JPH0331281B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field of invention] This invention relates to current circuits.

[Technical background of the invention]

Figure 2 shows the emitter operating currents of the two emitter follower transistors in a circuit that includes an emitter follower transistor that sets the input bias of the amplifier circuit and an emitter follower transistor that takes out the output of the amplifier circuit. This shows the current source circuit to be set. In the figure, the amplifier circuit 11
is composed of, for example, an operational amplifier, and a signal is applied to its non-inverting input terminal from a signal source 12 via a capacitor _C11 . The input bias of the amplifier circuit 11 is from the power supply V _cc to the emitter follower transistor Q ₁₁ ,
It is given through resistors R ₁₁ and R ₁₂ . Amplifier circuit 1
1 is taken out to the output terminal ₁₃ via the emitter follower transistor Q12.

The emitter operating currents of transistors Q ₁₁ and Q ₁₂ are respectively given by transistors Q ₁₃ and Q ₁₄ having common bases. The base potential of these transistors Q ₁₃ and Q ₁₄ is detected by a feedback transistor Q ₁₅ which has a common base with these transistors, and this
By being fed back to the bases _{of transistors Q 13 and} Q ₁₄ via Q 16, these transistors Q ₁₃ and
The base potential of Q ₁₄ is kept constant.

The emitter operating current of transistor _Q16 is set by diode-connected transistor _Q17 .

Now, assuming that the current amplification factor h _fe of the transistor is infinite and the base-emitter voltage is V _J , the collector current I _C15 of the transistor Q ₁₅ is: I _C15 = V _CC −2V _J /R ₁₃ + R ₁₄ + R ₁₇ ...(1) It is expressed as. The collector current _IC13 of the transistor _Q13 also matches this. In addition, the collector current I _C14 of the transistor Q ₁₄ is expressed as I _C14 = R ₁₇ / R ₁₆ · V _CC −2V _J / R ₁₃ + R ₁₄ + R ₁₇ ...(2), and the collector current I _C15 is
It is multiplied by the ratio of the emitter resistances R ₁₆ and R ₁₇ of Q ₁₄ and Q ₁₅ .

In the figure, R ₁₅ and R ₁₈ are emitter resistances of transistors Q ₁₃ and Q ₁₇ , respectively.

[Problems with background technology]

In the above configuration, when the gain of the amplifier circuit 11 is high, oscillation may occur. To explain this below, the output of the amplifier circuit 11 is transmitted to the baselines of the transistors _{Q 14} , Q ₁₃ , Q ₁₅ , and Q ₁₇ via the collector-base capacitance C _CB of the transistor Q 14 . When this leakage is at a relatively low frequency, the feedback action of transistors Q ₁₅ and Q ₁₆ works to suppress baseline fluctuations, making it difficult for the leakage to be transmitted. In other words, the baseline impedance becomes extremely small. However, at high frequencies, the feedback of transistors Q ₁₅ and Q ₁₆ does not become complete negative feedback, and the impedance of the baseline itself increases, and the
The fluctuation in the base potential of Q ₁₅ is amplified by a factor of R ₁₃ /R ₁₇ to determine the bias voltage of the amplifier circuit 11.
_Q11 's Emitsuta will tell you this. this transistor
Fluctuations in the emitter potential of _Q11 are transmitted to the amplifier circuit 11, but since the non-inverting input terminal, which is the signal input terminal, has low impedance, the above fluctuations are not transmitted thereto, resulting in a constant potential. Therefore, the above fluctuation is transmitted only to the inverting input terminal,
As this is amplified, oscillation occurs when the gain of this round loop is 0 dB or more.

[Purpose of the invention]

The present invention was made in order to cope with the above-mentioned circumstances, and an object of the present invention is to provide a current source circuit that is free from oscillation even if the gain of the amplifier circuit is large.

[Summary of the invention]

The present invention will be explained using the embodiment shown in _FIG . transistor Q ₁₁
Transistor Q ₁₃ to set the emitter operating current of
Although the transistor Q22 forms a feedback circuit, the transistor _Q22 , which sets the emitter operating current of the emitter follower transistor _Q12 on the output side, is driven in a current mirror configuration.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention. In the figure, the same parts as in FIG. 2 are given the same reference numerals.

First, the collector of the transistor (first transistor) _Q13 that determines the emitter operating current of the emitter follower transistor _Q11 is the transistor _Q11.
The emitter is connected to the emitter of , and the emitter is grounded through the resistor _R15 . Fluctuations in the base potential of the transistor _Q13 are detected by a feedback transistor (third transistor) _Q15 and fed back to the base of the transistor _Q13 via a transistor (fourth transistor) _Q16 . This allows the transistor
The base potential of Q ₁₃ is kept constant and the transistor
The emitter current of _Q11 is stabilized. Here, the base of transistor _Q15 is the transistor
It is common with the base of Q ₁₃ , and the emitter is resistor R ₁₇
and the collector is connected to the transistor _Q16
resistors R ₁₄ , R ₁₃
is connected to the power supply V _CC through a series circuit.

The collector of transistor Q ₁₆ is connected to the power supply V _CC , and the emitter is connected to the common base of transistors Q ₁₃ and Q ₁₅ . This common base includes
A common connection point between the base and collector of a diode-connected transistor _Q21 , which determines the emitter current of the transistor _Q16 , is connected via a resistor _R21 , and the emitter of this transistor _Q21 is grounded. Here, the base bias of transistor Q ₁₃ is _set by resistors R ₁₃ , R ₁₄ and transistor
given through Q ₁₆ and also the transistor Q ₁₁
The base bias of is applied from the power supply V _CC through resistor _R13 .

The collector of the transistor (second transistor) Q ₂₂ that determines the emitter current of the emitter follower transistor Q ₁₂ is connected to the emitter of transistor Q ₁₂ , the emitter is grounded, and the base is common to the base and collector of transistor Q ₂₁ . It is connected to the connection point and forms a current mirror circuit with this transistor _Q21 .

In the configuration of FIG. 1, the transistors Q ₁₅ ,
The collector currents I _C15 and I _C13 of Q ₁₃ are given by equation (1) as in FIG. 2 above. Further, the collector current I _C22 of the transistor Q ₂₂ is calculated as follows, assuming that the emitter area ratio of the transistors Q ₂₁ and Q ₂₂ is 1:N, I _C22 = NR ₁₇ /R ₂₁・V _CC −2V _J /R ₁₃ +R ₁₄ +R ₁₇ ……(3) is given by. Here, if R ₂₁ /N=R ₁₆ , the collector current I _C22 of the transistor Q ₂₂ becomes equal to the collector current I _C14 of the transistor Q ₁₄ .

Here, to further explain the circuit of FIG. 1, regarding the transistor _Q13 that determines the emitter operating current of the emitter follower transistor _Q11 on the input side, a feedback type base bias circuit is formed as in the conventional case. Emitsuta follower transistor on the output side
Transistor that determines the emitter operating current of _Q12
No feedback circuit is formed for _Q22 , and the emitter current of transistor _Q12 is obtained by connecting it to transistor _Q21 as a current mirror.

According to such a configuration, the base line of transistor Q ₂₂ is a diode-connected transistor
It has an impedance of _Q21 , and due to the collector-substrate capacitance of this transistor _Q21 , the baseline impedance becomes smaller at higher frequencies. Therefore, the output of the amplifier circuit 11 is unlikely to appear on the base line of the transistor _Q22 , and even if it does appear, by the time the fluctuation is transmitted to the base of the transistor _Q15 , r _e / _re + _R21
(However, r _e is the equivalent emitter resistance of transistor Q ₃ )
Since the voltage is divided into two, it becomes very small and has almost no effect on the emitter potential of transistor _Q11 . As a result, in the circuit shown in Figure 1,
Even if the gain of the amplifier circuit 11 is large, oscillation does not occur.

Moreover, the circuit shown in FIG. 1 has fewer elements than the circuit shown in FIG. 2, and is advantageous in terms of structure.

Generally, the emitter area ratio N of a transistor is less accurate than the resistance ratio, so in the configuration shown in Fig. 1, there is a risk that the accuracy of the collector current I _C22 of the transistor _Q22 will be degraded. The emitter current of the emitter follower transistor _Q12 does not require much precision, so there is no problem.

In the above explanation, the case where there is one amplifier circuit 11 has been explained as a representative, but if there are multiple amplifier circuits, it is sufficient to provide multiple transistors Q ₁₃ and Q ₂₂ according to the number, and transistors Q ₁₅ and Q ₁₆ , _Q21, etc., there is no need to provide a plurality of them, and they can be used in all circuits.

Furthermore, it goes without saying that a diode may be used as the diode characteristic element in place of the diode-connected transistor _Q21 .

In addition, in the above explanation, the emitter follower transistor that sets the input bias of the amplifier circuit 11 is
A case where the present invention is applied to a current source circuit that sets the emitter operating current of the above two emitter follower transistors in a circuit equipped with an emitter follower transistor that takes out the output of Q ₁₁ and the amplifier circuit 11 has been explained as a representative case. However, it goes without saying that the present invention can also be applied to other current source circuits.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide a current source circuit that is free from oscillation even if the gain of the amplifier circuit is large.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a circuit diagram showing the configuration of a conventional current source circuit. 11...Amplification circuit, 12...Signal source, 13...
Output terminals, _Q11 to _Q13 , _Q15 , _Q16 , _Q21 , _Q22 ...
Transistor, R ₁₁ ~ R ₁₅ , R ₁₇ , R ₂₁ ... Resistor,
C ₁₁ ... Capacitor, V _CC ... Power supply.

Claims (1)

[Claims] 1: a first transistor forming a first current source; a second transistor forming a second current source; a base connected to the base of the first transistor; a third transistor for detecting potential fluctuations; a base connected to the collector of the third transistor; an emitter connected to a common connection point of the bases of the first and third transistors;
a fourth transistor that feeds back the collector output of the transistor to the common base; a portion corresponding to the anode of the diode is connected to the common base of the first and third transistors via a resistor; A current source circuit comprising a transistor and a diode characteristic element connected in a current mirror.