appropriately modified and implemented within the range having the following effects.

[0016]

As apparent from the above description, the amplifier circuit according to the present invention can increase the common-mode component removal ratio of the amplifier having the differential input section. In order to obtain a particularly high in-phase component removal ratio, it is almost unnecessary to add a component accompanying the improvement as compared with the simplest conventional example. Therefore, the performance can be improved without increasing the cost of parts.

The fact that it is not necessary to increase the number of parts is advantageous in terms of production costs such as part management and manufacturing man-hours. As is clear from the above, the amplifier circuit of the present invention detects the emitter current of the first-stage input section and controls the active load of the second-stage amplifier section by the detected output, thereby increasing the common-mode component removal ratio with a simple circuit configuration. Therefore, it is possible to expect an effect that the performance of the amplifier can be improved without cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram illustrating a configuration of a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a modified example of the first embodiment.

FIG. 3 is a circuit diagram of an input unit of a conventional example.

FIG. 4 is a most basic circuit diagram of a conventional example.

FIG. 5 is a circuit diagram of another conventional example.

FIG. 6 is a circuit diagram of a further improved conventional example.

[Explanation of symbols]

 1 differential amplification input stage 2 current detection means of emitter circuit 3 second stage amplification element 4 second stage active load

Claims (2)

[Claims] 1. A differential amplification input stage (1), a current detection means (2) connected to a common emitter circuit of the differential amplification input stage (1), and the differential amplification input stage (1). Second-stage amplification element (3) that extracts and amplifies only one of the two amplified outputs of
And an second stage active load (4) controlled by the detection output of the current detecting means (2). 2. The current detecting means (2) comprises a resistor RE1 and a resistor RE.
2. The amplifier circuit according to claim 1, wherein the amplifier circuit comprises a series circuit of 2 and the voltage across the resistor RE2 is used as a detection output.