JPH01256806A - Differential amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent an input DC offset voltage from appearing at the output while DC voltage gain is eliminated by connecting emitters of two transistors(TRs) being component of a differential amplifier circuit by a capacitor and providing a constant current source corresponding to each of the two TRs. CONSTITUTION:A capacitor 7 is used for a constant current source and the capacitor 7 couples of both the TRs 1, 2 in terms of AC. Then constant current sources 10, 11 are provided respectively to the TRs 1, 2 individually. Through the constitution above, the voltage gain G is expressed in the following equation; G=R/{(kT/qI+1/(4pifC)}, where R is the resistance of resistors 5, 6, C is a capacitance of the capacitor 7, I is the current of the current sources 10, 11, (k) is the Boltzmann's constant, Q is the electric charge of an electron, T is the absolute temperature and (f) is a frequency. Then the gain G is illustrated as a curve shown in the frequency characteristic 2 and the G approaches zero as the frequency (f) is close to zero and then the gain is G=0 at the DC. In this case, the input DC offset voltage is not sent to the output.

Description

[Detailed description of the invention] Skin lift The present invention relates to a differential amplifier circuit.

As shown in Figure 5, the conventional differential amplifier circuit consists of two transistors 1 and 2, collector resistors 5 and 6 of these transistors 1.2, and emitter resistor 8.9. It is configured by a common constant current source 10. The bases 14.15 of both transistors serve as differential inputs, and the collectors 16.1
7 is a differential output. Note that 12 indicates a DC power supply.

Such a conventional differential amplifier circuit has the frequency characteristic 1 shown in FIG. 2, and it is clear that it has a voltage gain in direct current. Now, here, the input DC offset voltage of the previous stage (input DC offset voltage) is VIO1 and the output DC offset voltage is V. When the gain is G,
The following equation is obtained.

Vo = G V to ·” (1) As described above, when there is an input current offset voltage, there is a drawback that an output DC offset voltage appears at the output terminals 16 and 17 due to the voltage gain G.

SUMMARY OF THE INVENTION An object of the present invention is to provide a differential amplifier circuit that eliminates DC gain and prevents input DC offset voltage from appearing at the output.

According to the present invention, there is provided a differential amplifier circuit comprising two transistors having a base as an input and a collector as an output,
A differential amplifier circuit is obtained, comprising a capacitor provided to connect the emitters of the two transistors, and a constant current source provided corresponding to each of the two transistors. .

X Riyo Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of a 111th embodiment of the present invention, and parts equivalent to those in FIG. 5 are designated by the same reference numerals. A capacitor 7 is used in place of the emitter resistor 8.9 in FIG. 5, and the emitters of both transistors 1.2 are AC-coupled by this capacitor 7. Constant current sources 10.11 for each transistor 1.2 are provided individually.

The other configurations are the same as those shown in FIG. 5, and their explanation will be omitted.

In such a configuration, the resistance value of the collector resistor 5.6 is R
, the capacitance value of the capacitor 7 is C, and the current source 10.1
Assuming that 1 current source is 1, the voltage gain G is given by the following equation.

G=R/ + (kT/(t I+1/ (4πfC)
)...(2) In the above formula, k is Boltzmann's constant, q is the electron charge,
T is absolute temperature, then frequency.

The above equation (2) forms a curve like the frequency characteristic 2 shown in FIG. 2, and when the frequency f approaches zero, G also approaches zero, and therefore, in direct current, G=0. At this time, from equation (1), the output DC offset voltage V. is the following formula.

V o =G V Io = O (3) Therefore, the input DC offset voltage is not transmitted to the output.

FIGS. 3 and 4 are circuit diagrams showing other embodiments of the present invention, and parts equivalent to those in FIGS. 1 and 5 are designated by the same reference numerals. In both of these circuits, a resistor is inserted between the respective emitters to adjust the input dynamic range and gain, and as with the embodiment shown in Figure 1, no DC offset appears in the output. do not have.

According to the present invention, by configuring to eliminate the DC voltage gain, it is possible to prevent input DC offset voltage from appearing in the output.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a diagram comparing the frequency characteristics of the embodiment circuit of the present invention and a conventional circuit, and FIGS. 3 and 4 are diagrams of the present invention. FIG. 5 is a circuit diagram showing other embodiments, and FIG. 5 is a diagram showing a conventional differential amplifier circuit. Explanation of symbols of main parts 1.2...Transistor 7...Capacitor TO, IT...Constant current source Applicant: 1 person from NEC Corporation)

Claims (1)

[Claims] (1) A differential amplifier circuit consisting of two transistors with a base as an input and a collector as an output, including a capacitor provided to connect the emitters of the two transistors, and a capacitor provided to connect the emitters of the two transistors. What is claimed is: 1. A differential amplifier circuit comprising a constant current source provided corresponding to each of the constant current sources.