JPH04273707A - Level shift circuit - Google Patents

Abstract

PURPOSE:To obtain an AC coupling circuit whose level operated by a low source voltage is shifted. CONSTITUTION:The emitter of a transistor Q4 is connected to the base of a transistor Q5, and a resistor R3 is connected between the base and the emitter of the transistor Q5. The emitter of the transistor Q5 is grounded via a current source I1, and the collector is connected to the emitter of a transistor Q6. The collector of the transistor Q6 is connected to a power source Vcc, and also, a resister R4 is connected between the base and the collector of it. A capacitor C2 is connected between the base of the transistor Q6 and the emitter of the transistor Q5. The base of the transistor Q4 is set as a voltage input terminal, and the emitter of the transistor Q6 as a signal voltage output terminal.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a level shift circuit suitable for use in a chroma signal processing section of a TV receiver.

0003

2. Description of the Related Art FIG. 3 shows a conventional level shift AC coupling circuit. The emitter of transistor Q1 is connected to the base of transistor Q2 and one end of resistor R1, and the emitter of transistor Q2 is connected to the other end of resistor R1, one end of capacitor C1, and transistor Q3.
Connect to each collector. The base of transistor Q3 is connected to the other end of capacitor C1 and resistor R2, and the emitter of transistor Q3 is connected to GND via current source IO. The other end of resistor R2 is connected to constant voltage source Vref. The base of transistor Q1 is used as an input terminal, and the emitter of transistor Q3 is used as an output terminal.

In this circuit, transistors Q1 and Q2
is the Darlington configuration buffer, and the capacitor C1
and resistor R2 become a high pass filter (HPF). The HPF output voltage signal at the connection point between resistor R2 and capacitor C1 is outputted by the emitter follower of transistor Q3. I is the bias current of transistor Q3
0 flows from the collector of transistor Q3 to the emitter of transistor Q2, and also serves as a bias current for transistor Q2.

[0005] With this arrangement, only the alternating current signal can be taken out from the emitter of the transistor Q3, and since the bias current is shared, power consumption can also be reduced.

[0006] In recent years, there has been an increasing demand for circuits such as liquid crystal TVs that operate even when the power source is at a low voltage. For example, when using a 5V power supply, the minimum operating voltage is set to around 3V, and operation is guaranteed up to 60% even when powered by batteries. To operate at 3V, the number of transistors between Vcc and GND must be three. The standard base-emitter voltage (VF) during forward bias is 0.
．． If it is 75V, the power supply will be 2.3V in the case of 3 pieces.
It works, but when there are 4 of them, the dynamic range of the signal cannot be achieved with a 3V power supply.

Since the circuit shown in FIG. 3 includes three transistors between Vcc and GND, it can operate even with a 3V power supply, but it is not possible to connect an NPN transistor differential amplifier to the next stage. If an NPN transistor is connected to the next stage, there will be four transistors including the emitter, and the dynamic range of the signal will not be maintained as described above. Therefore, the circuit of FIG. 3 cannot actually use a 3V power supply, and has the disadvantage that it is unsuitable for low voltage circuits such as liquid crystal TVs.

[0008]

[Problems to be Solved by the Invention] As mentioned above, in the conventional circuit, although it is driven at a low power supply voltage, the
When a differential amplifier of PN transistors is connected, a dynamic range cannot be obtained due to the VF component of this transistor, making it unsuitable for low power supply voltages.

SUMMARY OF THE INVENTION The present invention provides a level-shifting AC coupling that operates at low supply voltages.

[Configuration of the invention]

[0011]

SUMMARY OF THE INVENTION The present invention eliminates the conventional AC coupling from high voltage to low voltage, and conversely couples AC from low voltage to high voltage.

0012

[Operation] By doing this, the base-emitter voltage of the Darlington buffer can be overlapped with the base-emitter voltage of the subsequent buffer, so it can be used even when the power supply is low. Furthermore, since the level is shifted to a higher voltage, even if an NPN transistor differential amplifier is used in the next stage, it can be used at a low power supply voltage.

[0013]

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

FIG. 1 shows an embodiment of the present invention. In Figure 1, the emitter of transistor Q4 and Q5
A resistor R3 is connected between the base and emitter of the transistor Q5. Transistor Q5
The emitter of Q6 is connected to GND via current source I1, and the collector is connected to the emitter of Q6. transistor Q
The collector of 6 is connected to the power supply Vcc. A resistor R4 is connected between the base and collector of the transistor Q6, and a capacitor C2 is connected to the emitter of the transistor Q5 and the base of Q6. The base of transistor Q4 is used as a voltage input terminal, and the emitter of transistor Q6 is used as a signal voltage output terminal.

Transistors Q4 and Q5 are emitter followers in a Darlington configuration, and resistor R3 provides bias current for transistor Q4. Only the alternating current component of the signal appearing at the emitter of transistor Q5 is transmitted to the base of transistor Q6 through the HPF of capacitor C2 and resistor R4. The signal based on the transistor Q6 is output by the buffer of the transistor Q6.

[0016] In this way, the signal at the emitter of transistor Q5 can be raised to the potential of power supply Vcc, so that the level can be shifted to a higher voltage. The output potential is Vcc-VF, and even if an NPN differential amplifier is connected to the next stage, there are only two VFs, and it can operate satisfactorily below 3V (can operate up to 1.8V).

Furthermore, as in the prior art, since the bias current I1 is shared by the transistors Q5 and Q6, current consumption does not increase.

In the circuit of FIG. 1, the power supply to which the resistor R4 is connected is set to Vcc, but FIG. 2 shows another embodiment of the present invention in which the power supply is set to Vcc-VF. transistor Q7
supplies the bias of Vcc-VF. In this case, the input voltage Vin must also be Vcc-VF so that the transistor Q5 is not saturated. Since the DC potential of the output voltage Vout is Vcc-2VF, the next stage is an NPN
When a transistor differential amplifier is installed, the differential amplifier
The dynamic range of the collector is wider than that of the circuit shown in FIG.

Furthermore, recently, when a circuit is built into an IC, two power supplies Vcc are provided, one for AC and one for DC, to reduce leakage of AC signals. Transistor Q7 as shown in Figure 2
If the base of the transistor Q7 and Q4 is connected to the DC Vcc and the collectors of the transistors Q7 and Q4 are connected to the AC Vcc, an AC signal will not be applied to the bias voltage of the emitter of the transistor Q7, and the transistor will operate well.

There are various other methods of applying the bias voltage to R4, but their explanation will be omitted.

[0021]

[Effects of the Invention] As explained above, according to the present invention,
It is possible to realize a level shift AC coupling circuit that can operate on a power supply of 3V or less while sharing a bias current as in the conventional case.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the invention.

FIG. 3 is a conventional circuit diagram.

[Explanation of symbols]

Q5, Q6......transistor R3, R4……Resistance I1......Power supply C2……Capacitor

Claims (3)

[Claims] 1. A first emitter follower, providing a signal from the emitter of the first emitter follower to the second emitter follower, and from the emitter of the first emitter follower to the base of the second emitter follower. an AC coupling circuit and a bias current source, connecting the collector of the first emitter follower and the emitter of the second emitter follower, sharing part or all of the bias current; A level shift circuit characterized in that the base of the follower is used as an input terminal, and the emitter of the second emitter follower is used as an output terminal. 2. The level shift circuit according to claim 1, wherein at least one of the first emitter follower and the second emitter follower includes a Darlington-connected transistor. 3. The level shift circuit according to claim 1, wherein the second emitter follower is supplied with a bias lower than the power supply voltage.