JPH0474881B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to an amplifier speed-up circuit used in a drive circuit for a viewfinder cathode ray tube.

2. Description of the Related Art In a drive circuit for a cathode ray tube for a viewfinder in a video camera, there is a demand for a faster start of operation to meet the demand for displaying an image at the same time as the power switch is turned on.

FIG. 2 shows a conventional differential amplifier and its feedback system used in this type of drive circuit. That is, the signal to be amplified is applied to the non-inverting input terminal (+) of the differential amplifier 2, the load resistor 4 is connected to the output terminal, and the connection between the inverting input terminal (-) and the output terminal is connected to the load resistor 4. There is a resistance 6 in between,
A feedback circuit consisting of 8, 10 and capacitor 12 is added.

A driving power source 14 is connected to the differential amplifier 2 via a power switch 16.

According to such a configuration, when the power switch 16 is turned on, the differential amplifier 2 becomes operational;
When the power switch 16 is turned on, a charging current flows through the capacitor 12, and until the charging potential of the capacitor 12 reaches a predetermined potential, normal amplification operation cannot be obtained. That is, in this type of differential amplifier 2, a non-inverting input terminal (+) and an inverting input terminal (-)
capacitor 12 until they reach the same DC potential.
is charged.

Problems to be Solved by the Invention In the differential amplifier 2 having a feedback circuit including such a capacitor 12, the low frequency limit of the amplification degree is determined by the value C of the capacitance C of the capacitor 12 and the resistance value R of the resistor 6.
Since it is determined by a time constant consisting of R, the capacitance C and the resistance value R are set large. Note that the resistance values of the resistors 8 and 10 are set smaller than the resistance value R of the resistor 6.

Therefore, when the power switch 16 is turned on, there is a drawback that it takes a considerable time for the potential of the inverting input terminal (-) of the differential amplifier 2 to rise. Therefore, it is not suitable for a drive circuit for a cathode ray tube for a viewfinder, etc., which requires a high speed start of operation.

Therefore, the present invention provides an amplifier speed-up circuit that forcibly charges a capacitor installed in the feedback circuit of an amplifier used in a drive circuit of a cathode ray tube for a finder in accordance with the power supply, thereby increasing the startup speed. purpose.

Means for Solving the Problems Namely, the amplifier speed-up circuit of the present invention includes an amplifier (differential amplifier 2) installed in a drive circuit of a cathode ray tube for a finder and having a feedback circuit including a capacitor 12, and this amplifier. The base is connected to the positive phase input terminal of the amplifier, and the first constant current source 34 is connected in series to the emitter via an offset setting resistor (resistance 30), so that the level of the positive phase input terminal of the amplifier is connected. A first transistor 24 that takes out a corresponding output, and a base connected to the negative phase input terminal of the amplifier, and a second constant current source 36 connected in series to the emitter of the first transistor 24 and the negative phase input terminal of the amplifier. a second transistor 26 that takes out an output according to the level of the second transistor; a comparator 22 that compares the outputs taken out from the first transistor and the second transistor and generates an output according to the magnitude relationship; When the level on the positive phase input terminal side is higher than the level on the negative phase input terminal side, conduction occurs due to the output of the comparator, and at the time of conduction, a closed circuit is formed between the power supply 14 and the capacitor, and the It is characterized by comprising a third transistor 28 for flowing a charging current into the capacitor, so that the capacitor is rapidly charged when the amplifier starts operating.

Effect: In this invention, a charging current is supplied to a capacitor installed in a feedback circuit of an amplifier in response to power supply to rapidly charge the capacitor, and at the same time detecting that the charging potential has reached a predetermined potential. By canceling the charging current, it is possible to speed up the start of operation.

Embodiments Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

FIG. 1 shows an embodiment of a speed-up circuit for an amplifier according to the present invention, and the same parts as those in the circuit shown in FIG. 2 are given the same reference numerals.

In FIG. 1, in response to the power switch 16 being closed, the quick charging circuit 20 has a non-inverting input terminal (+) for applying a positive phase input to the differential amplifier 2 and an inverting input terminal (+) for applying a negative phase input to the differential amplifier 2. The potential with the input terminal (-) is compared, and based on the level difference, a charging current is supplied to the capacitor 12 of the feedback circuit of the differential amplifier 2, and the non-inverting input terminal (+) and the inverting input terminal of the differential amplifier 2 are The charging current is configured to be released when the potential of the terminal (-) reaches the same potential or a potential difference within a predetermined range.

The quick charging circuit 20 includes a comparator 22, a first transistor 24, a second transistor 26, and a third transistor 24.
transistor 28, resistors 30, 32, first and second constant current sources 34, 36 are installed, and the first transistor 24, resistor 30 and constant current source 34 are installed.
The circuit consisting of the second transistor 26 and the constant current source 36 respectively constitute an impedance conversion circuit. That is, the potential at the non-inverting input terminal (+) of the differential amplifier 2 is applied to the base of the transistor 24, the potential at the inverting input terminal (-) of the differential amplifier 2 is applied to the base of the transistor 26, and each transistor 24 ,26,
A direct current flows according to the applied potential.

The transistors 24 and 26 constitute a differential amplifier, and a constant current source 34 is connected to the emitter side of the transistor 24 via a resistor 30 for offset setting, and a constant current source 34 is connected to the emitter side of the transistor 26. 36 are connected. The potential between the resistor 30 connected to the emitter of each transistor 24 and the constant current source 34 is applied to the non-inverting input terminal (+) of the comparator 22, and the potential at the emitter of the transistor 26 is applied to the inverting input terminal (+) of the comparator 22. These comparators 22 generate high level outputs until the applied potentials are the same or within a certain range, and the outputs are applied to the resistor 32.
It is added to the base of a transistor 28 as a switching element via a.

The transistor 28 is for supplying charging current to the capacitor 12 and is connected between the positive line of the power supply 14 and the non-grounded terminal of the capacitor 12.

The operation will be explained based on the above configuration.

When the power switch 16 is closed, the power source 14 is supplied to the differential amplifier 2 and the quick charging circuit 20. At this time, the potential at the non-inverting input terminal (+) of the differential amplifier 2 rises rapidly and reaches a predetermined DC potential, but the potential at the inverting input terminal (-) is left to the charging of the capacitor 12; Charging will be delayed by a predetermined time constant as explained in FIG.

That is, at the time when the power switch 16 is turned on, a considerable potential difference occurs between the non-inverting input terminal (+) and the inverting input terminal (-) of the differential amplifier 2. 26 base currents will be provided.

Therefore, the potential on the base side of transistors 24 and 26 is the same as that on the transistor 24 side.
A shift due to the emitter voltage and the voltage drop across the resistor 30; on the transistor 26 side, its base
It is shifted by the emitter voltage and applied to the comparator 22. As a result, the comparator 22 generates an inverted output, and the transistor 28 becomes conductive, so that a charging current is applied to the capacitor 12 through the transistor 28, and the capacitor 12 is rapidly charged. As a result, the DC potential at the inverting input terminal (-) of the differential amplifier 2 increases toward the steady potential.

When the terminal voltage of the capacitor 12 increases due to charging of the capacitor 12, the emitter potential of the transistor 28 increases, and when the potential exceeds the threshold level at which the transistor 28 becomes conductive, the transistor 28 becomes non-conductive, and the capacitor 12
charging is released.

Since such an operation is performed at the moment when the power switch 16 is closed, the capacitor 12 is charged instantly, and the differential amplifier 2 immediately enters a steady state, thereby speeding up the start of operation.

Therefore, the present invention can be applied to a circuit that requires speeding up the start of operation, for example, a drive circuit for a cathode ray tube. In this case, the load resistor 4 is removed and the resistor 8 is connected to the deflection coil of the cathode ray tube. shall be replaced.

In the case of this embodiment, the differential amplifier composed of transistors 24 and 26 has an offset set by the resistor 30, so that the non-inverting input terminal (+) and the inverting input terminal ( −)
When the potential of capacitor 1 reaches the specified range, capacitor 1
2 charging is released.

In the embodiment, the offset is set in the comparator 22 by the constant current value of the constant current source 34 and the value of the resistor 30, but the offset can also be set by level setting by resistance ratio division of a variable resistor or the like. It's okay.

Effects of the Invention As explained above, according to the present invention, the following effects can be obtained.

(a) When the power switch is closed, it is possible to shift to a steady state as quickly as possible, and it is possible to speed up the start of operation when the power is turned on for the drive circuit of the cathode ray tube for the viewfinder of the video camera.

(b) When configured with a semiconductor integrated circuit, there is no increase in the number of terminals and it can be provided at low cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a speed-up circuit for an amplifier according to the present invention, and FIG. 2 is a circuit diagram showing a configuration of a feedback system of a conventional differential amplifier. 2...Differential amplifier, 12...Capacitor, 14
... Drive power supply, 22 ... Comparator, 24 ... First
transistor, 26... second transistor,
28... Third transistor, 30... Resistor (offset setting resistor), 34... First constant current source,
36...Second constant current source.

Claims (1)

[Claims] 1. An amplifier installed in the drive circuit of a cathode ray tube for a finder and having a feedback circuit equipped with a capacitor, a base connected to the positive phase input terminal of this amplifier, and an offset setting resistor connected to the emitter. a first transistor to which a first constant current source is connected in series through the transistor to take out an output corresponding to the level of the positive phase input terminal of the amplifier; a first transistor having a base connected to the negative phase input terminal of the amplifier; , a second transistor whose emitter is connected in series with a second constant current source to take out an output according to the level of the negative phase input terminal of the amplifier; a comparator that compares the outputs of the input terminals and generates an output according to the magnitude relationship; and when the level on the positive phase input terminal side is higher than the level on the negative phase input terminal side, the output of the comparator conducts. , a third transistor forming a closed circuit between a power source and the capacitor when the transistor is conductive, and flowing a charging current into the capacitor, and rapidly charging the capacitor when the amplifier starts operating. An amplifier speed-up circuit featuring: