JPH0317243B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Technical Field of the Invention The present invention relates to a switching circuit, and particularly to a switching circuit for switching between an automatic gain control circuit (AGC circuit) and a manual gain circuit (MGC circuit) used in an amplifier. It is related to.

(b) Prior art and problems Amplifiers with automatic gain control are used in television signal relay equipment, but when testing this amplifier, it is necessary to automatically adjust the gain of this amplifier. It is necessary to conduct various tests by freely switching between manually controlled functions and manually controlled functions using a switching circuit.

FIG. 1 shows a block diagram of an amplifier including a conventional switching circuit.

In FIG. 1, the signal applied to the terminal IN is amplified to the required level by the amplifier AMP-1, and most of the signal is taken out from the terminal OUT through the hybrid circuit HYB. On the other hand, a part of the amplified signal is extracted by the hybrid circuit HYB and sent to the detector.
The wave is detected by DET and the detection output is the voltage comparator COM
is compared with the reference voltage contained in this comparator COM, and this difference voltage is amplified by an amplifier AMP-2 and applied to the control voltage input terminal C of the amplifier AMP-1.
The gain of the amplifier AMP-1 is automatically controlled to an appropriate value.

And to manually control the gain of this amplifier AMP-1, use the changeover switch _SW1 to manually control the gain of the amplifier AMP-1.
By switching the output terminal AGC of AMP-2 to the manual control supply voltage terminal MGC and changing the value of the variable resistor Rv, the external supply voltage V ₁ is divided into appropriate voltages, and this is connected to the control voltage input terminal. In addition to C, the gain of amplifier AMP-1 is controlled.

As mentioned above, the control voltage of the amplifier AMP-1 is controlled by the switching operation of the switch SW ₁ from the AGC circuit or
It is supplied from the MGC circuit, but this switching operation is performed even when the test signal is applied to this amplifier AMP-1, but under the following conditions, the terminal
Excessive voltage may occur at OUT. In other words, an amplifier to which the test signal is applied from the input terminal IN.
If AMP-1 is first supplied with control voltage from the MGC circuit and then supplied with control voltage from the AGC circuit with switch SW ₁ , the gain of amplifier AMP-1 will be maximized and overvoltage may occur at terminal OUT. be. In such a state, there is a problem in that overvoltage is applied to the wattmeter or power amplifier (not shown) connected to the terminal OUT, causing damage or deterioration.

(c) Purpose of the Invention The present invention has been made in view of the problems of the prior art described above, and an object of the present invention is to provide a switching circuit that does not generate overvoltage.

(d) Structure of the Invention The above object is to provide a control voltage input terminal of an amplifier whose gain is controlled according to the present invention in the case of automatic gain control.
A part of the output of the amplifier is given as a control voltage through a detector and a voltage comparator circuit that compares the reference voltage and the output of the detector, while in the case of manual gain control, the control voltage input terminal of the amplifier is given as a control voltage. In addition to applying the voltage directly, a predetermined voltage is applied to the capacitor connected to the output side of the detector as a voltage that is added to the output voltage of the detector, and the added voltage is the difference from the reference voltage in the voltage comparison circuit. the voltage is determined to be a control voltage approximately corresponding to the lowest gain of the amplifier, and the predetermined voltage is discharged from the capacitor through the resistor after switching from manual gain control to automatic gain control;
This is achieved by a switching circuit characterized by decay over time.

(e) Embodiment of the invention Figure 2 is a block diagram showing an embodiment of the invention.
AMP-1 is an amplifier, HYB is a hybrid circuit,
DET is a wave detector, COM is a voltage comparison circuit, AMP−
2 is an amplifier, SW ₂ and SW ₃ are switching circuits, RV ₁ is a variable resistor, R ₁ is a resistor, C ₁ is a capacitor, IN is an input terminal, OUT is an output terminal, C is a control voltage input terminal, 1 and 2 indicate terminals, respectively.

Embodiments of the present invention will be described in detail below with reference to FIG.

In Figure 2, the input terminal of the amplifier AMP-1 is connected to the input terminal IN, the output terminal of this amplifier is connected to the input terminal of the hybrid circuit HYB, and one output terminal of this hybrid circuit is output. terminal
OUT, the other output terminals are respectively connected to the input terminal of the detector DET, and the output terminal of this detector DET is connected through the input terminal of the voltage comparison circuit COM and the parallel connection of the resistor _R1 and the capacitor _C1 . The output terminal of the voltage comparator circuit COM is connected to the input terminal of the amplifier AMP-2, and the control voltage input terminal C of the amplifier AMP-1 is connected to the switching circuit.
It is connected to the output terminal of the amplifier AMP-2 via SW ₂ or to terminal 2 via a variable resistor.

Further, a switching circuit SW ₃ operating in conjunction with this switching circuit SW ₂ switches the connection between the terminal 1 and the detector DET. The portion surrounded by dotted lines is the portion added in the present invention.

Next, the operation of this block diagram is as follows.

When the switching circuits SW ₂ and SW ₃ are connected to the MGC circuit, the voltage supplied from the terminal 2 is applied to the terminal C of the amplifier AMP-1 via the variable resistor Rv ₁ , and the voltage supplied from the amplifier AMP- 1 amplifies the signal applied to this amplifier from the terminal IN with a certain gain, most of the amplified output signal is taken out from the terminal OUT, and the remaining part is detected by the detector DET. Charge capacitor _C1 .

On the other hand, the constant positive voltage applied from the terminal 1 charges the capacitor _C1 through the switching circuit _SW3 , so that the detected voltage and the positive voltage are added at the capacitor _C1 . This voltage is then compared with a reference voltage Vs included in the voltage comparator circuit COM, and a difference voltage is extracted.

This differential voltage is then amplified to an appropriate level by the amplifier AMP-2.

Here, the explanation of the operation of the voltage comparator circuit COM is based on the connection of the diode used in the detector DET.
As shown in the figure, the negative side of the diode is connected to the capacitor.
When connected to C ₁ and the diode is connected in the opposite direction as shown by the dotted line, the voltage applied to terminal 1 is 0, that is, grounded.

In other words, the voltage applied to the voltage comparator COM is set to a control voltage whose differential voltage corresponds to the minimum gain of the amplifier AMP-1.

In this state, next switchover circuits SW ₂ and SW ₃ are connected.
When switching from the MGC circuit side to the AGC circuit side, the amplifier
AMP-1, hybrid circuit HYB, detector
An automatic gain control loop is created that passes through DET, voltage comparator COM, and amplifier AMP-2. Drawer AMP-1
Since the gain of amplifier AMP-1 is controlled, the gain of amplifier AMP-1 first decreases to the minimum gain. but
Voltage comparator COM from terminal 1 when MGC circuit is operating
Since the voltage applied to the AGC circuit is no longer supplied by the AGC circuit operation, the voltage comparator
When the COM input voltage approaches the reference voltage Vs, the gain of the amplifier AMP-1 begins to increase and stops when the voltage difference is zero.

Figure 3 is a diagram showing the relationship between the gain change of amplifier AMP-1 and time, and point a is from the MGC circuit operation.
It shows the point in time when switching to AGC circuit operation, point b is the state where the voltage difference between the reference voltage and the voltage applied in the voltage comparator COM has become 0, and point c is the state where the charge stored in the capacitor C ₁ is low resistance R ₁ A voltage difference occurs between the reference voltage and the input voltage of the voltage comparator COM, and the gain of the amplifier AMP-1 starts to increase to reduce this difference. Each shows a state in which it becomes 0.

(f) Effects of the Invention As explained above, according to the present invention, when switching from MGC circuit operation to AGC circuit operation, the amplifier whose gain is controlled by the circuit does not generate excessive voltage. Therefore, damage to the power amplifier or wattmeter connected to the amplifier output terminal can be prevented, and work efficiency can be improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional switching circuit, Fig. 2 is a block diagram of a switching circuit of the present invention, and Fig. 3 is a block diagram of a conventional switching circuit.
2A and 2B are diagrams illustrating the operation of the block diagrams shown in FIG. In the figure, AMP-1 and AMP-2 are amplifiers, HYB
is a hybrid circuit, COM is a voltage comparison circuit,
DET is a detector, SW ₂ and SW ₃ are switching circuits, IN is an input terminal, OUT is an output terminal, 1 and 2 are terminals,
Rv ₁ is a variable resistor, R ₁ is a resistor, and C ₁ is a capacitor.

Claims (1)

[Claims] 1. In the case of automatic gain control, a part of the output of the amplifier is connected to the control voltage input terminal of the gain-controlled amplifier as a control voltage via a detector and a voltage comparator circuit that compares the reference voltage and the output of the detector. On the other hand, in the case of manual gain control, the manual control voltage is directly applied to the control voltage input terminal of the amplifier, and a predetermined voltage is added to the output voltage of the detector to the capacitor connected to the output side of the detector. The additive voltage is determined so that the difference voltage from the reference voltage in the voltage comparator circuit becomes the control voltage that approximately corresponds to the lowest gain of the amplifier, and the predetermined voltage can be changed from manual gain control to automatic gain control. A switching circuit characterized in that the capacitor discharges through the resistor after switching to the current state, and the switching circuit attenuates over time.