JPS59191913A - Differential amplifier - Google Patents

Abstract

PURPOSE:To reduce the in-phase gain component as far as possible by detecting the working current of one of a differential pair of transistors and flowing a current corresponding to the detected working current out of or into the other transistor. CONSTITUTION:When a transistor TRQ1 of one side has a change, the working current of a TRQ3 has a change in the opposite direction. As a result, the TRQ4 and Q5 constituting a current mirror circuit flow a current equal to the variation of the working current of the TRQ3 to a resistance R3'. This changes the working current of a TRQ2 of the other side in the opposite direction. Therefore the working currents of a differential pair of TRQ1 and Q2 are changed in the same direction. This reduces the in-phase gain component as far as possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a differential amplifier, and more particularly to one in which the common mode gain component thereof is reduced.

[Technical background of the invention and its problems]

2. Description of the Related Art Conventionally, a differential amplifier configured as shown in FIG. 1 is known as a differential amplifier widely used in general electronic equipment including audio equipment.

That is, the emitters of each other are coupled through a resistor R1 and a resistor R2.

Differential pair of transistors Ql, Q2 grounded via R3
An input signal vXN is applied between each pace of the differential output signal generator through an output deriving circuit (not shown) from each collector of the input signal vXN. . It is designed to derive .

In such a differential amplifier, the potential drop across resistors R2 and R3 is sufficiently large so that ΔvBE (change in pace-emitter voltage) with respect to change Δ■1N in input signal VIN can be ignored. Let us try to simplify the analysis of the circuit operation.

Now, if the base potential on the transistor Ql side changes by Δv4, the changes in the collector current of each transistor Qt+Q2, ΔIc(Ql) and ΔIC(Q2), are as follows.

According to these equations (1) and (2), the differential gain g
md and common mode gain gme are each as follows.

By the way, what is required for a differential amplifier is only the differential gain gmd in equation (3), and the common-mode gain gff in equation (4).
lc is not necessary.

Therefore, in order to make the common mode gain grrlc in equation (4) zero, it is necessary to set R2-ω.

However, in practice, it is common to use a current source instead of resistors R2 and R3 to bias the emitters of both transistors Ql and Q2, but since the impedance of the current source is finite, There is a problem in that it is unavoidable to have a certain amount of common mode gain gmc.

[Purpose of the invention]

Therefore, the present invention has been made in view of the above points, and an object of the present invention is to provide an extremely good differential amplifier that is improved so that the common mode gain component can be reduced as much as possible.

[Summary of the invention]

That is, the differential amplifier according to the present invention includes first and second transistors having a differential configuration in which their emitters are connected to each other via a resistor, and detects the operating current of one of the first and second transistors. and a second means for causing a current corresponding to the operating current detected by the first means to flow into or out of the other of the first and second transistors. It is characterized by the following:

[Embodiment of the Invention] An embodiment of the invention will be described in detail below with reference to the drawings.

That is, as shown in FIG.
1/ and each resistor R2' r
Differential pair transistor Q1 grounded via R3'
, Q2 are configured to apply the input signal V1N between each pace, and to derive differential outputs from their respective collectors via an output deriving circuit (not shown), which is similar to the conventional one. Providing a transistor Q3 whose base and emitter are connected correspondingly to the base and emitter of the transistor Q1,
The novel configuration is that transistors Q4 and Q5 are provided, the collectors of which are connected to the collector of the transistor Q3 and the emitter of the transistor Q2, respectively.

In this case, the transistors Q4 and Q5 are directly connected to each other, their emitters are commonly connected to the power supply VCC, and Q4 is diode-connected with its base and collector directly connected. It constitutes a count mirror circuit.

When the circuit operation of the differential amplifier configured as described above is analyzed under the same conditions as in the case of FIG. 1, the following relationship holds true.

However, in equations (5), (6), and (7), α is the current ratio between transistors Ql and Q3.

The differential gain gmd and common mode gain gme of these (5), (6), and (7) are respectively as follows.

(8) Here, it can be seen that the differential gain gmd component is equal to that obtained by the conventional equation (3).

On the other hand, the common mode gain gmQ is now R2' = 10 R1
It can be seen that when it is assumed that ', the value of equation (9) is reduced compared to that obtained by conventional equation (4) under the same conditions.

In this case, by appropriately selecting α (αs o, 5), the common mode gain gme component can be reduced to substantially zero.

Further, by further increasing the value of α, it is possible to provide a common mode gain gmc with a phase characteristic opposite to that of the conventional one.

FIG. 3 shows each transistor Ql of FIG. 2 as another embodiment.
, Q2, Q3 emitters R4+R5
+ R6 is inserted, and the value of α mentioned above can be arbitrarily set by appropriately selecting the ratio of these resistances.

It goes without saying that the present invention is not limited to the embodiments described above and illustrated, and that various modifications and applications can be made without departing from the gist of the invention.

For example, as shown by broken lines in FIGS. 2 and 3, the collector of the output side transistor Q5 of the current mirror circuit may be connected to the collector of the transistor Q2 instead of the emitter thereof.

〔Effect of the invention〕

Therefore, as described in detail above, according to the present invention, it is possible to provide an extremely good differential amplifier that is improved so that the common mode gain component can be reduced as much as possible.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of a conventional differential amplifier. FIGS. 2 and 3 are configuration diagrams showing one embodiment and other embodiments of a differential amplifier according to the present invention. Ql, Q2...differential pair transistor, Q3...
Common-mode gain reduction transistor, Q41Q5...Current mirror transistor, R1' to R3'...Resistor.

Claims (1)

[Claims] first and second transistors having a differential configuration whose emitters are connected to each other via a resistor; a first means for detecting an operating current of one of the first and second transistors; and second means for causing a current corresponding to the operating current detected by the first means to flow out or flow into the other of the first and second transistors. amplifier.