JPS6266706A - Signal coupling circuit - Google Patents

Abstract

PURPOSE:To obtain a distortionless signal coupling circuit by constituting the output circuit of the first circuit block as an emitter follower circuit and constituting the input circuit of the second circuit block as a grounded-base circuit and connecting both circuits with an impedance circuit. CONSTITUTION:An emitter follower circuit 110 is adopted as the output circuit of the first circuit block 100, and a terminal pin 111 is connected to an input terminal pin 210 of a circuit block 200 through an impedance circuit 300 consisting of a capacity 301 for attaining AC coupling and a resistance 302 which sets the input impedance. The circuit block 200 consists of a grounded- base amplifier and converts the voltage signal supplied to the input resistance 302 to a current to lead out a current output signal. Transistors Q11 and 12 of circuit blocks 100 and 200 constitute a differential amplifier; and therefore, even if distortion is generated in the output signal, distortion of even order is not generated because the variance of gm is symmetrical with an operating point as the center.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a signal coupling circuit, and is a circuit that suppresses distortion in a transmitted signal due to circuit coupling.

[Technical background of the invention]

When integrated circuit blocks are connected and a signal is transmitted from one circuit block to another, the distortion component of the signal may increase due to the circuit coupling.

An example of this type is a connection between an FM modulation circuit block and a recording circuit block of a video tape recorder. In a video tape recorder, a luminance signal separated from a chrominance signal is FM-modulated and then supplied to a video head system as a current output via a recording amplifier circuit.

Here, the recording amplifier circuit handles a large current to perform constant current recording, and is usually disposed near the cylinder. On the other hand, the FM modulation circuit is usually arranged at a location apart from the recording circuit, and its output is a voltage output.

Therefore, a coupling circuit with voltage-current conversion is required between the FM modulation circuit and the recording amplifier circuit.

FIG. 5 shows a conventional coupling circuit of this type. Generally, the output of the y modulation circuit is derived through an emitter follower circuit 11 having a transistor Q1 and having a low output impedance. The voltage output (FM signal) is supplied to the input terminal pin P2 on the recording amplifier circuit side via the output terminal pin P1 and the coupled impedance circuit 12. The signal of this input terminal pin P2 is supplied to a current mirror circuit 15 consisting of a current source 13, a diode 14, and a transistor Q2, and is converted into a current output. Although low distortion is required, the above-described circuit coupling poses a problem of distortion due to nonlinearity of the transistor.

That is, the 1m characteristic of the emitter follower circuit is shown in FIG. 6 and explained as follows. Jm (=Imm) corresponding to point A in the diagram of the input signal and 1m corresponding to point B
(gmb), on the output side, due to the nonlinearity of the transistor, #ma "r Jmb. Therefore, the relationship between points A and B on the input side is a linear relationship on the output side. This will not result in an increase in distortion.

In other words, the fact that Io changes asymmetrically with respect to an input signal that changes symmetrically around the operating point means that even-order distortion occurs, and as a result, further distortion is added to the output signal. Particularly in the recording circuit system of a video tape recorder, even order distortion has an adverse effect on the signal, so there is a need for a means that does not cause this type of distortion.

[Purpose of the invention]

This invention was made to deal with the above circumstances,
It is an object of the present invention to provide a signal coupling circuit that prevents distortion from occurring in signal transmission involving voltage-current conversion.

[Summary of the invention]

In the present invention, for example, as shown in FIG. 1, the output circuit of the first circuit block 100 is an emitter follower circuit,
The impedance circuit 300 is configured such that the input circuit of the second circuit block 200 is a pace ground circuit, and the transistors Qll and Q12 of these circuits form a differential amplifier circuit.
The above purpose is achieved by connecting the two.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows one embodiment of the present invention, in which a first circuit block 100 is, for example, a first integrated circuit, and a second circuit block 200 is a second integrated circuit.

As the output circuit of the circuit block 100, EMI.

A follower circuit 110 is employed. That is, the signal ν
i is supplied to the pace of transistor Qll, whose output is directed out of its emitter and to the output terminal pin of circuit block 100. The collector of transistor Qll is connected to the power supply line 112
The emitter is connected to a ground line 114 via a current source 113.

The terminal pin 111 has a capacitor 301 for obtaining AC coupling,
It is connected to the input terminal pin 210 of the circuit block 200 via an impedance circuit SOO consisting of a resistor 302 that sets the input impedance.

The input circuit 21 of the circuit block 200 has a current source 212.
, transistor Q12, and bias source 214;
12 has the same polarity as the transistor Qll. That is, the emitter of transistor Q12 is connected to ground line 213 via current source 212, the collector is connected to current output line 215, and the bias source 214 is connected to the transistor Q12.
is connected. The input signal from terminal pin 210 is then provided to the emitter of transistor Q12.

Therefore, this common base amplifier circuit converts the voltage signal supplied to the input resistor 302 into a current output signal (
t ) can be led to the collector of transistor Q12.

Here, in this invention, circuit blocks 100 and 20
Looking at the connection manner of transistors Qll and Q12 used in the output circuit and input circuit of 0, it can be seen that they form a differential amplifier.

The 1m characteristic of a differential amplifier is as shown in Figure 2, and if the input signal changes symmetrically around the operating point, then! ! , also changes symmetrically. In other words, the input signal diagram A
gm (gma) corresponding to the point and Jam (gmb) corresponding to the illustrated point B are equally Fma = &mb
becomes. This means that although distortion occurs in the output signal, even distortion does not occur because the change in pm is symmetrical about the operating point. This realizes a coupling circuit that performs voltage-current conversion that can reduce even-order distortion. Therefore, it is effective for recording circuit systems such as video tape recorders that require reduction of even-order distortion.

FIG. 3 shows another embodiment of the present invention], and this circuit replaces the current source 113 and 212 in the embodiment of FIG.
This is an example in which they are replaced with 0 and 220, respectively. Therefore, since the basic operation is the same as that in FIG. 1, the same reference numerals are given to the same functional parts and the explanation thereof will be omitted.

FIG. 4 shows yet another embodiment in which the voltage-current conversion resistor 302 shown in FIGS. 1 and 3 is replaced with a filter having an arbitrary transfer function. Therefore, in this case, the output signal can be set to have an arbitrary frequency characteristic by the transfer function of the filter 310.

〔Effect of the invention〕

As explained above, the present invention applies to the g of a transistor when performing signal transmission involving voltage-to-current conversion. It is possible to provide a signal coupling circuit that can eliminate distortion caused by the characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the invention, FIG. 2 is a diagram showing characteristics of the circuit shown in FIG. 1, and FIG. 3 is a diagram showing characteristics of the circuit shown in FIG. FIG. 4 is a circuit diagram showing another embodiment of the present invention, FIG. 5 is a diagram showing a conventional signal coupling circuit, and FIG. 6 is a diagram showing characteristics of the circuit shown in FIG. 100.200...1st. second circuit block, 30
0...Coupling circuit, Qll, Q12...Transistor @ Applicant's agent Patent attorney Takehiko Suzue Figure 3 Figure 4

Claims (4)

[Claims] (1) A first circuit block having an emitter follower circuit as a signal output circuit, and a common base circuit using a second transistor having the same polarity as the first transistor constituting the emitter follower circuit as a signal input circuit. and an impedance circuit connecting the emitters of the first and second transistors so that the transistors form a differential amplifier circuit. (2) The first and second circuit blocks are independent integrated circuit blocks, and the impedance circuit is connected to each integrated circuit block via a terminal pin. The signal coupling circuit according to item 1. (3) The signal coupling circuit according to claim 1, wherein the impedance circuit has filter characteristics. (4) The signal coupling circuit according to claim 1, wherein the first circuit block is an FM modulation circuit, and the second circuit block is a recording amplifier circuit.