JP2881770B2 - AGC circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an AGC circuit.

[Summary of the Invention]

According to the present invention, in an AGC circuit used for a radio receiver or the like, a time constant of an AGC is controlled in accordance with the intensity of a received signal, thereby reducing distortion and improving fading characteristics.

[Conventional technology]

The AM receiver and its AGC circuit are configured, for example, as shown in FIG.

That is, in the figure, (1) is an antenna tuning circuit, (2) is a high frequency amplifier, (3) is a mixer circuit,
(4) shows a local oscillation circuit, and an intermediate frequency signal obtained by frequency-converting the reception signal (broadcast wave signal) selected by the tuning circuit (1) is extracted from the mixer circuit (3).

The intermediate frequency signal is supplied to an intermediate frequency amplifier (5)
Is supplied to an AM detection circuit (6) to perform AM detection, and the detection output (AM detection voltage) Ea is supplied to a speaker (8) through a low-frequency amplifier (7). In this case, Ea = E + e E: DC component whose level changes in accordance with the level of the intermediate frequency signal e: Audio signal component

The detection voltage Ea is supplied to the low-pass filter (9), and the DC component E is extracted as an AGC voltage. The AGC voltage E is supplied to the amplifiers (2) and (5) as control signals for their gains. AGC is performed.

However, in the above-described AGC circuit, when the time constant of the low-pass filter (9) is small, that is, when the cutoff frequency is high, the low-frequency component of the audio signal component e becomes
It is included in the AGC voltage E supplied to the amplifiers (2) and (5).

When the low frequency component of the audio signal component e is included in the AGC voltage E, the gains of the amplifiers (2) and (5) also change due to the low frequency component. ) Increases the distortion of the audio signal e.

In order to eliminate such distortion, the time constant of the filter (9) is increased, and the AGC supplied to the amplifiers (2) and (5) is increased.
It is sufficient that the voltage E does not include the audio signal component e.

However, if the time constant of the filter (9) is increased, when a fading radio wave is received, the AGC cannot follow the fast-period fading, so that the volume changes according to the fading, and a stable volume can be received. Will be gone.

In this regard, as shown in FIG. 3, if the low-pass filter (9) is composed of two stages of low-pass filters (9A) and (9B), the cut-off characteristic of the low-pass filter becomes steep, so that the time constant It is possible to sufficiently attenuate the audio signal component e included in the AGC voltage E without increasing the value, and therefore, the distortion is reduced. Further, since the time constant can be reduced, reception can be performed with a stable sound volume even with fading.

However, when the two-stage filters (9A) and (9B) are used in this way, unless the ratio between the time constant of the filter (9A) and the time constant of the filter (9B) is set to about 10 times, the AGC Since the loop characteristic is quadratic, a peak occurs in the low frequency characteristic of the audio signal component e. In addition, the transient characteristics when the reception level of the reception signal changes become almost overshoot, and the feeling at the time of tuning deteriorates.

Further, as the number of components increases, the number of capacitors that need to be externally added becomes one when the IC is formed, and the number of external terminal pins of the IC also increases.

[Problems to be solved by the invention]

As described above, in the AGC circuit of FIG. 2, when the time constant of the low-pass filter (9) is small, the distortion increases, and when the time constant is large, the sound volume is not stabilized against fading.

Further, in the AGC circuit shown in FIG. 3, a peak occurs in the frequency characteristic, and the feeling at the time of tuning deteriorates. Furthermore, it is not suitable for IC conversion.

The present invention seeks to eliminate the above problems.

[Means for solving the problem]

Now, considering fading, this occurs when a long-distance broadcast is being received, and therefore, the reception level of the broadcast is small. In other words,
A high reception level is when a short-distance broadcast is received, and no fading occurs at this time.

The present invention focuses on the above points, and as shown in FIG. 1, for example, operational amplifiers A1 and A2 for converting an AM detection output into a pair of balanced AM detection outputs, and outputs of the operational amplifiers A1 and A2. And a pair of transistors Q3 and Q4 with sufficiently low input impedance
And a capacitor C1 that is charged and discharged by a difference current between the output currents of the transistors Q3 and Q4, and a pair of AM detection outputs obtained by voltage-current conversion by the resistors R1 and R2 are respectively applied to the transistors Q3 and Q4. Supplied and the terminal voltage E of the capacitor C1
s is supplied to the high frequency signal system 2 to the intermediate frequency signal system 5 so that AGC is performed, and the capacitor C
Control circuits Q15 and Q16 for controlling the magnitude of the current charged and discharged to 1 in accordance with the reception level of the reception signal, and changing the time constant of the AGC in accordance with the reception level of the reception signal It is characterized by:

[Action]

According to the present invention, the transient characteristics of the AGC include resistors R1 and R2,
Since it is determined by the time constant of the capacitor C1 and becomes independent of the loop gain of the AGC, it is possible to obtain the optimum AGC characteristic required for the receiver.

Also, the loop gain of the AGC has a magnitude corresponding to the ratio of the resistance values of the resistors R1 and R2 and the output impedances of the transistors Q3 and Q4, but since the output impedance of the transistors Q3 and Q4 is infinite in an ideal state, This ratio becomes very large. Therefore, since a sufficiently large loop gain can be obtained, good AGC is performed.

In addition, since a sufficiently large loop gain can be obtained, the amplifier can be a simple buffer amplifier having a gain of 1 or less. Therefore, the noise level in this amplifier becomes so small as to be negligible, so that the S / N does not decrease without providing a low-pass filter for removing noise in the subsequent stage.

Further, since a low-pass filter for removing noise is not required, an external terminal pin for externally connecting a capacitor of the low-pass filter is not required when the AGC circuit is formed into an IC, which is suitable for the IC.

Also, by controlling the magnitude of the current charged and discharged to the capacitor C1 according to the reception level of the reception signal, the AGC
Is changed in accordance with the reception level, so that distortion is small and the sound volume is received stably with respect to fading.

〔Example〕

In FIG. 1, A ₁ and A ₂ denote operational amplifiers. The amplifiers A ₁ and A ₂ are connected as shown in FIG. It operates as an inverted output terminal, a level detection circuit having an inverted output terminal, or a voltage comparison circuit (11).

Then, the non-inverting input of amplifier A _1, detection circuit (6)
With detection voltage Ea is supplied from a predetermined reference voltage Er to the non-inverting input terminal of the amplifier A ₂ is supplied.

Further, the emitters of the transistors Q ₁ and Q ₂ are connected to the power supply terminal T ₁ , and a predetermined bias voltage E ₁ is supplied to their bases, so that the transistors Q ₁ and Q ₂ are respectively constant current sources. Constant currents I and I are taken out of those collectors, respectively.

The bases of the transistors Q ₃ and Q ₄ are connected to the collector of a transistor Q _{13 to be} described later, so that the transistors Q ₃ and Q 4
₄ is a common base, and an output terminal of the amplifier A _1, A _2, between the emitter of the transistor Q _3, Q _4, resistors R _1, R ₂ (R ₁
= R ₂ ) and their emitters are connected to the collectors of transistors Q ₁ and Q ₂ , respectively.

Further, the emitters of the transistors Q ₅ and Q ₆ are grounded,
Their bases as a reference potential point ground is connected to the collector of the transistor Q _5, and is constructed current mirror circuit A ₃ to the input side of the transistor Q _5. And
The collectors of transistors Q ₅ and Q ₆ are connected to transistors Q ₃ and Q ₄
Connected to the respective collectors.

Further, between the ground and the collector of the transistor Q _6, the capacitor C ₁ is connected, the terminal voltage of the capacitor C ₁ Es
But is supplied to the base of the transistor Q _7, between the emitter and the terminal T ₁ of the transistor Q _7, the transistor Q ₈ is connected transistors Q ₇ for the constant current source is an emitter follower.

Thus, the emitter of the transistor Q ₇ is voltage Es is retrieved, the voltage Es is, the high-frequency signal system to the intermediate frequency signal system, in this example, a high frequency amplifier (2) and an intermediate frequency amplifier (5) to the gain It is supplied as a control signal.

In addition, the voltage Es from the emitter of the transistor Q ₇ is,
It is supplied to the base of the transistor Q ₁₂ through the transistor Q ₁₁ of the emitter follower.

The transistor Q _12, together with the transistor Q _13,
A differential amplifier type level detection circuit or a voltage comparison circuit (12) is configured. That is, transistors Q ₁₂ and Q
₁₃ and the emitter of between the terminal T _1, the transistor Q ₁₄ of the constant-current source is connected, the reference current Eo is supplied to the base of the transistor Q _13, between Kurekuta transistors Q _12, Q ₁₃ resistors _{R 3, R 4 (R 3} = R 4) are connected in series, the connection point is grounded via a resistor R _5.

Further, transistors Q ₁₅ and Q ₁₆ are provided, their emitters are connected to the emitters of transistors Q ₃ and Q ₄ , respectively, the collectors of transistors Q ₁₅ and Q ₁₆ are grounded, and their bases are connected to transistor Q _12. Connected to the collector.

In such a configuration, for simplicity, a transistor
And Q ₁₁ ~Q ₁₆ is not connected. It is also assumed that the received signal is in a non-modulated state, and therefore, e = 0 and Ea = E.

Then, the output terminal of the amplifier A _1, is taken out voltage (E-Er), the output terminal of the amplifier A _2, the voltage (Er-E) is taken out.

At this time, the transistors Q ₃ and Q ₄ function as a common base, and the input impedance seen from the emitter is sufficiently small.

Therefore, the resistor R ₁ R ₂ has the following relationship: I ₁ = (E−Er) / R ₁ I ₂ = (Er−E) / R ₂ = −I ₁ (∵R ₁ = R ₂ ) (i) As shown currents I ₁ and I ₂ flow, these currents
I ₁ and I ₂ also flow to the emitters of transistors Q ₃ and Q ₄ . Also, the transistors Q ₁ and Q ₂ work as a constant current source to
I and I are output, and these currents I and I also flow through the emitters of the transistors Q ₃ and Q ₄ . Therefore, the currents (I + I ₁ ) and (I + I ₂ ) are applied to the emitters of the transistors Q ₃ and Q _4.
Flows, and currents (I + I ₁ ) and (I + I ₂ ) are output from the collectors of the transistors Q ₃ and Q ₄ .

At this time, with the collector current of the transistor Q ₃ are also the collector current of the transistor Q _5, the transistors Q _5, Q ₆ constitute a current mirror circuit A _3, the collector of the transistor Q _6, the current (I + I ₁ )
Will flow in.

Thus, the capacitor C _1, and the current (I + I ₂₎ from the transistor Q _4, the current to the transistor Q ₆ (I +
I ₁₎ difference in current between _{Is Is = (I + I 2} ) - (I + I 1) = 2 (Er-E) / R 2 ...... (ii) (∵ (i) formula) so that the flows. That is, the capacitor C ₁
When> 0, the battery is charged by the current Is. When Is <0, the current Is is discharged. When Is = 0, charging / discharging is not performed.

Thus, the capacitor C _1, the terminal voltage Es obtained corresponding to the current Is, the amplifier (2) The voltage Es is through the transistor Q _7, are supplied as the AGC voltage to (5).

Then, if E = Er, Is = 0, and the so charging and discharging of the capacitor C ₁ is not performed, the terminal voltage Es becomes directly from (ii) expression, thus, the amplifier (2),
The gain of (5) is maintained as it is.

But now, if the reception level of the received signal is less even though E> Er, (ii) next Is <0 from equation, the current Is is discharged from the capacitor C _1, the terminal voltage Es Decreases. Then, if the AGC loop is closed, the voltage Es is reduced until the transistor Q ₆ is several tens mV enters the saturation region, but the voltage Es amplifier (2), the control signal of the gain (5) , The gains of the amplifiers (2) and (5) increase, and the level of the received signal or the intermediate frequency signal passing through the amplifiers (2) and (5) is increased.

Further, <if Er, Is from (ii) expression> Despite E reception level is greater of the received signal since been charged the terminal voltage Es rises by 0, the capacitor C ₁ is the current Is, The gain of the amplifiers (2) and (5) decreases, and the level of the received signal or the intermediate frequency signal passing through the amplifiers (2) and (5) is reduced.

Therefore, from the above, the gains of the amplifiers (2) and (5) are stabilized at a value where E = Er. Therefore, the detection output voltage Ea becomes constant regardless of the reception level of the reception signal. That is, AGC was performed.

At the time of actual reception, the received signal is modulated and e ≠ 0. Therefore, the current Is includes an AC component corresponding to the audio signal component e.
The output impedance of the transistors Q ₄ and Q ₆ and the capacitor C ₁ are removed by a low-pass filter. The terminal voltage Es does not include the audio signal component e, and the AGC is performed as described above.

Then, in addition to the above AGC operation, the transistors Q ₁₁ ,
The following operation is carried out by Q _16.

That is, the terminal voltage (AGC voltage) of the capacitor C ₁ Es
Since the base-emitter voltages of the transistors Q ₇ and Q ₁₁ have opposite polarities, the transistors have almost the same level.
It is supplied to the base of Q _12.

As described above, when a large reception level of the received signal is AGC voltage Es is increased, at the Es> Eo, the collector current of the transistor Q ₁₂ is reduced, the potential of the collector is low, transistor
With the impedance between the emitter and the collector of Q _15, Q ₁₆ decreases, the collector current of the transistor Q ₁₃ is increased, the impedance between the emitter and the collector of the transistor Q _3, Q ₄ increases. Therefore, a part of the current (I + I ₁ ) flowing from the transistor Q ₁ and the resistor R ₁ into the transistor Q ₃ is shunted through the transistor Q ₁₅ , and the shunt current increases, so that the current (I + I ₁ ) flowing into the transistor Q _{3 1} ) becomes smaller. For the same reason, the current flowing into the transistor Q ₄ (I +
I ₂ ) also decreases at the same rate.

Therefore, the currents (I + I ₁ ) and (I + I ₂ ) flowing through the collectors of the transistors Q ₄ and Q ₆ are reduced, and the difference current
Is also decreases, but the smaller the current Is, when the capacitor C ₁ is charged and discharged by the current Is, the change in the terminal voltage Es is slower, this capacitor C ₁ and the transistor
This is equivalent to an increase in the time constant of the output impedance of Q ₄ and Q ₆ . That is, when the reception level of the reception signal is high, the time constant for the AGC voltage Es increases.

At this time, the time constant of the AGC becomes large, so that the AGC cannot follow the fading.
As described above, the time constant increases when the level of the received signal is high, that is, when a short-range broadcast is being received. At this time, since no fading occurs in the received signal, There is no problem even if the constant is large.

On the other hand, when the reception level of the reception signal is low, AGC
The voltage Es decreases, but when Es <Eo, the transistor Q
_As the collector current of ₁₂ increases, the transistor
Since the collector current of Q ₁₃ is reduced, contrary to the case of Es> Eo, current shunting transistor Q _15, Q ₁₆ (I +
I ₁ ) and (I + I ₂ ) are reduced and the transistor
The currents (I + I ₁ ) and (I + I ₂ ) flowing into Q ₃ and Q ₄ increase.

Thus, current Is becomes large relative to the capacitor C _1, the change in the terminal voltage Es is faster, this AGC voltage E
This is equivalent to a smaller time constant for s.

At this time, the distortion of the audio signal e supplied to the amplifier (7) increases due to the decrease in the time constant of the AGC, but the time constant decreases when the reception level of the reception signal is low. In other words, this is generally the case when a long-distance broadcast is being received. At this time, since the quality of the received signal itself has been slightly deteriorated, there is no problem even if the distortion is slightly increased.

Thus, according to the present invention, AGC can be performed. In this case, in particular, according to the present invention, the time constant of the AGC is high for a reception signal having a large reception level, that is, for a short-range broadcast wave. Audio signal e
Can be minimized, and good listening can be achieved.

In addition, for a reception signal having a small reception level, that is, for a broadcast over a long distance, the time constant of the AGC becomes small.
Even with fading, you can listen to the broadcast at a stable volume.

Further, the low-pass filter for AGC voltage Es, so 1 order filter by the output impedance of the capacitor C ₁ and the transistor Q _4, Q _6, without or cause the overshoot transient characteristics, feeling during tuning It will be good. In addition, when an IC is used, this is easy, and there is no increase in external terminal pins. In addition, since the connection is in a balanced configuration, it is suitable for use in an IC from this point as well, and is stable against changes in operating voltage and temperature.

Also, the loop gain of the AGC is the ratio between the resistor R ₁ (R ₂ ) and the output impedance of the transistors Q ₄ and Q ₆ (which is infinite in an ideal state), and since this ratio is extremely large, , AGC has an extremely large loop gain, so that the AGC characteristics are good and the receiving characteristics are also improved.

Furthermore, in order to increase the AGC loop gain, the AGC
When the voltage Es is amplified by a high-gain amplifier, noise generated by the high-gain amplifier is included in the AGC voltage Es, and the noise changes the gains of the amplifiers (2) and (5). , As a receiver, the S / N is reduced, but in the above, the resistor R ₁ (R ₂ ) and the transistor
Q _4, the ratio of the output impedance of Q _6, since the greater the loop gain of the AGC, the AGC voltage Es need not be amplified by the high gain amplifier, thus, good S / N as a receiver Becomes

Furthermore, the transient characteristics of the AGC are determined by the time constant of the resistor R ₁ (R ₂ ) and the capacitor C ₁ and are independent of the loop gain of the AGC.
AGC characteristics can be obtained.

In the above description, the amplifiers (2) and (5)
The AGC can either be a delayed AGC or one can be omitted.

〔The invention's effect〕

According to the present invention, the time constant of the AGC is increased for a reception signal having a high reception level, that is, for a short-range broadcast wave, so that distortion to the audio signal e can be minimized, and good listening can be performed.

In addition, for a reception signal having a small reception level, that is, for a broadcast over a long distance, the time constant of the AGC becomes small.
Even with fading, you can listen to the broadcast at a stable volume.

Further, the low-pass filter for AGC voltage Es, so 1 order filter by the output impedance of the capacitor C ₁ and the transistor Q _4, Q _6, without or overshooting the transient characteristics, feeling during tuning It will be good. In addition, when an IC is used, this is easy, and there is no increase in external terminal pins. In addition, since the connection is in a balanced configuration, it is suitable for use in an IC from this point as well, and is stable against changes in operating voltage and temperature.

Also, the loop gain of the AGC is the ratio between the resistor R ₁ (R ₂ ) and the output impedance of the transistors Q ₄ and Q ₆ (which is infinite in an ideal state), and since this ratio is extremely large, , AGC has an extremely large loop gain, so that the AGC characteristics are good and the receiving characteristics are also improved.

Furthermore, in order to increase the AGC loop gain, the AGC
When the voltage Es is amplified by a high-gain amplifier, noise generated by the high-gain amplifier is included in the AGC voltage Es, and the noise changes the gains of the amplifiers (2) and (5). , As a receiver, the S / N is reduced, but in the above, the resistor R ₁ (R ₂ ) and the transistor
Q _4, the ratio of the output impedance of Q _6, since the greater the loop gain of the AGC, the AGC voltage Es need not be amplified by the high gain amplifier, thus, good S / N as a receiver Becomes

Furthermore, the transient characteristics of the AGC are determined by the time constant of the resistor R ₁ (R ₂ ) and the capacitor C ₁ and are independent of the loop gain of the AGC.
AGC characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a connection diagram of an example of the present invention, and FIGS. 2 and 3 are diagrams for explanation thereof. A ₁ and A ₂ are operational amplifiers, A ₃ is a current mirror circuit, (1
1) and (12) are voltage comparison circuits.

Claims (1)

(57) [Claims] 1. An operational amplifier for converting an AM detection output to a pair of balanced AM detection outputs, a pair of resistors for converting the output of the operational amplifier into a voltage and a current, and a pair of transistors having sufficiently low input impedance. And a capacitor that is charged and discharged by a difference current between the output currents of the pair of transistors. A pair of AM detection outputs, which are voltage-current converted by the resistor, are supplied to the pair of transistors, respectively. The terminal voltage of the capacitor is supplied to a high-frequency signal system or an intermediate frequency signal so that AGC is performed, and the magnitude of the current charged and discharged in the capacitor is determined according to the reception level of the reception signal. An AGC circuit having a control circuit for controlling, wherein the time constant of the AGC is changed according to a reception level of the reception signal.