JPH01279310A - Holding device - Google Patents

Abstract

PURPOSE:To stabilize the holding voltage even with a small-capacity capacitor for a differential amplifier having a current mirror circuit included in an active load circuit by controlling the current supplied to a capacitor which supplies the holding voltage. CONSTITUTION:The buffers of both TR Q12 and Q13 are added to a current mirror circuit 2 serving as an active load circuit of a differential amplifier 1. As a result, the Early's effect due to the potential difference between the bases and the collectors of both TR Q8 and Q9 is reduced. The collector and the base of the TR Q8 serve as the input 7 of the circuit 2 with the collector of the TR Q9 serving as the output 8 of the circuit 2 respectively. The output voltage of a capacitor C is applied to the base of a TR Q14 from the emitter of a TR Q10 serving as a 1st TR of an emitter follower and then taken out of the emitter of the TR Q14 in the form of voltage. A TR Q11 cascaded to the collector of the TR Q10 also serves as a TR which detects the base current. Then the base of the TR Q11 is connected to the collector of the TR Q8 serving as the input 7 of the circuit 2. The base current of the TR Q10 flows into the capacitor C from the collector of the TR Q9 for compensation of the base current.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a holding device used for holding voltage levels.

BACKGROUND OF THE INVENTION FIG. 2 shows a conventional hold device used in a killer detection circuit of a television receiver. Transistor Ql
, Q2, a color signal [see FIG. 3(a)] is applied to the base of transistor Q8. Q4. Q5. Q6
A color subcarrier signal synchronized with the burst signal B of the color signal is added to the base of the color signal. Transistor Q7 and resistor R
1 is a differential amplifier 1 formed by transistors Q1 to Q6.
A burst gate pulse BP as shown in FIG. 8(b), which is inverted to the "H" level only during the burst signal period, is applied to the base of the transistor Q7 to make the current source conductive. Transistors Q8-Q during the burst signal period
The chroma burst of the color subcarrier signal applied to the base of transistors Q1 and Q2 and the chroma burst of the color signals applied to the bases of transistors Ql and Q2 are detected, and a burst detection current flows as shown in FIG. 8(c).

The collectors of transistors QB, Q5 and transistor Q4. A detection signal of opposite phase appears at the collector of Q6.

The collectors of transistors Q8 and Q5 are connected to the input of a current mirror circuit 2 as an active load circuit. Current mirror circuit 2 includes transistor Q8. Q9, transistors Q4. The collector of Q6 is connected to the collector of the transistor Q9 as the output of the current mirror circuit 2.

Due to the action of current mirror circuit 2, transistor Q
A current having the same value as that of the transistors QB and Q5 flows through the collector of the transistor 9, and charges the capacitor C connected to the output of the differential amplifier 1. Since transistor Q7 is in an off state outside the burst gate period, transistor Q4,
Q6 and Q9 are turned off, and the charging/discharging current of capacitor C disappears, resulting in a hold state. The voltage held by the capacitor C is taken out from its emitter via a terminal 8 via a transistor QIO connected as an emitter follower.

Conventionally, this circuit is constructed of an integrated circuit, and in FIG. 2, 4 represents the inside of the integrated circuit.

Conventionally, the capacitor C is constructed by attaching a large capacitance externally to the integrated circuit via an external connection terminal 5.

Problems to be solved by integrated circuit technology In recent years, the development of integrated circuit technology has been remarkable.
is now being incorporated into the internal part 4 of the integrated circuit. In the case of external connection, the capacitor C should be several μF.
A large-capacity device was used, but if it is built-in, only a small capacity device of several + PF can be used at most.

Therefore, the base current of the transistor QIO, which has not been a problem in the past, becomes a problem. - In order to compensate for the drop in the holding voltage of the capacitor C due to the base current of the transistor QIO, another current mirror circuit 6 as shown in FIG. 4 is conventionally provided in addition to the current mirror circuit 2. In FIG. 4, a transistor Qll is connected in cascade to the transistor QIO in order to compensate the base current of the transistor QIO, and the base current of the transistor Qll is changed to the transistor Q1.
2, that is, the input of the current mirror circuit 6, and charges the capacitor C with a current equal to the base current of the transistor Qll from the output of the current mirror circuit 6, thereby compensating the base current of the transistor QIO. In this current mirror circuit 6, the transistor Q12,
In order to eliminate current errors due to the Early effect of Q18, a transistor Q14 is provided to keep the voltage between the collector and base of the transistor Q12 constant.

Providing the current mirror circuit that acts as an active load for the differential amplifier 1 and the current mirror circuit 6 for compensating the base current of the transistor QIO in this manner is not preferable because it increases the mounting area in an integrated circuit.

In particular, PNPs used in current mirror circuits 2 and 6
Since transistors have a large mounting area, using a large number of PNP (PNP) transistors increases the chip area.
There is a problem in that it leads to an increase in costs.

SUMMARY OF THE INVENTION An object of the present invention is to provide a hold device that can compensate for the base current of a transistor that detects the voltage of a capacitor with a circuit configuration that is simpler than the conventional one.

Means for Solving the Problems The hold device of the present invention includes a differential amplifier having a current mirror circuit as an active load circuit, a switch means for turning on/off the current of the current source of the differential amplifier, and a switch means for turning on/off the current of the current source of the differential amplifier. a capacitor connected to the output and holding the output voltage of the differential amplifier while the switch means is off; a first transistor acting as an emitter follower for taking out the voltage of the capacitor; and a first transistor connected in cascade to the first transistor. A second transistor is provided, and the base of the second transistor is connected to the input of the current mirror circuit.

According to this configuration, a compensation current corresponding to the base current of the first transistor flows from the current mirror circuit for the active load of the differential amplifier to the capacitor.

Example An embodiment of the present invention will be described below with reference to FIG.

Components having the same functions as in FIGS. 2 and 4 showing the conventional example will be described with the same reference numerals.

FIG. 1 shows a holding device of the invention. Here, a case is shown in which the present invention is employed in the killer detection circuit of a television receiver, which is the same as the conventional example. The current mirror circuit 2 as an active load circuit of the differential amplifier 1 includes the transistor Q8. In order to reduce the Early effect due to the potential difference between the base and collector of Q9, the transistor Q12
．． The configuration includes a buffer Q11, and the collector-base voltage of transistor Q12 is always equal to the collector-base voltage of transistor QLI, so
The early effect will no longer be affected. The collector and base of the transistor Q8 serve as the input port of the current mirror circuit 2, and the collector of the transistor Q9 serves as the output 8. In FIG. 1, the output voltage of the capacitor C is applied from the emitter of the transistor QIO as the first transistor of the emitter follower to the base of the transistor Q14, and taken out as a voltage from the emitter of the transistor Q14.

A second transistor Qll connected in cascade to the collector of the transistor QIO is a base current detection transistor, and the base of the transistor Qll is connected to the collector of the transistor Q8 which is the input cap of the current mirror circuit 2. has been done. As a result, the base current of the transistor QIO flows into the capacitor C from the collector of the transistor Q9, and the base current can be compensated.
The hold voltage can be maintained stably even in the case of small capacitance.

Moreover, PNP type transistors Q8, Q9, Q
12. Since only Qsa is required, PNP is not required as before.
Transistors of type Q8, Q9, Q12, Q
The number of required PNP transistors can be reduced compared to a circuit that requires IB and Q14, and the mounting area can also be reduced compared to the conventional circuit.

Note that the switch means in the claims is constituted by a transistor Q7 in FIG.

Effects of the Invention As described above, according to the present invention, there is provided a differential amplifier having a current mirror circuit as an active load circuit, a switch means for turning on/off the current of the current source of the differential amplifier, and a switch means for turning on/off the current of the current source of the differential amplifier. a first capacitor that is connected and holds the output voltage of the differential amplifier while the switch means is off; and a first emitter follower that takes out the voltage of this capacitor.
and a second transistor connected in cascade to the first transistor, and the base of the second transistor was connected to the input of the current mirror circuit, so that the base current flowing through the first transistor is transferred to the second transistor. It is detected as the base current of the transistor, and a large amount of current flows from the current mirror circuit to the capacitor by the detected current, which prevents the holding voltage from decreasing due to the base current of the first transistor, and only requires a small capacitance capacitor. Even in the case of an integrated circuit with a built-in capacitor that cannot be used, the hold voltage can be stably maintained without increasing the mounting area as in the conventional case, which is particularly effective.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the hold device of the present invention, Fig. 2 is a block diagram of a conventional hold device with an external capacitor, Fig. 8 is a waveform diagram of the main part of Fig. 2, and Fig. 4 is a block diagram of a conventional hold device with an external capacitor. FIG. 2 is a configuration diagram of main parts of a conventional hold device when built-in. 1...Differential amplifier, 2...Current mirror circuit, 7
...Input of current mirror circuit 2, 8...Output of current mirror circuit 2, Q7...Transistor [switch means], QIO, Qll...Transistor [first
．． second transistor], q... capacitor.

Claims (1)

[Claims] 1. A differential amplifier having a current mirror circuit as an active load circuit, a switch means for turning on and off the current of the current source of the differential amplifier, and a switch means connected to the output of the differential amplifier during a period when the switch means is off. A capacitor that holds the output voltage of the differential amplifier, a first transistor that works as an emitter follower that takes out the voltage of this capacitor, and a second transistor that is cascade-connected to the first transistor are provided. A hold device whose base is connected to the input of the current mirror circuit.