JPS63100876A - Video signal amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent unsharpened waveform of a signal due to clipping by adopting a differential amplification system for a comparator of a clip circuit. CONSTITUTION:The comparator 3' of the clip circuit 2 is of differential amplifier form. That is, a collector of a transistor (TR) Q5 of emitter connection is connected to an output terminal of an amplifier 1, that is, a base of a TR Q4 via a resistor R6 and a collector of the other TR Q6 is connected to a power supply Vcc. Moreover, a PNP TR Q7 is connected between the base of the TR Q4 and a grounding terminal, and the base is connected to the collector of the TR Q5. An output voltage of the TR Q4 is fed to the base of the TR Q5 while being divided by resistors R4, R5 and a clip reference voltage Vo is fed to the base of the TR Q6 from a constant voltage circuit 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a video signal amplification circuit suitable for use in a VTR or the like.

[Conventional technology]

A video signal amplification circuit used in a VTR or the like is usually provided with a clip circuit so that its output level falls within the dynamic range of a subsequent processing circuit.

FIG. 4 is a circuit diagram showing an example of such a conventional video signal amplification circuit, in which 1 is an amplifier, 2 is a clip circuit, 3 is a comparator, 4 is an input terminal, and 5 is an output terminal.

In the figure, the video signal input from input terminal 4 is
After being amplified by amplifier 1, it is supplied to output terminal 5 via transistor Q8 forming an emitter follower. Here, when the video signal is supplied from the amplifier 1 to the base of the transistor Q8, it is subjected to the action of the clip circuit 20.

This clip circuit 2 consists of two emitter-coupled transistors Q? , Q10 also have a comparator 3.

Here, the base and collector of transistor Q9 are connected to the output terminal of amplifier 1, and therefore the base of transistor Q8, and the collector of transistor Q10 is
Connected to the cc power supply. Also, transistor QI
A clip reference voltage v0 obtained by dividing the power supply voltage Vcc by a resistor R is applied to the base of O.

Therefore, a part of the output video signal of the amplifier 1 is also supplied to the base of the transistor Q9, and its voltage (2) is compared with the reference voltage Vo. When V is low, transistor Q10 is turned on and transistor Q9 is turned off, and the output video signal of amplifier 1 is directly supplied to the base of transistor Q8. However, when V≧V, transistor Q10 is turned off and transistor Q9 is turned off. Q9 is turned off and the base brazing of transistor Q8 is fixed at the clip reference voltage v0.

In this way, the output video signal of the amplifier 1 is clipped to the clip reference voltage V, and is supplied to the base of the transistor Q8.

[Problem that the invention seeks to solve]

By the way, in this clip circuit 2, if the output voltage V of the amplifier 1 is gradually rising as shown in FIG.
Immediately before the m pressure becomes equal to v0, collector current begins to flow through transistor Q9, and along with this, transistor Q1
0 collector current begins to decrease. When the collector current flows through the transistor Q9, it flows to the resistor that is the load resistance of the amplifier 1 [fi increases by one collector current of the transistor Q9, and the output voltage V decreases by the voltage drop across the resistor due to the increase r30 minutes. (As the collector 1 of the transistor Q11 further decreases, the output voltage ■ increases, but at the same time, the collector current of the transistor Q9 increases its power (accordingly, the collector current of the transistor QIO decreases by γ) Even though the output voltage V increases, this output voltage V will be lower by 1E1 voltage due to the resistor drawn by this collector current than if no collector current flows through the transistor Q9. .and,
Furthermore, when the collector current of transistor Q11 decreases, transistor Q9 becomes saturated and turns on. The collector current at this time is equal to the saturated collector current of transistor Q10 when it is on, and transistor Q10 turns off. Transistor Q9. Ql O is Tomo NPN
are transistors and have the same characteristics, therefore, transistor Q9. The base position of Q8 is the clip group at pressure ■. is equal to

In other words, when the output voltage (2) of the amplifier 1 gradually increases, the portion of this output voltage (2) that exceeds the clip reference voltage 70 is clipped, but the output voltage (V) due only to the collector's whirl current of the transistor Q11 (Fig. 5 1-1) is clipped. ) becomes equal to the clip reference voltage v0, a transient phenomenon occurs in the collector current of transistor Q9, and as shown in FIG.
As shown in FIG. 1, a waveform rounding occurs immediately before the output pressure V reaches the clip reference voltage v0.

Conversely, since the output voltage of the amplifier 1 is clipped at the clip reference voltage V0, the collector current of the transistor Q11 increases (and from when this collector current reaches a predetermined value, the transistor Q9 The collector current begins to decrease as the on-state transitions to the off-state.This process follows the above process in reverse, and as a result, as shown in FIG.
Output 1 pressure due to collector return flow only (Fig. 5 (4)
) becomes equal to the clip reference voltage v0, a waveform rounding occurs in the output voltage V supplied to the base of the transistor Q8 due to a transient phenomenon at the collector 1tfi of the transistor Q9.

Furthermore, in the case of a monolithic IC, each element incorporated therein has the following temperature coefficient.

That is, the temperature coefficient of the resistor is , and the temperature coefficient of the transistor is . However, ■B8: Between the base and emitter of the transistor! Pressure Echi: Energy gap of silicon (about t12eV)? =Electron charge amount (t6X10-1-coulombs) A:
Ho 7 ff y constant fi (t38X10-” a
J/K) T: Absolute temperature Since transistors and resistors have a temperature coefficient a, in Fig. 4, power supply voltage Vcc is expressed as resistance horse, R
The clipping reference voltage v0 obtained by dividing the voltage by , changes due to disturbances such as current/voltage fluctuations and temperature drift, and stable clipping is not performed.

An object of the present invention is to provide a video signal amplification circuit that eliminates the four points during this period, performs stable clipping, and prevents waveform distortion due to clipping.
be.

[Means for solving the four points] In order to achieve the above object, the present invention uses a differential quadrupler type comparator in the clip circuit.

[Effect]

When comparing the clip reference voltage and the video I signal voltage, a transient phenomenon occurs in the collector current of the transistor forming one comparator in the period immediately before and after the two match, as in the prior art described above. Because the comparator is of the differential amplifier type, the collector current is amplified. Therefore, the detection sensitivity of the comparator is greatly improved and waveform rounding does not occur.

〔Example〕

Hereinafter, embodiments of the invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the video signal amplification circuit 11 according to the present invention, in which 5° is a comparator, 6 is a bias circuit, and 7 is a constant voltage circuit, parts corresponding to FIG. are given the same symbol.

In FIG. 1, the video signal input to input terminal 4 is amplified by amplification 51, and clipped by clip reference 4 by clip circuit 2.
After the part above the voltage is clipped, the transistor Q4'
The output is supplied to the first child 5 via. The bias circuit 6 performs temperature compensation for the amplifier 1, so that the amplifier 1 outputs an interposed signal that is stable against disturbances.

Here, the comparator 3' of the clip circuit 2 is in the form of a differential amplifier. That is, the collector of one emitter-coupled transistor Q5 is connected via a resistor to the output terminal of amplifier 1, and thus to the base of transistor Q4, and the collector of the other transistor Q6 is connected to Vcc'gL#I. ing. Also, transistor Q4
A PNP) transistor Q is connected between the base and the ground terminal of
7 is connected, and its base is connected to the collector of transistor Q5. The base of the transistor Q5 is supplied with the output four voltages of the transistor Q4 divided by resistors R, 4, R, and the base of the transistor Q6 is applied with a clip reference voltage v-; from the constant voltage circuit 7.

Next, how does this clip circuit 2 work? explain.

The image g1 signal output from the emitter of transistor Q4 is divided by resistors it,, R, and then sent to transistor Q5.
is supplied to the base of the transistor Q6 and is compared with the clip base pressure V0 applied to the base of the transistor Q6 [marked 71I]. Now, if the base of the transistor Q5 is set at a potential V<the clip reference voltage V, the transistor Q5 is turned off, the transistor Q6 is turned on, and therefore the transistor Q7 is also turned off. As a result, the output video signal of the amplifier 1 is directly supplied to the base of the transistor Q4. On the other hand, when the voltage is V2V5, the transistor Q6 is turned off and the transistor Q5 is turned on, and a current flows through the resistor, turning on the transistor Q7. However, at this time, the emitter potential of the transistor Q7, and hence the base potential of the transistor Q4, is higher than the pace '1 of the transistor Q7 by one voltage vF between the emitter and the pace of the transistor Q7. In other words, the pace center position of transistor Q4 is lower than the base potential of transistor Q7 by voltage VF (approximately 17 t
el) will be clipped to 1st place, which is higher.

By the way, transistor Q5. The operation of Q6 is the same as that of the conventional comparator 3 shown in FIG. Therefore, a transient phenomenon occurs in the collector current of transistor Q5 during a period from immediately before to immediately after the pace potential V of transistor Q5 matches the clip reference voltage V.

However, in this embodiment, the base potential of the transistor Q7 generated by the collector current of the transistor Q5 flowing through the resistor is amplified by the transistor Q7 and fed back to the resistor. As a result, the collector current of transistor Q5 increases and is immediately saturated.

Therefore, the transient period of the collector flow of the transistor Q5 is significantly shortened, and the response sensitivity (transient response) of the comparator 3 is the same as above. The video signal obtained at the terminal 5 has no waveform rounding, as shown in +bl of FIG. Moreover,
Since the leakage current when the transistor Q7 is off is almost zero, the clip level is stabilized.

In this embodiment, further, in order to stabilize the clip reference voltage v0 applied to the pace of the transistor Q6,
This clip reference subsoil V0 is generated by a constant voltage circuit 7.

FIG. 5 is a circuit diagram showing a band cap constant voltage circuit as an example of the constant voltage circuit 7. In FIG. As shown in the figure, the voltage between the collector and emitter of the transistor Q5 is the output voltage Vout of this bandgap constant voltage circuit. Here, the base-emitter l1J1El of the transistor QS
If the voltage is vBI3 and the leakage voltage of the resistor R2 is V, then this output voltage Vout is expressed as follows.

Vout −VBE3+V, =・−・=−A1
1 Generally, one voltage VB between the pace and emitter of a transistor
The temperature coefficient of E is, as explained earlier, approximately -18-v
/ 0C, but since the voltage V at both ends has a positive temperature coefficient due to the resistance R, the output voltage V can be adjusted by appropriately setting the value of the resistance R1. The temperature coefficient of ut can be made zero. By the way, one current flowing through the resistors R, , R, is connected to I, , I, and transistor Q1 .
Since the direct pressure between the base and emitter of Q2 is respectively and , the equation (1) becomes from the equation t21131.

Therefore, by selecting L>It, the second term on the right side of the above equation (4) becomes positive, and by appropriately selecting the head of the resistor R1, the temperature coefficient of the output '1 pressure Vout becomes zero, that is,
Output voltage V. ut can be made constant against @box changes.

This multiplication factor is that the output voltage Vout is the forbidden band of silicon,
In other words, it is obtained when the voltage is equal to the bandcap value (approximately tZ(V)).For this reason, the constant voltage circuit shown in FIG. 3 is called a bandcap constant voltage circuit.

In FIG. 1, such a constant voltage circuit 7 has a clip reference voltage ■. By setting , the output video signal of the amplifier 1 is clipped at a constant potential higher than the base potential of the transistor Q7 by the base Φ emitter voltage vF of the transistor Q7, and furthermore, no waveform rounding occurs.

Moreover, since the amplifier 1 is temperature-compensated by the bias circuit 6, the loss width and DC operating point of the output video signal of the 4m amplifier 1 will fluctuate due to disturbances such as temperature drift and fluctuations in the power supply voltage. Therefore, clipping is always performed correctly at the correct level with respect to the DC operating point of this output video signal.

〔Effect of the invention〕

As described above, according to the present invention, the transient response of the clipping circuit is greatly improved, the rounding of the signal waveform due to clipping can be prevented, and the S/N is significantly improved.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the video signal amplification circuit according to the present invention, Fig. 2 is a hexagonal diagram showing its clipping operation, and Fig. 3 is a specific example of the constant voltage circuit in Fig. 1. FIG. 4 is a circuit diagram showing an example of a conventional video increasing circuit, and FIG. 5 is a waveform diagram showing its clipping operation. 1...Amplifier, 2...Clip circuit, 3°...Comparator, 4...Input terminal, 5...Output terminal, 6...
Bias circuit, 7... constant voltage circuit.

Claims (1)

[Claims] 1. In a video amplification circuit comprising an amplifier and a clip circuit, and configured to clip the output video signal of the amplifier to a desired clip level by the clip circuit, the comparator of the clip circuit is differentially connected. A video signal amplification circuit characterized in that it is in an amplification format. 2. The video signal amplification circuit according to claim 1, wherein a clip reference voltage serving as a comparison reference of the comparator is set by a constant voltage circuit.