JPH03101408A - Attenuate circuit - Google Patents

Abstract

PURPOSE:To prevent the deterioration in the frequency characteristic and to attain broad band circuit by connecting outputs of a voltage follower circuit of a pre-stage and a post-stage in series via two resistors and extracting an attenuated signal from the connecting point of the resistors. CONSTITUTION:Outputs of a pre-stage feedback type voltage follower circuit and of a post-stage feedback type voltage follower circuit are connected in series by resistors R14 and 15. An attenuated signal is extracted from a connecting point between the resistors R14 and 15. A DC voltage of an input signal inputted to a transistor(TR) Q11 is ensured at an output signal side outputted from a TR Q28 and the output impedance of the pre-stage and post- stage voltage follower circuits is reduced. Thus, the deterioration in the linearity of the output signal and the increase in the distortion thereof are prevented and broad band circuit configuration is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION (Field of Industrial Application) The present invention relates to an attenuate circuit in a linear integrated circuit used in television receivers, video tape recorders, and the like.

(Prior Art) Conventionally, an attenuate circuit used in a linear integrated circuit has a configuration as shown in FIG. 3, for example. That is, this is an example in which the amplifier circuit including the transistor Q1 is used as an attenuate circuit by setting the resistance R2≦R1.

However, since the emitter resistance of the transistor Q1 fluctuates, there is a drawback that distortion of the output signal and deterioration of linearity are noticeable.

Figure 4 shows another example of the conventional attenuate circuit, which consists of transistors Q2 and Q3, and output signals are passed through resistors R3 and R.
The input signal is attenuated by taking it out from the dividing point of 4. In this example, the variation in the emitter resistance of the transistor Q2 due to the rise and fall of the amplitude of the input signal Vin is canceled out by the variation in the emitter resistance of the transistor Q3, thereby reducing distortion and linearity deterioration of the output signal.

Here, in both the circuits shown in FIGS. 3 and 4 above, when it is important to preserve the DC voltage as the input signal Vin, for example, when a clamped video signal is input as the input signal, the output side with respect to the input side There was a problem that the DC voltage of the device fluctuated greatly.

In order to eliminate this inconvenience and maintain the input DC voltage, it is necessary to install a DC voltage level shift circuit, but since there are variations in the level shift circuit, it is necessary to strictly preserve the input DC voltage. The disadvantage is that it is difficult to carry out.

FIG. 5 is a diagram showing an example of a conventional attenuator circuit that takes into account preservation of the DC voltage of an input signal. In this example, when an input signal as shown in FIG. 6A is applied to the base of transistor Q4, the current corresponding to the input signal flowing through the emitter of transistor Q4 becomes the base current of transistor Q6.

Since the current corresponding to the emitter current of the transistor Q6 corresponding to this base current becomes the base current of the transistor QIO, the current shown in FIG. 6(B) corresponding to the input signal ends up being
An output signal as shown in is output from the emitter of transistor QIO. Here, as the output signal of transistor Q4, the sink chip is clamped to 2.0■ IV
I) When the video signal of I) is input, V84 (2, OV where the clamp circuit is used) is given as the base DC voltage of transistor Q5, and in the figure, t1 = i2 and i3""t5 are established. . Furthermore, transistor Q4~
Assuming that the base-emitter voltage ■BE up to Q10 is almost equal, the sink tip voltage of the output signal output from the emitter of transistor QIO is approximately 2. OV, and the DC voltage of the input signal is preserved quite strictly.

However, in the circuit shown in FIG. 5, the emitter currents of transistors Q4 and Q5 vary depending on the signal level input to the base of transistor Q4, so the linearity of the signal generated at the base of transistor Q6 deteriorates.
There was a drawback that distortion occurred. For example, R5=R
6 = 10Ω, il = i2 = 0.1 (mA>), and ignoring the base current of transistor Q6, when the sink tip of the signal input to the base of transistor Q4 is 2.0cm, l4E = 5E (4E and 15F are the emitter currents of transistors Q4 and Q5), so the base of transistor Q6 is 2.0〒V4B
E=1,3 (V). (v4BE is the base-emitter voltage of transistor Q4). However, when the signal level input to the base of transistor Q4 reaches 3.0V, a voltage (3,0-2,0>
/ (10Ω + 10Ω) = 50 (μA) current flows, so 4E”0.15 (mA), l5E = 0.0
5 (mA>. Therefore, the emitter resistance of transistor Q4 is r4e=30/I 4E"200 (Ω)
, the emitter resistance of transistor Q5 is r5F=30/5F=600 (Ω), so the base of transistor Q6 is (3,0-2,O)X (10+016)/('
10+0.6+10+0.2)=0.51, which should be 0.5, but due to the change in emitter current, it becomes 0.
．． It has fluctuated to 51.

That is, the more the input signal level fluctuates, the more the linearity of the attenuate signal appearing at the base of transistor Q6 deteriorates and the distortion increases.

This tendency becomes more pronounced as the resistances R5 and R6 are made smaller, or as the currents L1 and 12 in the figure are made smaller. vice versa,
In order to improve the linearity, the current L1. L2
or resistor R5.

Either increase R6. However, the current i
1. Increasing L2 is contrary to reducing the power consumption of the circuit, but on the other hand, increasing the resistors R5 and R6 causes the frequency characteristics to deteriorate due to the capacitance on the base side of transistor Q6, making it difficult to widen the band of the circuit. It had the disadvantage of becoming

(Problems to be Solved by the Invention) As described above, in the attenuating circuit used in conventional linear integrated circuits, when a video signal or the like is attenuated, the DC voltage of the input signal becomes the DC voltage (DC voltage) of the output signal. There were problems such as misalignment and variation. Further, a circuit in which the DC voltage of the input signal can be secured on the output signal side has the disadvantage that the linearity of the output signal deteriorates and distortion increases.

Therefore, the present invention eliminates the above-mentioned drawbacks, and provides a wide band that can accurately secure the DC voltage of the input signal at the output side5, and prevents deterioration of output signal linearity and increase of distortion without increasing power consumption. The purpose is to provide an attenuate circuit.

[Structure of the invention]

(Means for Solving the Problems) The attenuator circuit of the present invention includes first and second transistors forming a differential pair, and a first transistor that supplies a constant current to a common emitter of the first and second transistors. Supplying voltage Vcc to a constant current source, a third transistor whose base is connected to the emitter of the second transistor and whose emitter is connected to the base of the second transistor, and the collectors of the first and third transistors. a voltage source; a second constant current source that supplies a constant current to the collector of the second transistor; and a third constant current source that supplies a constant current to the base of the second transistor and the emitter of the third transistor. a first voltage follower circuit comprising a first voltage follower circuit, and a fourth voltage follower circuit constituting a differential pair.
, a fourth constant current source that supplies a constant current to a common emitter of the fourth and fifth transistors; a fourth constant current source whose base is connected to the emitter of the fifth transistor; 6th emitter connected
a voltage source that supplies a voltage Vcc to the collectors of the fourth and sixth transistors, a fifth constant current source that supplies a constant current to the collector of the fifth transistor, and a base of the fifth transistor. a second voltage follower circuit comprising a sixth constant current source that supplies a constant current to the emitter of the sixth transistor; and a first voltage follower circuit that connects the base of the second transistor and the base of the fifth transistor. and a second resistor, and a seventh transistor whose base is connected to the connection point of these first and second resistors to form an emitter follower.

(Function) In the attenuate circuit of the present invention, the first voltage follower circuit outputs an output signal equal to the input signal to the first voltage follower circuit.
Output to a series circuit consisting of a second resistor. The first and second resistors attenuate the output signal according to the ratio of these resistors, and output the attenuated signal from the connection point to the base of the seventh transistor. If a voltage equal to the base voltage of the first transistor, which is the input stage of the first voltage follower circuit, is applied to the base of the fourth transistor, which is the input stage of the second voltage follower circuit, the second
The voltage follower circuit outputs a base voltage substantially equal to the base voltage of the first transistor to the DC circuit, and the DC voltage at the connection point becomes equal to the DC voltage applied to the input terminal of the first voltage follower circuit. . Furthermore, since feedback is applied to the third transistor and the sixth transistor, their output impedances are kept low. The seventh transistor outputs the DC voltage and signal at the connection point of the resistor as an emitter follower.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the attenuate circuit of the present invention. Transistor Q11 configuring a differential pair. Q
12. and transistor Q15, and further transistor Q1
A first constant current source consisting of a transistor Q16 and a resistor R11, a second constant current source consisting of a transistor Q16 and a resistor R12, and a third constant current source consisting of a transistor Q14 and a resistor R13 are connected to the feedback voltage of the previous stage. It constitutes a follower circuit. Transistor Q17 constituting a differential pair. Q1
8 and transistor Q19, a fourth constant current source consisting of transistor Q20 and resistor R16, a fifth constant current source consisting of transistor Q21 and resistor R18, and a sixth constant current source consisting of transistor Q26 and resistor R17. , which constitutes a feedback voltage follower circuit in the latter stage. The outputs of these front-stage and rear-stage voltage follower circuits are connected in series by two resistors R14 and R15.
An attenuated signal is taken out from the connection point between resistors R14 and R15, and this signal is applied to the base of transistor Q22.

Here, the transistors Q22 to Q2B constitute an emitter follower output stage for the attenuated signal, and the output signal is taken out from the emitter of the transistor Q28. Note that the transistor Q23 and resistor R19 constitute a seventh constant current source, the transistor Q27 and resistor R20 constitute an eighth constant current source, and the transistor Q29 and resistor R21 constitute a ninth constant current source.

Further, the attenuate ratio is R15/(R14+R15).

Next, the operation of this embodiment will be explained. transistor Q
When a video signal with a sync chip of 2V and a level ■p-p as shown in FIG. 2(A) is input to the base of 11,
A signal equal to the video signal input to the base of transistor Q11 appears at the emitter of transistor Q15, which is the output of the feedback voltage follower circuit at the previous stage. At the same time, the emitter of transistor Q19, which is the output of the subsequent voltage follower circuit, is connected to transistor Q.
A base voltage V82 of 17 is generated. Here, if a voltage equal to the sync tip DC' [voltage (2, OV)] of the video signal input to the base of the transistor Q11 is applied as B2, the emitter of the transistor Q2B has a level of 2.0 V at the sync tip. is 0.5Vp-p (
When R4=R5=10 and Ω), a video signal is obtained.

In addition, if the input signal is sync-chip clamped, V81=■ can be easily determined by using the clamp voltage.
, 2 can be achieved.

In this case, the output impedance of transistors Q15 and Q19 that drive resistor R14 and resistor R15 has decreased considerably, so even if the linearity of the video signal deteriorates due to a change in the output impedance of transistor Q15 and transistor Q19, Even if the emitter current of transistor Q15 and transistor Q19 changes due to a change in the signal level and the emitter voltage of transistor Q15 and transistor Q19 changes, the transistor Q15 is a feedback voltage follower circuit.
The emitter voltage of transistor Q19 and the emitter voltage of transistor Q19 do not change. Therefore, resistor R14 and resistor R
By reducing the voltage change across the terminal 15, deterioration of linearity can also be reduced.

According to this embodiment, the DC voltage of the input signal input to the transistor Q11 can be accurately ensured on the output signal side output from the transistor Q28. In addition, since the output impedance of the voltage follower circuits in the front and rear stages is quite small, it is possible to prevent deterioration of signal linearity and increase in distortion due to fluctuations in output impedance, and therefore it is also possible to reduce the circuit current in each part. .

Moreover, series resistors R14 and R15 for level attenuation
Since the value of can also be made small, it is possible to prevent deterioration of the frequency characteristics of the attenuate circuit, making it possible to widen the band. Further, the output signal of the attenuate circuit of the above embodiment is switched with another video signal whose sync chip is clamped at 2.0 (this is the same as the clamp voltage of the video signal input to the attenuate circuit). When the input signal is input to the next stage amplifier circuit, the output DC voltage shifts, so it is necessary that the sync chip voltage of the input signal and the output signal of the above attenuate circuit be equal.In such a case, the above attenuate circuit is effective. It can be used for.

〔Effect of the invention〕

As described above, according to the attenuate circuit of the present invention, the DC voltage of the input signal can be accurately secured on the output side, and the deterioration of the linearity of the output signal and the increase in distortion can be prevented without increasing power consumption. In addition, it has the effect of making it possible to widen the band.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the attenuate circuit of the present invention, Fig. 2 is a diagram showing an example of input and output signals of the circuit shown in Fig. 1, and Fig. 3 is a conventional attenuate circuit. FIG. 4 is a circuit diagram showing an example of the conventional attenuate circuit. FIG. 5 is a circuit diagram showing still another example of the conventional attenuate circuit, and FIG. 6 is a diagram showing examples of input signals and output signals of the circuit shown in FIG. Q11-Q29...Transistor R11-R21...Resistor

Claims (1)

[Claims] first and second transistors forming a differential pair;
a first constant current source that supplies a constant current to a common emitter of the second transistor; and a third transistor whose base is connected to the emitter of the second transistor and whose emitter is connected to the base of the second transistor. , a voltage source that supplies a voltage V_c_c to the collectors of the first and third transistors, a second constant current source that supplies a constant current to the collector of the second transistor, and a voltage source that supplies a voltage V_c_c to the collectors of the first and third transistors; The third one supplies a constant current to the emitter of the transistor.
a first voltage follower circuit comprising a constant current source; fourth and fifth transistors constituting a differential pair; and a first voltage follower circuit that supplies a constant current to a common emitter of the fourth and fifth transistors. a sixth transistor whose base is connected to the emitter of the fifth transistor and whose emitter is connected to the base of the fifth transistor;
, a voltage source that supplies a voltage V_c_c to the collector of the sixth transistor, a fifth constant current source that supplies a constant current to the collector of the fifth transistor, and a base of the fifth transistor and an emitter of the sixth transistor. a second voltage follower circuit comprising: a sixth constant current source that supplies a constant current; a first resistor connecting the base of the second transistor and the base of the fifth transistor; 1. An attenuate circuit comprising: a series circuit composed of resistors; and a seventh transistor whose base is connected to a connection point between these first and second resistors to form an emitter follower.