JPH0324813B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a multi-stage configuration used in an AM receiver.
Regarding AGC circuit.

Figure 1 is a schematic block diagram of an AM receiver whose main parts are IC-based. 1 is an antenna, 2 is a first-stage radio frequency (RF) amplifier, 3 is a next-stage RF amplifier, 4 is a mixer, and 5 is a receiving station. 6 is the intermediate frequency (IF) of the first stage
Amplifier, 7 is the next stage IF amplifier, 8 is the detector, 9
is an AGC (automatic gain control) amplifier. The main parts of RF amplifiers 2 and 3 are voltage-controlled variable gain amplifiers.
These are VCA1 and VCA2, and the main part of the IF amplifier 6 is also a similar amplifier VCA3. The AGC of this receiver is the control voltage of AGC amplifier 9.
Vc to amplifier VCA1, VCA2 and amplifier VCA
This is done by making a common return to step 3. However, the changes in the attenuation amount ATT of each stage are differentiated as in the experiment shown in FIG. 2, and settings are made so that appropriate gain distribution can be achieved. That is, as the control voltage Vc increases, the final stage VCA3 starts to attenuate first, then the VCA2, and finally the first stage VCA1. In this way, deterioration of S/N and occurrence of distortion are prevented.

However, if the characteristics of each VCA are such that there is no limit to the attenuation amount ATT as shown in Figure 2, VCA3 has almost no limit.
AGC becomes stiff and VCA1 is used even during large input.
Regardless of AGC, it will exceed the dynamic resin of VCA1 and become distorted.

The present invention attempts to solve the above-mentioned drawbacks by simply adding one resistor to the circuit.

The present invention includes a first high-frequency amplifier that amplifies an antenna input signal, a second high-frequency amplifier that amplifies the output signal of the first high-frequency amplifier, and an intermediate frequency signal that converts the output signal of the second high-frequency amplifier into an intermediate frequency signal. In the AGC circuit, each of the intermediate frequency amplifiers that amplify the intermediate frequency signal after converting it into a variable gain amplifier is configured to reduce the gain sequentially from the downstream amplifier, and the first high frequency amplifier is a transistor that receives the antenna input signal. , a resistor connected to its emitter side, and a pair of transistors whose emitters are commonly connected to the collectors of the transistors and receive a reference voltage at one base and a control voltage for AGC at the other base,
The second high frequency amplifier and intermediate frequency amplifier are configured to take out the output from the collector of the one transistor without limiting the range of gain change, and the second high frequency amplifier and the intermediate frequency amplifier include a transistor receiving an input signal and a resistor connected to the emitter side of the transistor. and a pair of transistors whose emitters are commonly connected to the collectors of the transistors, and whose bases receive a reference voltage and whose other bases receive an AGC control voltage, and which are connected between both collectors of the pair of transistors. and a resistor connected between the collector of the transistor receiving the control voltage and a power supply, and the resistor connected between the collector and the power supply limits the range of gain change and controls the reference voltage. It is characterized by being configured so that the output is taken out from the collector of the transistor receiving the signal.
This will be explained in detail below with reference to the illustrated embodiments.

FIG. 3 is a circuit diagram of a variable gain amplifier VCA according to the present invention, where _R1 is a resistor added according to the present invention. Vi is the AC input voltage, V ₀ is its amplified output,
Vc is the previous DC control voltage, V is the reference voltage,
Tr ₁ and Tr ₂ are transistors forming a differential pair, Tr ₃
is a transistor that converts voltage Vi into current, _R3 is its emitter resistance, and _R2 is a resistance connected to the collector side of transistor _Tr2 . The reference voltage V is
Specifies the point at which AGC starts. Traditional VCA
Since the connection point between the collector of transistor Tr ₁ and resistor R ₂ is directly connected to the power supply Vcc, the attenuation amount is
ATT can take values up to infinity. However, in general, ATT of about 10 to 20 dB is sufficient, so in the present invention, the resistance _R1 is inserted to limit the range of change in ATT.

In other words, if Vc<V, the transistor _Tr1 is off and no AGC is applied. The gain at this time is V _p /V _i =R ₁ +R ₂ /R ₃ (1). On the other hand, when Vc>V, the transistor Tr ₂
When is completely turned off, the AGC is applied to the highest level, and the gain at this time is V _p /V _i =R ₁ /R ₂ (2). During normal operation, the gain changes between equations (1) and (2).

In the conventional circuit, since R ₁ = 0 in equation (2), the gain is 0, and therefore ATT =, but in the present invention, by setting the value of R ₁ , the ATT limit of each VCA can be individually limited. . FIG. 4 shows an example of characteristics when the circuit shown in FIG. 3 is applied to VCA2 and VCA3.

FIG. 5 shows a high-frequency stage implemented as an IC, and the configuration similar to that in FIG. 3 is adopted for the amplifier VCA2 in the subsequent stage. Since the first stage amplifier VCA1 does not limit the range of gain variation, its circuit configuration is not modified.
In other words, the amplifier VCA1 is made up of the differential pair transistors Tr ₄ and Tr ₅ and the transistor Tr ₆ on the common emitter side.
is configured. Ist RF IN is the AC input to VCA1, and its amplified output is passed from transistor Tr ₄ to external inductance L to the next stage amplifier.
Supplied to VCA2. 2nd RF OUT is the amplified output of this amplifier VCA2. Transistor Tr ₁ and
Control voltage Vc is commonly supplied to Tr ₅ . This is obtained by appropriately shifting the output of AGC amplifier 9 using an OP amplifier using diode _D1 . The input of this amplifier 9 is the output of the detection circuit 8, and the input of the detection circuit 8 is an intermediate frequency transformer.
This is an intermediate frequency signal obtained from IFT. Reference voltage V ₁ applied to transistor Tr ₄ and transistor
A difference in the forward voltage of the diode D 2 is provided between the reference voltage V ₂ applied to the Tr ₂ and the reference voltage V ₂ . This V ₁
>V ₂ , VCA2 in the latter stage starts attenuation first. Regarding the IF stage amplifier VCA3
A resistor _R1 similar to VCA2 is added, but the explanation of this part will be omitted.

As described above, according to the present invention, each stage of the AGC circuit uses a multi-stage variable gain amplifier.
Since we have added a limit to the input signal, we can expect stable operation without distortion, and the input signal level can be maximized at the time when the AGC of each amplifier starts to be heard, further improving the S/N ratio. Can be done.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the AGC circuit of an AM receiver, Figure 2 is an explanatory diagram of conventional AGC characteristics, and Figure 3 is a block diagram showing the AGC circuit of an AM receiver.
FIG. 4 is a circuit diagram of a main part of the present invention, FIG. 4 is an AGC characteristic diagram thereof, and FIG. 5 is a circuit diagram showing an embodiment of the present invention. In the figure, VCA1 to VCA3 are variable gain amplifiers,
Tr ₁ to _{Tr 3} are transistors, R ₁ to _{R 3} are resistors, Vi is an input, V ₀ is an output, V is a reference voltage, Vc is a control voltage, and Vcc is a power supply.

Claims (1)

[Claims] 1. A first high-frequency amplifier that amplifies an antenna input signal, a second high-frequency amplifier that amplifies an output signal of the first high-frequency amplifier, and an output signal of the second high-frequency amplifier that is amplified by a mixer. In the AGC circuit, in which the intermediate frequency amplifiers that amplify the intermediate frequency signal after converting it into an intermediate frequency signal are each configured with a variable gain amplifier, and the gain is decreased in order from the downstream amplifier, the first high frequency amplifier is configured to convert the intermediate frequency signal into an intermediate frequency signal and then amplify the intermediate frequency signal. a resistor connected to its emitter side, and a pair of transistors whose emitters are commonly connected to the collectors of the transistors and receive a reference voltage to one base and a control voltage for AGC to the other base. The second high frequency amplifier and the intermediate frequency amplifier are connected to the transistor receiving the input signal and the emitter side thereof, and the second high frequency amplifier and the intermediate frequency amplifier are connected to the transistor receiving the input signal and to the emitter side thereof. a pair of transistors whose emitters are commonly connected to the collectors of the transistors and receive a reference voltage at one base and a control voltage for AGC at the other base; and a resistor connected between the collector of the transistor receiving the control voltage and the power supply, and the range of gain change is limited by the resistor connected between the collector and the power supply. An AGC circuit characterized in that the AGC circuit is configured to take out an output from the collector of a transistor that receives the reference voltage.