JPH03235481A - Clamping circuit - Google Patents

Abstract

PURPOSE:To realize the stable reproduction of a direct current by providing a voltage detection means detecting that the DC level of an input signal suddenly rises and a switch means which temporarily forms the discharge route of coupling capacitors for clamping. CONSTITUTION:When the base current of a transistor 9 is set to be smaller than a current value I, the potential B of a signal line 7 rises from a clamping potential Vref with the change of the input signal A. When it becomes higher than V2, the transistor 13 of the voltage detection circuit 12 operates and a capacitor 11 is instantaneously charged at a current value I by current mirrors 16 and 16'. When the DC component of the input signal suddenly rises in a clamping circuit 1, for example, the voltage detection circuit 12 detects it and the current mirrors 16 and 16' charge the capacitor 11. Since the discharge route of the coupling capacitor 3 is temporarily formed by the turning-on of the transistor 9, the clamping potential is speedily stabilized to a prescribed reference value.

Description

[Detailed description of the invention] A. Industrial field of application B9 Summary of the invention C1 Prior art [Figure 3] D0 Problem to be solved by the invention [Figure 4] E0 Means for solving the problem F. Implementation Example [Fig. 1, Fig. 2] 80 circuit [Fig. Specifically, the present invention provides a novel lamp circuit that can quickly stabilize the clamp potential at a predetermined reference potential even when the DC level of an input signal increases abnormally.

3. Detailed Description of the Invention (B. Summary of the Invention) The clamp circuit of the present invention detects a change in the DC component of an input signal in a clamp circuit in which a coupling capacitor is provided on a signal path to which a human input signal is supplied. A voltage detection means and a switch means for temporarily forming a discharging path for the coupling capacitor in response to a signal from the voltage detection means are provided. When detected, the coupling capacitor is discharged by the switch means, thereby discharging the unnecessary charge stored in the coupling capacitor when the input signal level increases, and quickly stabilizing the clamp potential to a predetermined reference value. It was made so that it could be done.

(C, Prior Art) [Figure 3] The DC component of a signal may fluctuate due to various factors, and a clamp circuit is used to fix this DC component to a predetermined level. 24846
There is one shown in the publication.

FIG. 3 shows a basic configuration example a of such a lamp circuit.

In the figure, the input terminal is connected to the base of the emitter follower NPN transistor d via a coupling capacitor C for clamping.

e is a constant current source provided at the emitter of the transistor d.

f is an NPN transistor provided for diode clamping, and its emitter is connected to the base of transistor d, and its base is supplied with a predetermined reference voltage (the voltage between the base and emitter of transistor f) by a constant voltage source g. When "vBE"'V ret + V
BEJ to do. ) has been added.

(D. Problem to be Solved by the Invention) [Figure 4] By the way, in the above-mentioned lamp circuit a, if the DC component of the human input signal increases abnormally for some reason, the base potential of the transistor d There is a problem in that it takes time for the voltage to rise and stabilize to the clamp potential Vref, and stable DC component regeneration cannot be performed during that time.

Such a situation occurs when the operation of a circuit provided before the clamp circuit a suddenly starts, or when signal transmission from an external device such as a video camera to a device starts.

FIG. 4 schematically shows the signal waveform at this time.

In the figure, "A" indicates a human input signal, and rBJ indicates the base potential of transistor d.

As can be seen from the figure, when a stepwise change occurs from 0 (■) to V to (V), the base potential of transistor d changes from the reference voltage Vref to V r@(+V 1
The waveform becomes such that the level of Vr and f gradually rises and then gradually decreases to approach Vr and f.

This is because there is only a path for the charge stored in the coupling capacitor C to be discharged as the base current of the transistor d at the rise of the human input signal A, and the voltage of the waveform B is Vra? This is because it takes a long time to return to (this falling time is 'ttJ).

(E. Means for Solving the Problems) Therefore, in order to solve the above problems, the clamp circuit of the present invention includes a voltage detection means for detecting a sudden rise in the DC level of an input signal, and a voltage detection means for detecting a sudden rise in the DC level of an input signal. A switch means is provided for temporarily forming a discharge path for a coupling capacitor for clamping in response to a signal from the clamping capacitor.

Therefore, according to the present invention, when the voltage detecting means detects that the DC component of the input signal has increased to a predetermined value or more, the coupling capacitor is discharged by the switching means, and the useless waste accumulated when the input signal level rises is discharged by the switching means. Since the electric charge is swept away, the time required for the clamp potential to shift to a predetermined reference potential is short, and stable DC component regeneration can be realized.

(F, Embodiment) [Figures 1 and 2 below are Embodiment 1 illustrating the details of the clamp circuit of the present invention.
Explain according to the following.

(A, Circuit) [Figure 1] Reference numeral 2 is an input terminal, which is connected to the base of an emitter follower NPNI transistor 4 provided in the output stage via a coupling capacitor 3, and is connected to the emitter side of the transistor 4. A constant current source 5 is provided.

6 is an NPt1 transistor for clamping, the emitter of which is connected to the signal line 7 connecting the coupling capacitor 3 and the base of the transistor 4; When the predetermined reference voltage 'V ref + V B
EJ is applied by a constant voltage source 8.

Reference numeral 9 denotes an NPN transistor, which is provided so that a discharge path for the coupling capacitor 3 is formed when the transistor 9 is turned on. That is, the collector of the transistor 9 is connected to the signal line 7, the emitter thereof is grounded via a resistor 10, and a capacitor 11 is provided between the base and the ground line. When the potential of the capacitor 11 reaches a certain value, the transistor 9 is turned on and a discharge path for the coupling capacitor 3 is formed. At this time, the charge in the capacitor 11 is discharged as the base current of the transistor 9, but the discharge time constant is set to be a larger value than the discharge time constant of the coupling capacitor 3 due to the switching operation of the transistor 9. There is.

Reference numeral 12 denotes a voltage detection circuit, which is provided to determine whether the potential of the signal line 7 has passed through a predetermined potential.

That is, NPN)-transistors 13.1 constituting a differential pair.
3' is grounded via a constant current source 14 (the current value is "I"), and the base potential of one transistor 13' is set to a predetermined level by a constant voltage source 5. voltage (this will be referred to as "V2").

The base of the other transistor 13 is connected to the signal line 7, and the collector thereof is connected to the collector of the PNPNPt transistor.

The transistor 16 forms a current mirror together with the PNP transistor 16', and their bases are commonly connected to the collector of the transistor 16, and each emitter is connected to a power supply terminal (not shown). The collector of the transistor 16' is connected to the capacitor 11, and when the potential of the signal line 7 exceeds 2, the capacitor 11 is charged by this current mirror.

(b. Operation) [Fig. 2] However, in the clamp circuit 1, the operation when the input signal suddenly changes is performed as follows.

In addition, Fig. 2 schematically shows the signal waveforms of each part.
A'' is the input signal, rBJ is the potential of signal line 7, r(J
represent the potential of the capacitor 11, respectively.

As shown in the figure, consider a case where the human power signal changes stepwise from 0 (■) to ■1 (V).

Assuming that the base current of the transistor 9 is smaller than the current value I, the potential B of the voltage line 7 rises from Vr@y as the input signal A changes, and when this becomes higher than 2, the voltage detection circuit Twelve transistors 13 operate, and the capacitor 11 is instantly charged with a current value I by the current mirror (16, 16').

Therefore, the potential C of the capacitor 11 rises to a potential sufficient to turn on the transistor 9.

When the transistor 9 is turned on, the charge stored in the coupling capacitor 3 is discharged, and the potential B decreases.

Then, the potential B becomes lower than v2, but the charge in the capacitor 11 is only discharged as the base current of the transistor 9, and its time constant is larger than the discharging time constant of the coupling capacitor 3. The transistor 9 continues to be on until the discharge of the transistor 3 is completed.

In this way, the potential B of the signal line 7 reaches the reference potential V r with a shorter fall time Tt (<tt) than in the conventional case.
Returning to ef, the clamp potential becomes stable.

(c. Effect) In the above-mentioned clamp circuit 1, when the DC component of the input signal suddenly increases, the voltage detection circuit 1
is detected, the capacitor 11 is charged by the current mirror (16, 16'), and a discharge path for the coupling capacitor 3 is temporarily formed by turning on the transistor 9, so that the clamp potential quickly stabilizes to a predetermined reference value. do.

(G. Effects of the Invention) As is clear from the above description, the clamp circuit of the present invention is a clamp circuit in which a coupling capacitor is provided on a signal path to which an input signal is supplied. A voltage detecting means for detecting a change, and a switching means for receiving a signal from the voltage detecting means and temporarily forming a discharge path for a coupling capacitor are provided, and the DC level of the input signal rises above a predetermined value. The present invention is characterized in that when the voltage detection means detects the voltage, the coupling capacitor is discharged by the switch means so that the clamp potential is stabilized at a predetermined reference value.

Therefore, according to the present invention, when the voltage detecting means detects that the DC component of the input signal has increased to a predetermined value or more, the coupling capacitor is discharged by the switching means, and when the human input signal level rises, the coupling capacitor is discharged. Since unnecessary charges are wiped out, the time required for the clamp potential to shift to a predetermined reference potential is short, and stable DC component regeneration can be achieved.

[Brief explanation of drawings]

1 and 2 show an example of the implementation of the clamp circuit of the present invention. FIG. 1 is a circuit diagram, FIG. 2 is a waveform diagram schematically showing signal waveforms at each part, and FIG. 3 is a conventional FIG. 4 is a circuit diagram showing an example of a clamp circuit, and is a schematic waveform diagram for explaining the problem. Explanation of symbols 1...clamp circuit, 3...coupling capacitor, 9...switch means, 12...voltage detection means a1 time photograph diagram circuit l! l (Conventional example) Figure 3 mQ waveform diagram to clarify problem Stt

Claims (1)

[Claims] A clamp circuit in which a coupling capacitor is provided on a signal path to which an input signal is supplied, comprising: voltage detection means for detecting a change in the DC component of the input signal; and a signal received from the voltage detection means. switch means for temporarily forming a discharging path for the coupling capacitor, and when the voltage detection means detects that the DC level of the input signal has risen above a predetermined value, the coupling capacitor is discharged by the switch means and the clamp potential is increased. A clamp circuit characterized by being stabilized at a predetermined reference value.