JPH0522060A - Variable gain amplifier circuit - Google Patents

Abstract

PURPOSE:To provide the variable gain amplifier circuit which can sufficiently load automatic gain control (AGC) even when the control current of a pin diode is decreased. CONSTITUTION:At this variable gain amplifier circuit, plural amplifiers having fixed gain are connected through the attenuating means of pin diodes 30-32 so as to constitute an amplifier circuit 100 for input signals, the output of the amplifier in a final step is detected, the change of the detected voltage is converted to the control current change of the pin diode and fed back, and the output signal level of the amplifier circuit is kept constant by controlling the current of the pin diode. Then, the variable gain amplifier circuit is equipped with an amplifying means 183 to receive the current of the pin diode, to compare it with a reference value, to amplify the differential signal and to output it, control means 167 to control the level of the output from this amplifying means and level display means 81 to receive the output after this level control and to display the level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AGC (automatic gain control) amplifier circuit, and in particular, a variable gain amplifier circuit having a stable output level and a wide AGC range in an intermediate frequency AGC amplifier circuit. Regarding

[0002]

2. Description of the Related Art In a radio device or the like, the received electric field strength is monitored, and this monitor normally utilizes the AGC circuit output of a main intermediate frequency amplifier of the radio device.

Incidentally, the intermediate frequency AGC amplifier circuit has conventionally been constructed as shown in FIG. That is, 1 is a signal input terminal of the AGC amplifier circuit, 100 is a variable amplifier circuit section, 200 is a gain control section, 300 is a meter instruction circuit section, and 400 is a reference power supply circuit section.

The variable amplifier circuit section 100 is composed of operational amplifiers (OP amplifiers) 18, 19, 20, 21 having fixed gains connected in series. The output side of these operational amplifiers 18 and the input side of the operational amplifier 19 are constituted. Is connected in series with the capacitor 3, the pin diode 30 and the capacitor 4, and between the output side of the operational amplifier 19 and the input side of the operational amplifier 20, the capacitor 5 and the pin diode 3 are connected.
1 and a capacitor 6 are connected in series, and a circuit in which a capacitor 7, a pin diode 32 and a capacitor 8 are connected in series between the output side of the operational amplifier 20 and the input side of the operational amplifier 21. Each is connected.

The input side of the operational amplifier 18 is connected to the input terminal 1 via the capacitor 2, and the output side of the operational amplifier 21 is connected to the output terminal 91 via the capacitor 9. Also, the capacitor 3 and the pin diode 3
0, between the capacitor 5 and the pin diode 31,
The capacitor 7 and the pin diode 32 are grounded via resistors 50, 52, and 54, respectively, and between the pin diode 30 and the capacitor 4, between the pin diode 31 and the capacitor 6, and between the pin diode 32. One ends of resistors 51, 53, 55 are connected between 32 and the capacitor 8, respectively.

The other end of the resistor 55 is connected to the capacitor 12
The resistor 55 and the capacitor 12 are connected to the other end of the resistor 56 via the series circuit of the choke coils 72, 71 and 70. The other end of the resistor 53 is connected to the connection point of the choke coils 72, 71, and this connection point is further grounded via the capacitor 11. The other end of the resistor 51 is connected to the choke coils 71, 7
It is connected to the connection point of 0, and this connection point is further connected to capacitor 1
It is grounded through 0. Here, the pin (PI
N) The diodes 30, 31 and 32 are diodes whose resistance value changes with a change in current.

The gain control unit 200 includes an NPN transistor 80, resistors 57 to 63 and 65, a variable resistor 64, a Zener diode 33, detection diodes 35 and 36, an operational amplifier 82, capacitors 14 to 17, a choke coil 73, The transistor 80 has a collector connected to the other end of the resistor 56 and an emitter connected to the power supply terminal 92 through a series circuit of the resistor 58 and the Zener diode 33.

The base side of the transistor 80 is connected to the output side of the operational amplifier 82 via the resistor 90, and the resistor 59 and the capacitor 13 are connected in parallel between the inverting side input terminal and the output terminal of the operational amplifier 82. The circuit is connected.

Further, the detection diodes 35 and 36 are respectively arranged in anti-parallel, and then the anode side of the diode 35 and the cathode side of the diode 36 are connected, and this connection point is connected through a series circuit of the capacitor 17 and the resistor 57. Is connected to the output side of the operational amplifier 21.

The cathode side of the diode 35 and the anode side of the diode 36 are connected to each other through a series circuit of a choke coil 74 and a resistor 61, and this connection point is connected via a resistor 60 to an operational amplifier. It is connected to the inverting side input terminal of 82. The cathode side of the diode 35 is grounded via the capacitor 14, and the anode side of the diode 36 is grounded via the capacitor 15.

Furthermore, the connection point between the coil 74 and the resistor 61 is connected to the variable side terminal of the variable resistor 64 and is also grounded via the resistor 16. In addition, the operational amplifier 82
The non-inverting side input terminal is grounded via a series circuit of a resistor 62 and a resistor 65, and one end of the fixed side terminal of the variable resistor 64 is connected to a connection point of the resistor 62 and the resistor 65. Furthermore, the other end of the fixed side terminal of the variable resistor 64 is connected to the power supply terminal 92 via the resistor 63. The variable resistor 64 is a gain adjusting resistor for setting L3 in FIG. 4, and the operational amplifier 82 constitutes a DC amplifier of an AGC loop.

The reference power supply circuit section 400 comprises a resistor 66, zener diodes 37, 38, diodes 39, 40 and a variable resistor 69, and one end of the resistor 66 is connected to a power supply terminal 92. The other end of the resistor 66 is connected to the anode side of the series diode circuit including the Zener diodes 37 and 38 connected in the forward direction, and the cathode side of the series diode circuit is connected to the diodes 39 and 40 connected in the forward direction. Is connected to the cathode side of the series diode circuit, and the anode side of the series diode circuit is grounded. The Zener diodes 37 and 38 and the diodes 39 and 40 together form a temperature compensation circuit having a negative coefficient.

In the series circuit of the Zener diodes 37 and 38 and the diodes 39 and 40, the variable resistor 6 is arranged in parallel with the series circuit.
9 is connected to the fixed side terminal, and the variable resistor 69 is connected to the power supply terminal 92 through the resistor 66. Therefore, the DC voltage applied to the power supply terminal 92 is applied to the variable resistor 69 by the zener diodes 37, 38. The temperature is compensated by the temperature compensating circuit composed of the diodes 39 and 40 and applied. The variable resistor 69 sets zero (μA) in L1 of FIG.

The meter indicating circuit 300 includes a resistor 68, a diode 34, a meter (μA meter) 81, and a variable resistor 67.
The meter 81 has one end connected to the other end of the resistor 56 through a series circuit of a backflow preventing diode 34 and a resistor 68, and the other end of the meter 81 is a variable resistor 69. It is connected to the side terminal. A variable resistor 67 is connected in parallel to both ends of the meter 81 for zero adjustment of the pointer.

The conventional circuit having the above-mentioned configuration has the input terminal 1
The input signal is amplified through the variable amplification circuit section 100 and output to the output terminal 91. The gain control section 200 detects the output of the variable amplification circuit section 100 and the operational amplifier 82.
When the output of the variable amplification circuit section 100 increases, the collector current of the transistor 80 decreases and the input level is inverted and output by applying it to the inverting side input terminal of the transistor 80. It increases as the output of the amplifier circuit section 100 decreases.

Therefore, by such control of the collector current i5 of the transistor 80, the variable amplifier circuit section 100 can be controlled.
The current i4 of the resistor 56 becomes a current value corresponding to the level of the collector current i5 of the transistor 80, and the current values flowing through the pin diodes 30, 31, 32 are also proportional to this. Therefore, each pin diode 30, 31, 3
The current values (i1, i2, i3) flowing in 2 are controlled in a decreasing direction when the output of the variable amplifier circuit section 100 is large, and are controlled in an increasing direction when the output of the variable amplifier circuit section 100 is small.

As a result, when the input level of the variable amplifier circuit section 100 becomes high, the pin diodes 30, 31, 32 are pinned.
The current flowing through the pin diode 30, 31,
When the internal impedance of 32 is increased to increase the amount of attenuation between stages, and conversely when the input level is decreased, the current flowing through the pin diodes 30, 31, 32 is increased, and the internal impedance of the pin diodes 30, 31, 32 is increased. Is reduced to reduce the amount of attenuation between stages so that the output level becomes constant.

However, when the control current flowing through the pin diode becomes very small, that is, in FIG. 3, the current flowing through the resistor 51 is i1, the current flowing through the resistor 53 is i2, the current flowing through the resistor 55 is i3, and the resistance is The current flowing through 56 is i4, the current i4 is shunted into the transistor 80, i5, and the current flowing into the meter 81 is i6.
Then, i6> i5 (where i1 + i2 + i3 = i4
, I4 = i5 + i6) holds, the current flowing to the meter indicating circuit becomes larger than the control current, and the AGC is no longer effective, and the input L2 of A in FIG.
When the input level becomes larger, the level will change beyond the reference level of L3 that should be maintained. In this phenomenon, the meter indication current is transferred to the other end side P of the resistor 56
The more you get from the point, the more inevitable it will occur.

The characteristic A in FIG. 4 is a plot of the relationship between the input level and the output level, and the characteristic B is a plot of the relationship between the input level of the variable amplification circuit section 100 and the meter indicating current (i6). , L3 is the output level to be maintained by the variable amplification circuit section 100, L1 is the minimum input level of the variable amplification circuit section 100, and L2 is the input standard level of the variable amplification circuit section 100.

[0020]

In a wireless device,
The received field strength is monitored, which utilizes the AGC circuit output of the main intermediate frequency amplifier of the wireless device.

As described above, in the conventional device, the variable amplifier circuit section has a plurality of operational amplifiers connected in cascade, and a pin diode is interposed between each operational amplifier, and each pin is output in accordance with the level of the output of the final operational amplifier. The current value of the diode is controlled so that the output level of the final operational amplifier is constant. This utilizes the characteristic of the pin diode that the internal impedance increases as the current value decreases.

On the other hand, in the conventional device, the meter indicating circuit for monitoring the input level of the AGC amplifier utilizes the current of this pin diode, and therefore, is related to the pin diode control current system, and Since the instruction current is relatively large, there is a problem that AGC is not applied when the input level becomes high and the pin diode control current becomes less than the current value required for meter indication.

Therefore, the object of the present invention is to
It is an object of the present invention to provide a variable gain amplifier circuit in which AGC is sufficiently applied even when the input level increases and the control current of the pin diode decreases.

[0024]

In order to achieve the above object, the present invention is configured as follows. That is, a plurality of amplifiers having a fixed gain are connected via an attenuator by a pin diode to configure an amplifier circuit for an input signal, the output of the final stage amplifier is detected, and the change in the detected voltage is detected by the pin diode. In the variable gain amplifier circuit, which converts the control current into a change and feeds it back to control the current of the pin diode so as to keep the output signal level of the amplifier circuit constant, receives the current of the pin diode and obtains a reference value. Amplifying means for comparing and amplifying the difference signal and outputting the difference signal, adjusting means for adjusting the level of the output of the amplifying means, and level display means for receiving the level-adjusted output and displaying the level are provided.

[0025]

In the above structure, the output of the final stage of the amplifier circuit is detected, the change of the detected voltage is converted into the control current change of the pin diode and fed back, and the current of the pin diode is controlled to control the amplifier circuit. The output signal level of the level display means is kept constant, and the output of the amplification means is adjusted by using the output of the amplification means for receiving the current of the pin diode and comparing it with a reference value to amplify the difference signal. The level is adjusted by the means and then given to the level display means.

As described above, it is possible to avoid that the display output of the level display means depends entirely on the current flowing through the pin diode, so that the attenuation amount of the attenuation circuit by the pin diode increases in proportion to the input level. Since the AGC can be controlled during the operation, the AGC is possible for an extremely wide range of input levels.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In FIG. 1, 1 is a signal input terminal of the variable amplification circuit unit, 100 is a variable amplification circuit unit, 200 is a gain control unit, and 500 is a meter instruction circuit unit.

The variable amplifier circuit section 100 is constructed by connecting operational amplifiers (OP amplifiers) 18, 19, 20, 21 having a fixed gain in cascade, and the output side of these operational amplifiers 18 and the input side of the operational amplifier 19 are connected. Is connected in series with the capacitor 3, the pin diode 30 and the capacitor 4, and between the output side of the operational amplifier 19 and the input side of the operational amplifier 20, the capacitor 5 and the pin diode 3 are connected.
1 and a capacitor 6 are connected in series, and a circuit in which a capacitor 7, a pin diode 32 and a capacitor 8 are connected in series between the output side of the operational amplifier 20 and the input side of the operational amplifier 21. Each is connected.

The input side of the operational amplifier 18 is connected to the input terminal 1 via the capacitor 2, and the output side of the operational amplifier 21 is connected to the output terminal 91 via the capacitor 9. Also, the capacitor 3 and the pin diode 3
0, between the capacitor 5 and the pin diode 31,
The capacitor 7 and the pin diode 32 are grounded via resistors 50, 52, and 54, respectively, and between the pin diode 30 and the capacitor 4, between the pin diode 31 and the capacitor 6, and between the pin diode 32. One ends of resistors 51, 53, 55 are connected between 32 and the capacitor 8, respectively.

The other end of the resistor 55 is connected to the capacitor 12
The resistor 55 and the capacitor 12 are connected to the other end of the resistor 56 via the series circuit of the choke coils 72, 71 and 70. The other end of the resistor 53 is connected to the connection point of the choke coils 72, 71, and this connection point is further grounded via the capacitor 11. The other end of the resistor 51 is connected to the choke coils 71, 7
It is connected to the connection point of 0, and this connection point is further connected to capacitor 1
It is grounded through 0. Here, the pin (PI
N) The diodes 30, 31 and 32 are diodes whose resistance value changes with a change in current.

The gain controller 200 includes an NPN transistor 80, resistors 57 to 63 and 65, a variable resistor 64, a Zener diode 33, detection diodes 35 and 36, an operational amplifier 82, capacitors 14 to 17, choke coils 73, The transistor 80 has a collector connected to the other end of the resistor 56 and an emitter connected to the power supply terminal 92 through a series circuit of the resistor 58 and the Zener diode 33.

The base side of the transistor 80 is connected to the output side of the operational amplifier 82 via the resistor 90, and the resistor 59 and the capacitor 13 are connected in parallel between the inverting side input terminal and the output terminal of the operational amplifier 82. The circuit is connected. Further, the detection diodes 35 and 36 are respectively arranged in anti-parallel, and then the anode side of the diode 35 and the cathode side of the diode 36 are connected to each other, and this connection point is calculated through the series circuit of the capacitor 17 and the resistor 57. It is connected to the output side of the amplifier 21.

The cathode side of the diode 35 and the anode side of the diode 36 are connected to each other via a series circuit of a choke coil 74 and a resistor 61, and this connection point is connected to the operational amplifier via a resistor 60. It is connected to the inverting side input terminal of 82. The cathode side of the diode 35 is grounded via the capacitor 14, and the anode side of the diode 36 is grounded via the capacitor 15.

Furthermore, the connection point between the coil 74 and the resistor 61 is connected to the variable side terminal of the variable resistor 64 and is also grounded via the resistor 16. In addition, the operational amplifier 82
The non-inverting side input terminal is grounded via a series circuit of a resistor 62 and a resistor 65, and one end of the fixed side terminal of the variable resistor 64 is connected to a connection point of the resistor 62 and the resistor 65. Furthermore, the other end of the fixed side terminal of the variable resistor 64 is connected to the power input terminal 92 via the resistor 63.
The variable resistor 64 is a gain adjusting resistor for setting L3 in FIG. 4, and the operational amplifier 82 constitutes a DC amplifier of an AGC loop.

The meter indicating circuit section 500 includes a reference voltage circuit section including a resistor 66, Zener diodes 37 and 38, diodes 39 and 40, and a variable resistor 69, a resistor 168, a diode 134, and a meter (μA meter) 8
1, a variable resistor 167, an operational amplifier 183, capacitors 118 and 119, and resistors 175 and 176.

In the reference voltage circuit section of the meter indicating circuit section 500, one end side of the resistor 66 is connected to the power supply terminal 92, and the other end side of the resistor 66 is connected in series forward direction Zener diodes 37, 38. Is connected to the anode side of the series diode circuit, and the cathode side of the series diode circuit is connected to the series forward diode 3
It is connected to the cathode side of the series diode circuit constituted by 9, 40, and the anode side of the series diode circuit is grounded.

The Zener diodes 37 and 38 and the diodes 39 and 40 together form a temperature compensation circuit having a negative coefficient. The fixed side terminal of the variable resistor 69 is connected in parallel to the series circuit of the Zener diodes 37, 38 and the diodes 39, 40, and the variable resistor 69 is connected to the power supply terminal 92 via the resistor 66. Therefore, the DC voltage applied to the power supply terminal 92 is applied to the variable resistor 69 after being temperature-compensated by the temperature compensation circuit including the Zener diodes 37, 38 and the diodes 39, 40.

In the meter control circuit section, the output side of the operational amplifier 183 constituting the differential amplifier circuit is connected to one end side of the meter 81 via the series circuit of the diode 134 and the variable resistor 167, and the other end side of the meter 81 is connected. Grounded. The output side of the operational amplifier 183 is connected to the inverting side input terminal of the operational amplifier 183 via the resistor 176, and the inverting side input terminal is connected to the variable side terminal of the variable resistor 69 via the resistor 175. There is.

Further, the non-inverting side input terminal of the operational amplifier 183 is connected to the resistor 5 of the variable amplifier circuit section 100 via the resistor 168.
6 is connected to the other end side. Reference numerals 193 and 194 denote negative and positive DC power source input terminals,
The power supply of 3 is supplied. Also, the capacitor 11
8 and 119 are for stabilizing the power supply.

In the present apparatus having such a configuration, the signal input from the input terminal 1 is amplified through the variable amplification circuit section 100 and output to the output terminal 91, but the gain control section 200 outputs the output of the variable amplification circuit section 100. Is detected and applied to the inverting side input terminal of the operational amplifier 82 to invert the input level and output, which is applied to the base of the transistor 80.
Therefore, the collector current of the transistor 80 decreases as the output of the variable amplification circuit section 100 increases.
It increases as the output of the variable amplification circuit section 100 decreases.

Therefore, by controlling the collector current i5 of the transistor 80 in this way, the variable amplifier circuit section 100 can be controlled.
The current i4 of the resistor 56 becomes a current value corresponding to the level of the collector current i5 of the transistor 80, and the current values flowing through the pin diodes 30, 31, 32 are also proportional to this. Therefore, each pin diode 30, 31, 3
The current values (i1, i2, i3) flowing in 2 are controlled in a decreasing direction when the output of the variable amplifier circuit section 100 is large, and are controlled in an increasing direction when the output of the variable amplifier circuit section 100 is small.

Although a part of the current i4 also flows into the meter indicating circuit section 500 (current i6), this current is converted into a voltage signal through the input resistor 168, and then the non-inverting side of the operational amplifier 183. It is given to the input terminal.

The operational amplifier 183 is supplied with the set reference voltage from the variable resistor 69 of the reference voltage circuit section, and amplifies and outputs the difference signal between the two with reference to this. Then, this difference signal is sent to the meter 81 via the diode 134 and the variable resistor 167, and the meter 81 is swung corresponding to this difference signal. Basically, the operation is as described above, but in this apparatus, the fine adjustment is first set as follows and then used.

That is, in setting, first, an intermediate frequency signal generator for generating an intermediate frequency signal is provided at the signal input terminal 1, a high frequency level meter for measuring the output level at the signal output terminal 91, and a power source input. Terminals 92, 1
DC power supplies of specified voltages are connected to 93 and 194, respectively.

Then, the frequency of the intermediate frequency signal generator is adjusted to the designated frequency, and the level is set to L1 in FIG. 2, that is, the input intermediate frequency signal level is set to the input level corresponding to the lowest level of the electric field strength under the use condition. When you do, meter 8
Set the variable resistor 69 to the meter 8 so that the indication of 1 becomes zero.
Adjust the runout of 1 to zero.

Here, the characteristic "a" in FIG. 2 is obtained by plotting the relation between the input level and the output level.
The characteristic b is a plot of the relationship between the input level of the variable amplification circuit section 100 and the meter instruction current i7, L3 is the output level of the variable amplification circuit section 100, L1 is the minimum input level of the variable amplification circuit section 100, and L2 is variable. Amplifier circuit section 100
The input standard level, L4, indicates the maximum input level of the variable amplification circuit section 100.

Next, L2 of FIG. 2, that is, the fluctuation of the meter 81 when an input level corresponding to the maximum received electric field strength under the use condition is given is adjusted by the variable resistor 67, Further, under the input condition of the maximum electric field strength, the bias level of the detection signal given to the operational amplifier 82 by the variable resistor 64 of the gain control unit 200 so that the output level of the variable amplification circuit unit 100 becomes the level L4 in FIG. Adjust.

In the present apparatus, the meter indicating circuit section 50
In the case of 0, the collector side of the transistor 80 of the gain control unit 200 that controls the currents of the pin diodes 30 and 32 is supplied with the current for meter indication.
Instead of swinging the meter 81 by the divided current itself, it is used as an instruction current of the meter 81 after being amplified by the differential amplifier circuit by the operational amplifier 183. Therefore, the pin diode control current i4 consumed on the meter indicating circuit section 500 side can be very small.

Moreover, the indication range of the meter 81 is adjusted by the variable resistors 69 and 167, and the gain control section 200 is adjusted.
Since the bias level of the detection signal of the output of the variable amplification circuit section 100 which is given to the operational amplifier 82 is adjusted by the variable resistor 64 of FIG. The transistor 80 can be controlled so that the output level of the amplifier circuit section 100 can be maintained at L3, and the pin diode control current i4 can be maintained so that the AGC operation is performed.

Therefore, thereafter, the input level of the intermediate frequency signal input to the signal input terminal 1 is from L1 to L4 (L4
2), the characteristics of a and b in FIG. 2, that is, the swing of the meter 81 is linear, and the output level of the variable amplification circuit section 100 is level L3 and flat characteristics are obtained.

This is because even if the input level of the intermediate frequency signal to the variable amplification circuit section 100 increases, the gain control section 20
This is because the relationship that the AGC current i5 of 0 is sufficiently larger than the input current i6 to the meter indicating circuit unit 500 is maintained, and the control current corresponding to the input level of the intermediate frequency signal is the pin diodes 30, 31, 32. To each of the operational amplifiers 18 and 19, 19 and 2 so as to keep the output level constant.
This is for variably controlling the attenuation amount of the attenuation circuit by the pin diode provided between 0, 20, and 21.

Therefore, AGC can be performed by utilizing this action. In this device, the output signal level of the variable amplifier circuit unit is kept constant by controlling the current of the pin diode, but the indicator output of the meter indicator circuit unit receives the current of the pin diode of the variable amplifier circuit unit, Using the output of the amplification means for comparing the value and amplifying the difference signal,
Since the output of the amplifying means is level-adjusted and then given to the meter, it is possible to avoid that the indicating output of the meter depends entirely on the current flowing through the pin diode.

Therefore, the control current i4 flowing through the pin diode is dominated by i5 and is not affected by i6. still,
If the currents flowing in the pin diodes are i1, i2, and i3, they can be expressed as i1 + i2 + i3 = i4, i4 = i5 + i6. However, if i5 is sufficiently larger than i6,
Due to these relationships, i4 is approximately equal to i5.

Therefore, in the range where i4 is in a linear relationship, the gain control is performed so that the output signal level of the variable amplification circuit section is maintained at a predetermined level even if the level of the intermediate frequency input signal increases. You can

The diode 134 prevents the reverse swing of the meter indication. In addition, the diodes 37, 38, 39, 4
Each of 0 has a negative temperature coefficient and compensates for the temperature characteristic of the meter indicating circuit system.

As described above, in the present apparatus, a plurality of amplifiers having fixed gains are connected via attenuating means by pin diodes to configure an input signal amplifying circuit and detect the output of the final stage amplifier. In the variable gain amplifier circuit, the change of the detected voltage is converted into a control current change of the pin diode and fed back, and the output signal level of the amplifier circuit is kept constant by controlling the current of the pin diode. An amplification means for receiving the current of the pin diode and comparing it with a reference value to amplify and output the difference signal, an adjustment means for adjusting the level of the output of the amplification means, and a level display for receiving the output after the level adjustment. And a level display means.

In this configuration, the present device detects the output of the final stage of the amplifier circuit, converts the change in the detected voltage into the control current change of the pin diode, and feeds it back to control the current of the pin diode. By doing so, the output signal level of the amplification circuit is kept constant, but the display output of the level display means uses the output of the amplification means that receives the current of the pin diode and compares it with a reference value to amplify the difference signal, The output of the amplifying means is level-adjusted by the adjusting means before being given to the level display means.

As described above, it is possible to avoid that the display output of the level display means depends entirely on the current flowing through the pin diode, so that the attenuation amount of the attenuation circuit by the pin diode increases in proportion to the input level. Since the AGC can be controlled during the operation, the AGC is possible for an extremely wide range of input levels. still,
The present invention is not limited to the embodiments described above and shown in the drawings, and can be appropriately modified and implemented within the scope of the invention.

[0060]

As described above, according to the present invention, in the AGC amplifier using the pin diode, when the monitor voltage or current according to the conversion of the input level is obtained,
Since it does not depend on the current flowing in the pin diode, it can be controlled while the attenuation of the attenuation circuit by the pin diode increases in proportion to the input level, so it is possible to control the input level over a very wide range. Since AGC is possible, it is possible to provide a variable gain amplifier circuit that is optimal for the AGC circuit of the main intermediate frequency amplifier of a wireless device that needs to monitor the received electric field strength.

[Brief description of drawings]

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

FIG. 2 is a diagram showing operating characteristics of the device of the present invention.

FIG. 3 is a circuit diagram showing a conventional example.

FIG. 4 is a diagram showing operating characteristics of a conventional device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Signal input terminal, 18, -21,82,183 ... Operational amplifier (OP amplifier), 30, -32 ... Pin diode, 33,37,38 ... Zener diode, 35,3
6, 39, 40, 134 ... Diodes, 64, 69, 1
67 ... Variable resistor, 80 ... Transistor, 81 ... Meter, 91 ... Output terminal, 92, 193, 194 ... Power supply input terminal, 100 ... Variable amplification circuit section, 200 ... Gain control section,
500 ... Meter instruction circuit section.

Claims (1)

Claim: What is claimed is: 1. A plurality of amplifiers having a fixed gain are connected through an attenuating means by a pin diode to constitute an amplifier circuit for an input signal, and an output of a final stage amplifier is detected. In the variable gain amplifier circuit, which converts the change of the detection voltage into a control current change of the pin diode and feeds it back to control the current of the pin diode to keep the output signal level of the amplifier circuit constant, Amplifying means for receiving the current of the diode and comparing it with a reference value to amplify and output the difference signal, adjusting means for adjusting the level of the output of the amplifying means, and level for displaying the level after receiving the output after the level adjustment. A variable gain amplifier circuit, characterized in that display means is provided.