JP2831718B2 - Voltage follower circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a voltage follower circuit in a linear integrated circuit used for electronic equipment and the like.

(Prior Art) FIG. 3 is a circuit diagram showing an example of a conventional voltage follower circuit of this type. The transistors Q1 and Q2 form a differential amplifier circuit, and the common emitter of the transistors Q1 and Q2 is grounded via a constant current source including the transistor Q3 and the resistor R1. The collector of the transistor Q2 is connected to a voltage source Vcc via a constant current source including a transistor Q4 and a resistor R3. Further, the base of the transistor Q2 is grounded via a constant current source consisting of the transistor Q6 and the resistor R2, and the voltage source Vc is connected via the transistor Q5.
Connected to c. The base of the transistor Q5 is connected to the collector of the transistor Q2. A video signal of 2 V _pp as shown in FIG. 4A is input to the base of the transistor Q1, and its collector is directly connected to the voltage source Vcc. An output video signal as shown in FIG. 4C is taken out from the base of the transistor Q2.

And in the circuit _{Vcc = 5V, V B2 = 0.9V} , and V _B3 = 4.1 V,
It is assumed that the sync tip of the video signal input to the base of the transistor Q1 is clamped at 2.0V. Therefore, 100% white becomes 4.0V. In this case, assuming that the voltage follower circuit described above is operating normally,
From the base of transistor Q2, sink tip 2.0V, 100
A video signal of 4.0% is output. However, at the collector of the transistor Q2, as shown in FIG. 4 (B), the voltage of the sink tip and the 100% white voltage become almost 2.7V and 4.7V, so that the transistor Q4 is saturated and the voltage follower circuit As a result, normal operation cannot be performed. That is, the voltage follower circuit shown in FIG. 3 has a drawback that the input video signal is not output in a normal form because the dynamic range of the transistor Q2 is small. So the transistor
Although the clamp voltage of the video signal input to the base of Q1 can be reduced slightly (about 0.3V) to increase the dynamic range of the transistor Q2, this is still insufficient.

FIG. 5 is a diagram showing an example of a circuit which can be used as a voltage follower circuit because the input signal is almost the same as the output signal, although it is originally a level shift circuit. The transistors Q1 and Q2 form a differential pair, and the common emitter is grounded via a constant current source including the transistor Q3 and the resistor R1. The voltage Vcc is applied to the collector of the transistor Q1, and a video signal as shown in FIG. 6A is input to the base. The collector of the transistor Q2 is connected to the voltage Vcc via a constant current source consisting of the transistor Q4 and the resistor R2. Further, the collector and the base of the transistor Q2 are shared, from which a video signal as shown in FIG. 6 (B) is extracted. In this circuit, a sufficient dynamic range can be secured at Vcc = 5V, and normal operation as a voltage follower circuit can be performed. However, when the resistor R3 is connected as a load to the output side of this circuit, the emitter resistance of the transistor Q1 and the emitter resistance of the transistor Q2 fluctuate with the DC level of the video signal input to the base of the transistor Q1, so that the transistor Q2 However, there is a disadvantage that the linearity of the video signal output from the base and the collector deteriorates and distortion occurs. Therefore, in order to improve the linearity, the currents i ₁ and i ₂ must be increased or the resistance R3 must be increased. However, the resistance R3 is often uniquely determined in relation to other circuits, and as a result, the currents i ₁ and i ₂ are increased, so that there is a disadvantage that power consumption increases.

(Problems to be Solved by the Invention) As described above, the conventional voltage follower circuit has a shortcoming in that the dynamic range is insufficient and the normal voltage follower circuit cannot operate. Therefore, a level shift circuit that can secure a dynamic range can be used as a voltage follower circuit, but this circuit has a drawback that when a resistor is connected as a load, the linearity of an output signal is degraded.

Therefore, an object of the present invention is to eliminate the above-mentioned disadvantages and to provide a voltage follower circuit that can secure a sufficient dynamic range and has good linearity.

[Configuration of the invention]

(Means for Solving the Problems) A voltage follower circuit according to the present invention comprises a first and a second transistor forming a differential pair, and a first and a second transistor connected to a common emitter of the first and the second transistors. A constant current source; an operating voltage source connected to the collector of the first transistor; a third transistor having a collector connected to the operating voltage source and a base connected to the collector of the second transistor; A diode having a cathode connected to the emitter of the third transistor, and an anode connected to the base of the second transistor; a second constant current source connected to the collector of the second transistor; A third constant current source connected to the emitter of the first transistor, a fourth constant current source connected to the base of the second transistor, and the first transistor It has a configuration having an input terminal connected to the base of the transistor and an output terminal connected to the base of the second transistor.

(Operation) In the voltage follower circuit of the present invention, a signal input from the base of the first transistor appears at the base of the second transistor. At this time, the signal from the emitter of the third transistor to the base of the second transistor. Since the feedback is performed via the diode, the second transistor does not saturate. In addition, even when a resistor is connected as a load to the base of the second transistor, the base operates as a feedback terminal, so that there is no deterioration in signal linearity due to the influence of the resistor.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing one embodiment of the voltage follower circuit of the present invention. The transistors Q1 and Q2 form a differential pair, and their common emitter is grounded via a first constant current source including a transistor Q3 and a resistor R1. The collector of the transistor Q1 is connected to the voltage source Vcc, and the collector of the transistor Q2 is connected to the voltage source Vcc via a second constant current source including the transistor Q4 and the resistor R3. The collector of transistor Q2 is connected to the base of transistor Q5, the collector of transistor Q5 is connected to voltage source Vcc, and the emitter is transistor Q6 and resistor R2.
And is grounded via a third constant current source. The emitter of the transistor Q5 is connected to the base of the transistor Q2 via a feedback transistor Q7 (operating as a diode). The base of the transistor Q2 is connected to a voltage source Vcc via a fourth constant current source including a transistor Q8 and a resistor R4. The base of the transistor Q1 is a signal input terminal, and the base of the transistor Q2 is a signal output terminal.

FIG. 2 is a diagram showing an example of a potential waveform of each part of the circuit shown in FIG. (A) shows the potential of the base of the transistor Q1, (B) shows the potential of the collector of the transistor Q2,
(C) shows the potential of the emitter of the transistor Q7, and (D) shows the potential of the base of the transistor Q2.

Next, the operation of this embodiment will be described. The above-described circuit is configured to apply feedback from the emitter of the transistor Q5 to the base of the transistor Q2 via the transistor Q7 in addition to the configuration of the conventional voltage follower circuit shown in FIG. Immediately, the third constant current source is connected to the emitter of the transistor Q5, the fourth constant current source is connected to the base of the transistor Q2, and the potential is increased by DC level shift via the transistor Q7 to raise the potential. Feedback is applied to the base of the transistor Q2 from the shift end.

Therefore, when a video signal of 2 V _pp as shown in FIG. 2A is input to the base of the transistor Q1, the transistor whose dynamic range is insufficient in the conventional circuit is used.
As shown in FIG. 2 (B), the potential at the collector end of Q2 is 2.0V at the time of the sync tip and 4.0V at the time of 100% white, so that a sufficient dynamic range is secured. Therefore, Vcc = 5V
Can handle the video signal shown in FIG. 2A sufficiently. If the base-emitter voltage V _BE of the transistor Q5 is equal to the base-emitter voltage V _BE of the transistor Q7, the base and the collector of the transistor Q2 always have the same potential, so that the transistor Q2 does not saturate. Moreover, even if a resistor R5 is connected as a load to the output terminal, since this output terminal is a feedback terminal, the collector current i _{1 of the} transistor Q8> Vo
As long as ut / R5 is satisfied and the circuit operates as a voltage follower circuit, the deterioration of the linearity of the output signal by the transistor Q7 does not occur.

According to the present embodiment, it constitutes the output side of the voltage follower circuit as a direct current level shift circuit, because it constitutes a voltage follower circuit over the feedback from that level shift end to the base of the transistor Q2, 2V _pp Video A sufficient dynamic range can be secured for signals, and a normal voltage follower circuit operation can be performed. Further, since the output terminal is a feedback terminal, a high-quality video signal can be obtained without deteriorating the linearity of the output signal. It is assumed that the video signal input to the circuit is a video signal of 2 V _pp from the sync chip to 100% white, but a video signal having 120% white may be actually input. Actually 2.4V
_{Although it} is necessary to secure the dynamic range for the _pp video signal, the above circuit can sufficiently cope with the video signal of this level.

〔The invention's effect〕

As described above, according to the voltage follower circuit of the present invention, a sufficient dynamic range can be secured and the linearity of the output signal can be secured.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the voltage follower circuit of the present invention, FIG. 2 is a potential waveform diagram of each part of the circuit shown in FIG. 1, and FIG. 3 is an example of a conventional voltage follower circuit. 4 is a circuit diagram showing another example of a conventional voltage follower circuit, and FIG. 6 is a circuit diagram showing another example of a conventional voltage follower circuit. FIG. 6 is a circuit diagram showing another example of the conventional voltage follower circuit. FIG. 4 is a potential waveform diagram at each part of the circuit shown. Q1-Q7: Transistors R1-R5: Resistance

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03F 3/45 H03F 3/50-3/52

Claims (1)

(57) [Claims] 1. A first and a second transistor forming a differential pair, a first constant current source connected to a common emitter of the first and second transistors, and a collector of the first transistor. A third transistor having a collector connected to the operating voltage source, a base connected to the collector of the second transistor, and a cathode connected to the emitter of the third transistor. A diode having an anode connected to the base of the second transistor, a second constant current source connected to the collector of the second transistor, and a third diode connected to the emitter of the third transistor. A constant current source; a fourth constant current source connected to the base of the second transistor; an input terminal connected to the base of the first transistor; Voltage follower circuit, characterized in that of comprising a base connected to an output terminal of the transistor.