JPS62173807A - Constant current source bias circuit - Google Patents

Abstract

PURPOSE:To keep stably the gain of a differential amplifier against the variation of supply voltage and that of temperature by controlling the current of a constant current source if the supply voltage or the temperature is changed. CONSTITUTION:A transistor TR 10 is connected in series to a current source 9 which supplies a bias current, and a resistance 7 is connected between the collector and the base of the TR 10, and a resistance 8 and a diode-connected TR 11 are connected in series between the base and the emitter of the TR 10, thus constituting a constant current source bias circuit of a base grounded circuit of the differential amplifier. Even if a power source 12 is varied to change the current flowing to the current source 9, the influence upon a base potential V0 of a TR 5 is reduced to 1/beta where beta is the current amplification factor of the TR. Even if the temperature is changed, the gain of the differential amplifier is stabilized by selecting properly collector currents of TRs 10, 11 and 5 and values of resistances 7 and 8.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a constant current source bias circuit for a differential amplifier.

[Conventional technology]

A differential amplifier with a conventional grounded base circuit as a constant current circuit was presented at the 1981 IEICE General Conference, 43
4 is shown in FIG. 2, and the structure shown in the document ``Operational Amplifier Applied Technology Handbook'' (Ohmsha) is shown in FIG. 3. In Figure 2, 1 is an input signal terminal, 2
1 is an output signal terminal, and 3a and 3b are transistors whose emitters are connected to each other to form a differential amplifier circuit. 4a and 4b are resistors connected to the collectors of transistors 3a and 3b, respectively, forming a differential amplifier circuit. 5.13 is a transistor, 6, 14.15 is a resistor, 1
2 is a power source. In addition, in FIG. 3, 1 to 6.12 indicate the same things as in FIG. 2, 10 is a transistor, 7,
8 is resistance.

Next, the operation will be explained. A signal input from input signal terminal 1 is differentially amplified and output to output signal terminal 2. The gain G of the differential amplifier at this time is expressed by the following equation.゛kT Here, e is the elementary charge, IO is the current flowing through the constant current source composed of the transistor 5 whose base is grounded, and Rc is the resistor 4a, 4 connected to the collector of the transistor 3a, 3b.
b is the resistance value. k is the Boltzmann constant, T is the absolute temperature, and α is the ratio of emitter current to collector current.

The reason why a common base circuit is used as a constant current source is because the output impedance of the common base circuit is very high, so that the common mode suppression ratio of the differential amplifier can be increased.

In the configuration shown in FIG. 2, the current I flowing through the constant current source (i is the base-grounded circuit given by the resistor 14.15 and the base-emitter voltage of the transistor 13 whose base and collector are short-circuited and connected as a diode). It is determined by the bias voltage of , and the resistor 6, and is expressed by the following equation.

RO R3+R4 Here, R3, R4, and RO are resistances of 14 and 15.6, respectively.
, VBE3. VBE4 is the base-emitter voltage of the transistor 5, 130, and CC is the voltage of the power supply 12. From this second equation, it can be seen that in the configuration of FIG. 2, the current 0 changes because VQ changes due to changes in the power supply voltage, changes in resistance value due to temperature changes, and changes in the voltage between the base and emitter of the transistor.

The configuration shown in FIG. 3 is designed to suppress changes in the potential V□0 due to fluctuations in the power supply voltage. The effect on the current amplification factor of the transistor 10 is β
If so, it will be suppressed to 1/β. Further, the current IQ flowing through the constant current source is determined by the base-emitter voltage of the transistor 5.10 and the resistor 7.8 (R1, R2), and is expressed by the following equation.

This is the voltage across the transmitter. In the configuration shown in Equation 3 and Figure 3, the potential VQ is stable against changes in the power supply voltage, but
It can be seen that in response to a temperature change, the change in the base-emitter voltage of the transistor 10 is multiplied by (1+R1/R2), so the change in the potential VQ is large and the current IQ changes.

[Problem that the invention seeks to solve]

Since the conventional differential amplifier circuit is configured as described above, when the current voltage or temperature of the differential amplifier changes, the current IQ flowing through the constant current source, the resistance Rc, and the absolute temperature T in the first equation change. Therefore, there was a problem in that the gain of the differential amplifier fluctuated.

The present invention has been made to solve the above problems, and provides a constant current source bias circuit that can keep the gain of a differential amplifier stable against fluctuations in both power supply voltage and temperature. With the goal.

Means for Solving Problem c] The constant current source bias circuit according to the present invention controls the current of the constant current source so as to stabilize the gain of the differential amplifier when the power supply voltage and temperature change. This is what I did.

[Effect]

In this invention, when the power supply voltage and temperature change, the constant current source current can be controlled by changing the bias of the constant current source to stabilize the gain of the differential amplifier.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows the configuration of a differential amplifier according to an embodiment of the present invention,
In the figure, 1 is an input signal terminal, 2 is an output signal terminal, and 3 is an input signal terminal.
Transistors a and 3b have their emitters connected to form a differential amplifier circuit. 4a and 4b are resistors connected to the collectors of transistors 3a and 3b forming a differential amplifier circuit. A transistor 5 whose base is grounded serves as a constant current source for the differential amplifier. 6゜7.
8 is a resistor, 9 is a current source, 10.11 is a transistor, 1
2 is a power source.

Next, the operation will be explained.

In this embodiment, when the power supply voltage and temperature fluctuate, the constant current source current o in the first equation indicating the gain of the differential amplifier, the resistor Rc connected to the collector, and the absolute temperature T change.
Here, a stable gain can be obtained by controlling the constant current source current IQ.

In FIG. 1, if the base potential of transistor 5 constituting a constant current circuit with a common base is VQ, then vo
is expressed as follows.

Here, R1 and R2 indicate the values of resistors 7 and 8, respectively,
VBEl and VBE2 are transistors 10.
11 shows the base-emitter voltage of No. 11. When the power supply 12 fluctuates, even if the current flowing through the current source 9 changes, the effect on VQ is 1 if the current amplification factor of the transistor is β.
/β, changes in the constant current source current IQ due to voltage fluctuations of the power supply 12 are largely suppressed, and the gain of the differential amplifier is stable.

Further, a current IQ flowing through a constant current source configured with the base of the transistor 5 being grounded is expressed as follows using the base potential VQ of the transistor 5 expressed by the fourth equation.

Here, RQ is the resistor 6, and VBE3 is the base-emitter voltage of the transistor 5. From equation 1.4.5, the gain G of the differential amplifier is...(6) Here, ICI, IC2, and IC3 are the collector currents of transistors 10 and 11.5, respectively.

When the temperature changes, the resistance value changes because the resistance has a temperature coefficient, but if the temperature coefficient remains the same, the resistance ratio remains constant regardless of the temperature. In Equation 6, the temperature coefficients can be canceled by setting the collector currents so that the temperature coefficients of the terms representing the ratio of collector currents have the same sign, and adjusting the values of resistors R1 and R2. In this way, in this embodiment, the gain of the differential amplifier expressed by the sixth equation can be kept stable against fluctuations in both the power supply voltage and temperature.

In the above embodiment, a current source is used as the bias circuit of the common base current circuit, but a resistor may be used instead.

〔Effect of the invention〕

As described above, according to the constant current source bias circuit according to the present invention, the gain of the differential amplifier is configured to be stable against fluctuations in both power supply voltage and temperature at the same time.
This has the advantage that the differential amplifier can be used over a wide temperature and power supply voltage range.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a differential amplifier according to an embodiment of the present invention, and FIGS. 2 and 3 are block diagrams of differential amplifiers each having a conventional common base circuit as a constant current circuit. In the figure, 1 is an input signal terminal, 2 is an output signal terminal, and 3 is an input signal terminal.
is a transistor, 4 is a resistor, 5 is a transistor, 6, 7
．． 8 is a resistor, 9 is a current source, 10°11 is a transistor,
12 is a power supply, 13 is a transistor, and 14.15 is a resistor. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] (1) In the constant current source bias circuit of the common base circuit of a differential amplifier, there is a current source that supplies the bias current, a transistor connected in series to this current source, and a transistor connected between the collector and base of this transistor. A constant current source bias circuit comprising: a resistor; and a resistor and a diode-connected transistor connected in series between the base and emitter of the transistor.