JPS63279607A - Level shift circuit - Google Patents

Abstract

PURPOSE:To attain multiple-stage connection of a directly coupled broad band and high gain amplifiers by synthesizing an output signal and a signal with only a low frequency component existed therein through a differential input circuit, thereby eliminating a DC bias voltage to be included in an output signal. CONSTITUTION:A signal outputted from an amplifier 12 includes a DC bias voltage. The signal is divided into one path through which the signal is fed to a transistor(TR) 14 and the other path through which a signal with only a low frequency component including the offset of the DC voltage extracted therefrom by a voltage follower circuit comprising an operational amplifier 19, and a low pass filter is inputted to a TR 15, and the signals of each path are synthesized by the differential input circuit comprising the TRs 14, 15. Since amplifiers are to be coupled directly, a capacitor inserted between amplifiers is not required and an amplifier circuit ensuring the gain from over a frequency sufficiently lower than a conventional low frequency is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Fields] The present invention relates to the output signal of a monolithic IC wideband high-interest amplifier, for example, a video amplifier using a single-ended remote amplifier circuit system. This relates to a circuit that shifts the level of the DC bias voltage included in the. Roughly speaking, the present invention enables direct multi-stage connection without reducing the dynamic range of such wideband high-gain amplifiers, and sufficiently widens the frequency range from several hertz in the low range to the frequency band required for use in the high range. The present invention relates to a level shift circuit including a wideband high gain amplifier secured to

[Conventional technology] In recent years, there has been a large demand for amplifiers in the tJHF band and microwave bands in the communication equipment field, CATV-related fields, TV tuners, and other consumer equipment fields. , sold by each ■C manufacturer. By using these ICs, it is possible to easily realize a broadband amplifier circuit with a high gain of 17. It is also more compact than discrete assembly, consumes less power, is relatively inexpensive, and operates reliably. Wideband high gain amplifier, such as
For example, a wideband video amplifier includes a DC bias voltage in its output signal, and its input/output impedance is set to 50 ohms for widening the band. For this reason, when trying to perform multi-stage connection by direct connection, the dynamic range may decrease, or in extreme cases, direct connection may become impossible. When operating wideband video amplifiers in multi-stage connection or in a 50-ohm system with impedance matching, capacitors are inserted at the input and output of these amplifiers to cut the DC bias voltage. In this case, the capacitance ω of the capacitor must be selected to a value that provides sufficiently low impedance at high frequencies. Also, in order to avoid damaging the high-frequency characteristics of the amplifier, small-sized chip capacitors and the like are used. For this reason, it is inevitable that a capacitor with a large capacity cannot be used. However, in the case of multi-stage connection using capacitor coupling, an amplifier is constructed that mainly handles high frequencies, and there is no gain at low frequencies, for example, frequencies on the order of several tens of hertz. Therefore, in order to operate such an amplifier while ensuring the gain 1q from the low frequency range as described above, a capacitor with a large capacity is required. However, when the capacitance of a capacitor is increased, the shape of the capacitor also becomes larger. If this happens, the frequency characteristics of the amplifier in the high range will be impaired. In order to connect such wideband high-interest (1q) amplifiers in multiple stages, which include a DC bias voltage in the output signal, coupling by capacitors is required.For this reason, even though it is a general-purpose wideband high-interest 1q amplifier, the frequency range of use, especially Since the amplifier had to be used with a limited range of low frequencies, it could not be used as an amplifier with sufficiently wide frequency characteristics.

This problem will be specifically explained below using FIGS. 4 and 5.

FIG. 4 shows a single-ended feedback amplifier circuit for high frequency amplification. In this case, the DC input voltage level is the sum of the pace-emitter voltage of transistor 93 and the voltage across resistor 94.

Further, the DC output voltage level is the power supply voltage V. The voltage is lower than 0 by the voltage valves at both ends of the resistor 95. As is clear from this, since there is a difference in the DC voltage level between the input terminal 11 and the output terminal 92, the output signal includes the DC bias voltage. The amplifier shown in FIG. 4 is designed to have an input and output impedance of 500 to enable operation at high frequencies. Therefore, even if these amplifiers are directly connected in multiple stages and subjected to a 50Ω load, an excessive operating current will flow due to the DC bias voltage included in the output signal. This results in a reduction in dynamic range. Furthermore, in extreme cases, the system could become inoperable, and this would lead to a situation where it could not be used in multi-stage connections.

In order to avoid such problems, multi-stage connections are made as shown in FIG. Input terminal 11 and output terminal 61
Since the DC voltage component at
6 and the input impedance of the amplifiers 103 and 105, the gain of signal components below the frequency determined by the input impedance of the amplifiers 103 and 105 is lost. In other words, this capacitor limits the usable frequency band, especially the lower limit frequency.

For this reason, it could only be used as a so-called AC amplifier.

[Problems to be Solved by the Invention] When trying to operate a high-band, high-gain amplifier whose output signal includes a DC bias voltage Iν by directly connecting it to the output signal, the operating point inside the amplifier becomes abnormal and the dynamic range deteriorates. There is a decrease in 4f! In extreme cases, there is a problem that the system becomes inoperable. Further, although capacitor coupling enables multi-stage connection, there is a problem in that the band is limited because of this.

[Means for solving the problem] A broadband high-interest signal whose output signal includes a DC bias voltage.
The reason why the amplifiers of ' and i cannot be directly connected in multiple stages is due to their DC bias voltage.

Therefore, the DC bias voltage is removed to bring the level of the DC bias voltage that is applied to the output signal to 0. For example, by obtaining an inverted signal of the DC bias voltage and combining this signal with the above DC bias voltage, or by extracting the low frequency component of the DC bias voltage and calculating the difference between this signal and the DC bias voltage. By synthesizing using an amplifier, the level of the DC bias voltage of the output signal of such an amplifier can be obtained. This provides a shift circuit.

[Function] The present invention differentiates the output signal of a broadband high-gain amplifier with a DC bias voltage and the signal obtained by passing the output signal through a voltage follower circuit and a low-pass filter circuit to reduce only low frequency components. It acts to remove the DC bias voltage contained in the output signal of the amplifier by combining the dynamic inputs.

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

In Fig. 1(a), 12 is a broadband high gain amplifier, 19 is an operational amplifier, 14.15 is a high-speed PNP transistor that constitutes a differential input circuit, and 16.17 is the operating point of the transistor. Resistance that determines the city. 13 is a constant current source of the differential input circuit. A low-pass filter 18 is connected via an operational amplifier 19 (voltage follower) and is used to extract only the low frequency components of the signal output from the amplifier 12.

The signal output from amplifier 12 includes a DC bias voltage. One path inputs this signal to the transistor 14, and the other path inputs the signal, in which only the low frequency component including the offset of the DC voltage is extracted by the voltage follower circuit of the operational amplifier 19 and the low-pass filter, to the transistor 15. A route can be created. The signals passing through each path are combined by a differential input circuit made up of transistors 14 and 15, and output to output terminals 21 and 2.
2, a signal with the DC bias voltage component canceled is output. Output terminal 21 has an in-phase signal, and output terminal 22
A signal is outputted which is a negative phase signal and has a low frequency characteristic that shows no amplification with respect to the frequency passed by the low-pass filter 18.

Figure 1(a) shows the circuit when the DC bias voltage is a positive voltage, while Figure 1(b) shows the circuit when the DC bias voltage is a negative voltage. Using 43 NPN transistors, it has the same effect as in FIG. 1(a).

Modified Example The circuit explained in FIGS. 1(a) and (b) uses a discrete differential input circuit, but FIGS. 2 and 3 show an example using an operational amplifier. This is explained below.

Part 1 In Figure 2, 12 is a broadband high gain amplifier, and 19 is a wideband high gain amplifier.
．． 60 is an operational amplifier, 53, 54, 57.58.59
is a resistor for setting the degree of amplification, and 18 is a low-pass filter. The signal output from the amplifier 12 contains a DC bias voltage, and this signal is directly input to the resistor 57 through one path and an inverting amplifier circuit with a gain of 1 by an operational amplifier 19 to the low-pass filter 18. and an inverted signal via the other path, which is input to the resistor 58 via the signal line. Due to the signals of these two paths, the output signal of the amplifier 12 becomes a signal in which the DC bias voltage component is canceled by the addition circuit of the operational amplifier 60, and is obtained at the output terminal 61. Although the operational amplifier 19 may be of a low band type, the operational amplifier 60 needs to be of a sufficiently wide band type.

Part 2: In Figure 3, the adder circuit in Figure 2 is replaced with a subtracter circuit, and the differential input circuit shown in Figures 1(a) and (b) is replaced with an operational amplifier. The same effect as in the case of FIG. 2 can be obtained. Resistor 5 that determines the degree of amplification
The ratio of 7 and 59 is set equal to the ratio of resistors 76 and 77.

It is clear that the operational amplifier 19 and the low-pass filter 18 in FIGS. 1(a), 1(b) to 3 may be integrated.

As explained in the above examples, the present invention can be implemented in the following embodiments.

(1) A level shift circuit in which the low frequency means according to the claims is a circuit including a low-pass filter circuit.

(2. A level shift circuit in which the synthesis amplification means according to the claims is constituted by a circuit including a differential input amplification circuit using transistors.

(3) A level shift circuit in which the synthesis amplification means according to the claims is comprised of a circuit including a subtraction circuit using an operational amplifier.

(4) A level shift circuit in which the synthesis amplification means according to the claims is comprised of a circuit including an addition circuit using an operational amplifier.

[Effects of the Invention] As is clear from the above explanation, the present invention provides broadband high interest rate 1
Since the DC bias voltage contained in the output signal of the amplifier q is level-shifted to, for example, O Amplification becomes possible. Also,
Since direct connection is possible, there is no need for a capacitor to be inserted between each amplifier, and an amplifier circuit that secures gain from a low frequency sufficiently lower than the conventional low frequency can be provided. According to an embodiment of the present invention, by using a relatively inexpensive and small-sized wideband high-gain amplifier integrated into an IC, gain is secured from the low frequency range, and the high-frequency characteristics of the integrated integrated circuit amplifier used are impaired. The effect is remarkable when trying to realize an amplifier that does not

[Brief explanation of drawings]

FIGS. 1, 2, and 3 are circuit diagrams showing embodiments of the present invention, and FIGS. 4 and 5 are circuit diagrams for explaining a conventional example. 11...Input terminal 12...Amplifier 13...
- Constant current source 14.15... Transistor 16.17... Resistor 18... Low pass filter 19... Operational amplifier 21.22... Output terminal 34.35... Transistor 53.54, 57, 58.59... Resistor 60... Operational amplifier 61... Output terminal 76.77... Resistor 92... Output terminal 93... Transistor
94.95...Resistance 102.104,106...
Capacitor 103.105...Amplifier.

Claims (1)

[Scope of Claims] Amplifying means for amplifying from direct current to high frequency; low frequency means for obtaining a low frequency component containing direct current from the output signal of the amplifying means; the low frequency signal containing direct current and the amplification. and a synthesis amplification means for shifting the level of the output signal of the amplification means in a low frequency region including the direct current by synthesizing the output signal of the amplification means.