JPS6051809B2 - amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to amplifiers, and particularly to a relay amplifier used in a cable television system or the like.

As shown by the solid line 1 in Figure 1, the coaxial cable has Vf (f:
It has an attenuation characteristic that is proportional to the transmission frequency. Therefore, in order to make the output level constant at each frequency as shown by the dotted line 2, the relay amplifier as described above is provided with a negative feedback circuit including a reactance element, as shown by the dotted line 3.
As shown in , there was a device with a frequency characteristic that was inversely proportional to /' f. However, although the coaxial cable is exaggerated in the figure, the attenuation characteristics change with temperature changes, as shown by solid line 4. For example, if the temperature changes and the attenuation of the coaxial cable increases at frequency f, then if we simply increase the gain to compensate for this, we get
Since the relay amplifier has the same amount of feedback, for example, the frequency characteristic is obtained by moving the dotted line 3 in parallel, as shown by the dotted line 5,
The output level is not constant at each frequency level, as shown by a two-dot chain line 6. Therefore, by changing the amount of feedback, the dotted line 7
As for the frequency characteristics shown by , it is necessary to make the output level constant at each frequency as shown by the dashed line 2. Conventionally, for this purpose, a variable resistor was provided in the negative feedback circuit to adjust the amount of feedback, and a gain adjustment circuit was used to adjust the gain. However, it is troublesome to adjust the gain and feedback amount every time the temperature changes. j An object of the present invention is to provide an amplifier whose gain and feedback amount are automatically adjusted when the temperature changes.

This invention will be explained below based on one embodiment shown in FIG. In the figure, reference numeral 10 denotes a preamplifier with a frequency characteristic of 17 elements, the input side of which receives television signals and pilot signals from the previous stage amplifier via a coaxial cable, and the output side of the preamplifier 12 of the present invention. It is connected to the input terminal 14. Input terminal 14 is connected to the base of transistor 18 via coupling capacitor 16 . This base is grounded via a resistor 20 and connected to +■ via a resistor 22. The emitter of this transistor 18 is grounded through a series circuit of resistors 24 and 26, and also through a series circuit of resistors 27 and 28 and a variable capacitor 30, as well as a capacitor 32 and a variable resistor 34. The collector of this transistor 18 is connected to +■ via an impedance matching transformer 36. Note that 38 and 40 are high frequency bypass capacitors. The base of the transistor 18 and the intermediate tap of the matching transformer 36 are connected to the DC blocking capacitor 4.
2. Negative feedback circuit 4 Connected via a series circuit of a 4-pin diode 48 and a DC blocking capacitor 50.

Negative feedback circuit 44 by DC blocking capacitors 42 and 50
and the pin diode 48 are connected to the transistor 18 at high frequency.
is connected to the base and collector of. Negative feedback circuit 4
4 is composed of a series circuit of a negative feedback reactance 52 and a resistor 54. The anode of the pin diode 48 is connected to a control voltage input terminal 60 via a high frequency blocking reactance 56 and a resistor 58. The cathode of the pin diode 48 is grounded via a high frequency blocking reactance 62. The cathode of the pin diode 48 is DC-grounded by the high-frequency blocking reactance 62, and when a DC voltage is supplied to the control voltage input terminal, a current j1 corresponding to the magnitude of the DC voltage flows as shown in FIG. , will be. The high frequency resistance value of the pin diode 48 becomes a value corresponding to the magnitude of this current. These resistors 5
8, reactances 56 and 62 and pin diode 48 constitute an automatic variable resistance circuit 63. Note 6
4 is a high frequency bypass capacitor! There is. The middle tap of the matching transformer 36 is a DC blocking capacitor 66,
It is connected to an output terminal 72 via a pin diode 68 and a DC blocking capacitor 70.

The pin diode 68 is connected in series at high frequency to the transmission line on the output side of the amplifier 12 through the DC clamping capacitors 66 and 70 . This output terminal 72 is connected to the main amplifier 74.
connected to the input side of the The frequency characteristics of the main amplifier 74 are also configured to be constant. The anode of the pin diode 68 is connected to a control voltage input terminal 80 via a high frequency blocking reactance 76 and a resistor 78. The cathode of the pin diode 68 is a reactance element 81 for high frequency blocking.
is grounded through. High frequency blocking reactance 81
As a result, the cathode of the pin diode 68 is DC grounded, and when a DC voltage is supplied to the control voltage input terminal 72, a current J2 corresponding to the magnitude of the DC voltage flows as shown in FIG. The high frequency resistance value of the pin diode 68 becomes a value corresponding to the magnitude of this current. The resistor 78, the reactances 76 and 81, and the pin diode 68 constitute an automatic gain control circuit 83. Note that 82 is a high frequency bypass capacitor. A pilot signal detection circuit 84, a pilot signal amplifier 86, and a control voltage generation circuit 88 are connected in series to the output side of the main amplifier 74. The pilot signal detection circuit 84 detects the amplified pilot signal from the output signal of the main amplifier 74.
The pilot signal is amplified by a pilot signal amplifier 86 and supplied to a control voltage generation circuit 88 . The output side of the control voltage generation circuit 88 is connected to the control signal input terminals 60 and 80, respectively. This circuit 88 decreases the control voltage when the input increases, and increases the control voltage when the input decreases. Furthermore, the output side of this main amplifier 74 is connected to one end of a coaxial cable. Next, the operation of this amplifier 12- will be explained.

Assume that the coaxial cable is now at a predetermined temperature and has an attenuation characteristic as shown by the solid line 1 in FIG. At this time, when the television signal and pilot signal are input to the preamplifier 10, they are amplified by the preamplifier 10, and further amplified by the amplifier 12 and the main amplifier 74. Pilot signal detection circuit 84 detects a pilot signal from the output of main amplifier 74, and pilot signal amplifier 86 amplifies this.
The control voltage generation circuit 88 inputs the output of the pilot signal amplifier 86 and outputs the control voltage to the control voltage input terminals 60 and 80.
Enter. This control voltage causes resistor 58, reactance 56, pin diode 48 and reactance 62
The current 11 flows through the pin diode 48, the high frequency resistance value of the pin diode 48 becomes a predetermined value, and the amount of feedback is adjusted. Also, a resistor 78, a reactance element 76, a pin diode 68
A current J2 flows through the reactance 81, the high frequency resistance value of the pin diode 68 becomes a predetermined value, and the gain is adjusted. By adjusting the gain and feedback amount in this way, the frequency characteristics of the amplifier 12 become as shown by the dotted line 3 in FIG. 1. On the other hand, since the coaxial cable has a frequency characteristic as shown by the solid line 1 in FIG. 1, the frequency characteristic of the main amplifier 74 is approximately flat as shown by the dashed line 2 in FIG. When the temperature changes and the attenuation characteristics of the input side coaxial cable change as shown by the solid line 4 in the figure, and the amount of attenuation increases, the output of the main amplifier 74 decreases and the control voltage of the control voltage generation circuit 88 increases. do. Then, 11 increases, the high frequency resistance value of the pin diode 48 decreases, and the amount of feedback increases. On the other hand, J2 also increases, the high frequency resistance value of the pin diode 68 decreases, and the gain increases. Due to the increased feedback amount and gain, the amplifier 12 has a frequency characteristic as shown by the dotted line 7 in the figure, and the frequency characteristic of the main amplifier 74 becomes almost flat as shown by the dashed line 2 in the figure. As the temperature changes and the attenuation of the coaxial cable decreases, the control voltage decreases.

As a result, i1 decreases, the high frequency resistance value of the pin diode 48 increases, and the amount of feedback decreases. On the other hand, I2 also decreases, and the high frequency resistance value of the pin diode 68 also increases,
Gains decrease. Due to the decrease in the amount of feedback, the frequency characteristics of the amplifier 12 change, and the frequency characteristics of the main amplifier 74 become as shown by the dashed line 2 in the figure. This amplifier 12
According to , even if the attenuation characteristics of the input coaxial cable change due to temperature changes, the control voltage automatically changes the gain and feedback amount at the same time to compensate for the change in attenuation. Therefore, the output of the main amplifier 74 is always maintained at a predetermined level, which is convenient because there is no need to adjust it every time.

In the above embodiment, the gain adjustment circuit using the pin diode 68 is provided on the output side of this amplifier, but it may be provided on the input side. Furthermore, in the above embodiment, the automatic variable resistance circuit 6
3 and the automatic gain control circuit 83 are each provided with one pin diode 48 and one pin diode 68, but the pin diodes are arranged in a T-shape or a π-shape, similar to a known variable attenuation circuit using a pin diode. It's okay.

[Brief explanation of the drawing]

FIG. 1 is a table showing the attenuation characteristics of a coaxial cable and the frequency characteristics of a relay amplifier, and FIG. 2 is a circuit diagram of an amplifier embodying the present invention. 12...Amplifier, 44...Negative feedback circuit, 63...
automatic variable resistance circuit, 83... automatic gain control circuit, 88
...7 control voltage generation circuit.

Claims (1)

[Claims] 1. An amplifier circuit having an input side into which a pilot signal is input together with the signal to be amplified, and a feedback circuit including a reactance element between the input side and the output side, and a high-frequency an automatic variable resistance circuit including at least one first pin diode connected in series with each other, and the first pin diode is connected in direct current between a reference potential point and a first control voltage input terminal; At least one second pin diode is provided in series at high frequency in the signal transmission path on the input side or output side of the amplifier circuit, and the second pin diode is connected between the reference potential point and the second control voltage input terminal. an automatic gain control circuit connected in direct current to the amplifier circuit; a pilot signal detection circuit for detecting the pilot signal amplified by the amplifier circuit from the output of the amplifier circuit; and an amplifier for supplying a large control voltage to the first and second control voltage input terminals, respectively, when the output of the detection circuit is small.