JPS6251307A - Feedforward reception circuit - Google Patents

Abstract

PURPOSE:To obtain a stable characteristic even at the input of a low level by providing a level detection means detecting an output level (or input level) of a reception circuit and a control means controlling the stop/restart of the feedforward operation of a feedforward circuit. CONSTITUTION:Since the high frequency signal inputted to a mixer circuit 4 has a level sufficiently low because of the attenuation of an attenuator 2, an intermediate frequency signal only exists at the output side of the mixer circuit 4 and no other distortion exists. A part of an output signal of the mixer circuit 3 is extracted and the intermediate frequency signal including the distortion component is attenuated to the same level as that of the intermediate frequency signal at the output side of the mixer circuit 4, the result is added in opposite phase by a differential amplifier 7 via a switching circuit 11 and only the distortion component generated by the mixer circuit 3 is outputted to the output side of the differential amplifier 7. The distortion component is amplified to the same level as the distortion component at the output side of the mixer circuit 3 by the differential amplifier 7, added in opposite phase with the signal at the output side of the mixer circuit 3 by a differential amplifier 8 so as to obtain an intermediate frequency signal at the output terminal 9 in which the distortion component is suppressed completely.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to the configuration of an AGC circuit in a feedforward amplifier and a feedforward mixer.

[Background of the invention]

A typical conventional example of a feedforward circuit is the one published in Japanese Patent Application Laid-open No. 5
It is described in Japanese Patent No. 5-57011. Here, a low distortion feedforward amplifier is described.

A feedforward amplifier is an amplifier that extracts only the distortion component generated by the amplifier and adds it in reverse phase to the original signal containing the distortion component to obtain a good signal component without distortion. If this feedforward circuit is used in a small signal amplification circuit or mixer circuit used in a tuner or converter, it will work effectively for input signals at a level that causes distortion in the amplifier circuit or mixer circuit, but the amplifier circuit When a low-level signal that does not cause distortion is input to the feedforward circuit or mixer circuit, the noise generated by the adder or differential amplifier circuit in the feedforward circuit is added to the signal, causing deterioration of characteristics. Ta.

Further, when this circuit is used in a tuner or a converter, there is a similar drawback in that characteristics deteriorate when a low-level signal is input.

[Purpose of the invention]

An object of the present invention is to provide a feedforward circuit that can obtain stable characteristics even when a low level input is applied.

Another object of the present invention is to provide a tuner that is suitable for IC implementation and has stable characteristics using this feedforward circuit.

[Summary of the invention]

Feedforward circuits are effective circuits for removing distortion generated in amplifiers and mixer circuits, but they have the following drawbacks when the signal input level to the circuit is low enough that distortion does not occur.

Second, the feedforward circuit itself is a source of noise and deteriorates the characteristics of the output signal.

In order to eliminate these drawbacks, the output level of the circuit (
Alternatively, depending on the input level, the feedforward operation may be switched on or off.

By using this circuit, a feedforward circuit that operates stably even at low input levels can be obtained.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail according to embodiments shown in the drawings.

FIG. 1 shows an embodiment of the present invention. This separates the high frequency signal inputted through the terminal 1 into two signals, one of which is input to the mixer circuit 3 and the other to the mixer circuit 4 through the attenuator 2. Local oscillation signals from the local oscillation circuit 5 are injected into the mixer circuits 3 and 4. Since a high-level radio frequency signal is input to the mixer circuit 3, various distortion components are present in addition to the intermediate frequency signal on the output side.

On the other hand, since the high frequency signal input to the mixer circuit 4 is attenuated by the attenuator 2 and has a sufficiently low level, only the intermediate frequency signal exists on the output side of the mixer circuit 4.
There are no other distortion components.

Here, a part of the signal on the output side of the mixer circuit 3 is extracted, and the intermediate frequency signal containing this distortion component is sent to the attenuator 6, so that the level is the same as that of the intermediate frequency signal on the output side of the mixer circuit 4. The signals are attenuated to a maximum of 100 nm, and added in reverse phase to the differential amplifier 7 via the switching circuit 11, and only the distortion components generated in the mixer circuit 6 are output to the output side of the differential amplifier 7. This distortion component is amplified by the differential amplifier 7 to the same level as the distortion component on the output side of the mixer circuit 3, and added in reverse phase to the signal on the output side of the mixer circuit 3 in the differential amplifier 8. To obtain an intermediate frequency signal with completely suppressed distortion components.

Here, the transmission gain of attenuator 2 is set to A1. The conversion gain of mixer 3 is A2+the conversion gain of mixer 4 is A3. The transmission gain of the attenuator 6 is set to A4. The amplification degree of the differential amplifier 7 is set to A5. Differential amplifier 8
If the amplification degree of is A6, then A,+A5=A2+A4...
・(1) A4+ A5= 0
・・・・・・The two equations in (2) must be satisfied, A5
Adjustment is made by making either A or A4 variable.

Further, since the input level to the differential amplifier 7 is sufficiently low, no distortion occurs. Furthermore, in order not to generate distortion components in the differential amplifier 8, the differential amplifier 8 is used as a mere adder, and A=0.

Further, as an AGC circuit, a part of the signal is extracted from the output of the output differential amplifier 8, subjected to DC detection in the detection circuit 10, and this detected voltage is fed back to the switching circuit 11.

The operating principle and features of this AGC circuit will be explained below.

Now, in mixer circuit 3, the lowest input level at which distortion occurs is Pmi. Then, the input level is P. ,.

In the following cases, no distortion occurs in mixer circuit 3, and mixer circuit 4. Attenuator 2, 6. A feedforward circuit consisting of the differential amplifier 7 and the like becomes unnecessary.

On the other hand, if the feedforward circuit is operated with the input level below P,i□ and no distortion occurs,
There is a very high possibility that malfunction or noise will occur in the differential amplifier 7 or the like, which will be added to the signal and degrade its characteristics.

It is clear that noise generation is random and occurs with the same probability regardless of the signal input level, or that when the signal input level is low, the influence of the noise is particularly large and 9 deteriorates.

In order to eliminate the disadvantages of the feedforward circuit described above, the feedforward circuit is switched depending on the output level of the differential amplifier circuit 8. In other words, the detected voltage at the detector 10 when the input signal level is Psin is vai. Then, when the detected voltage is equal to or higher than Vsin, both ends of the switching circuit 11 are conductive and normal feedforward operation is performed, but when the detected voltage is Vai
When it becomes less than n, the impedance seen from the differential amplifier circuit 7 to the switching circuit will not be short-circuited or open, the feedforward circuit will be cut off, and a stable DC voltage will be supplied to the output side of the differential amplifier circuit 7. Therefore, a stable output signal from the terminal 9 can be obtained without noise or malfunction occurring in the feedforward circuit being added to the signal.

A circuit example of the switching circuit 11 is shown in FIG.

Terminal 14 is connected to the output of attenuator 6 and mixer circuit 4, terminal 15 is applied to differential amplifier 7, and terminal 16 is applied with a detection voltage from detection circuit 10.

Reference numeral 17 is a reference voltage generation circuit, and the output voltage value can be freely set.

Now, assuming that the output voltage of the reference voltage generator 17 is the above-mentioned Vain, when the detection voltage of the detection circuit 10 is equal to or higher than V, □n, the diode 18 and the transistor 19 are turned on, and the diode 18 and the transistor 19 are turned off. Tonanashi, the feedforward circuit works. Conversely, the input level from terminal 1 decreases and the detection voltage of the detection circuit decreases.
/sin or less, the diode 18. Transistor 19 is turned off, diode 18' and transistor 19' are turned on, the feedforward circuit is cut off, and the output of differential amplifier circuit 7 becomes a stable DC voltage (terminal 9). It will be done.

The second embodiment of the feedforward circuit of the present invention is described in the third embodiment.
As shown in the figure.

This is because, whereas the switching circuit 11 in FIG. 1 separately controls the two terminals input to the differential amplifier circuit 7, the switching circuit 12 is provided between the differential amplifier circuits 7. It is something.

A detailed circuit diagram of the switching circuit 12 is shown in FIG. Similar to the explanation in FIG. 2, this means that the reference voltage generation circuit 17 is
. Then, the diode 20 becomes O.
N, the feedforward circuit is cut off, and the output of the differential amplifier circuit 7 becomes a stable DC voltage.

FIG. 5 shows a third embodiment. This is Figure 1.

Unlike the embodiment shown in FIG. 3, a switching circuit 13 is provided on the output side of the differential amplifier circuit. This switching circuit 13 is a type that short-circuits at high frequency when the detected voltage is less than Vain. . This switching circuit 13
The circuit shown in FIG. 2 (b) or (d) is used for this purpose.

The terminal 14 is connected to the output side of the differential amplifier 7, the terminal 15 is connected to the input of the differential amplifier 8, and the terminal 16 is connected to the detection circuit 10. The output of the reference voltage generation circuit 17 is vsi. The detection voltage of the detection circuit 10 is set to vman
In the above case, when the diode 18' is turned off, the feedforward circuit operates and the input signal level drops to 9.
When the detection voltage becomes less than Vain, the diode 18' becomes O.
As a result, the output circuit of the differential amplifier 7 is short-circuited at high frequency, and a stable DC voltage is supplied to the differential amplifier 8. Therefore, a more stable output signal can be obtained from the output terminal 9.

In FIG. 1, the switching circuit 11 is not limited to this location, but is located between the contact 21 and the attenuator 2, between the attenuator 2 and the mixer 4, and between the attenuator 6 and the contact 22.
It is clear that similar characteristics can be obtained even if the arrangement is between the two.

In addition, in FIGS. 1, 3, and 5, the signal detection position is between the differential amplifier 8 and the output terminal 9, but it may be between the input terminal 1 and the mixer circuit 3 or between the mixer circuit 3. It is clear that similar characteristics can be obtained even if the circuit is set between the circuit 3 and the differential amplifier circuit 8.

Further, although the present invention has been described with respect to a mixer circuit, it is clear that similar effects can be obtained with an amplifier circuit.

Embodiments in which the present invention is applied to feedforward amplifiers are shown in FIGS. 6, 7, and 8. Figure 6.

Figures 7 and 8 are the mixer 3 of Figures 1, 3, and 5.
is replaced with amplifier 26, and attenuator 2. The mixer 4° has a configuration in which the local oscillation circuit 5 is removed, and the switching circuits 30, 3
2.35 are switching circuits 11 and 12.15 respectively
has the same function. Similar to the mixer circuit described above, this feedforward amplifier circuit can also obtain a good output signal even when a low level signal is input.

FIG. 9 shows the configuration of a tuner using the feedforward circuit described above. A high frequency signal is input through the terminal 34, and a variable or fixed bandpass filter 3
6 to a feedforward amplifier 37.

The feedforward amplifier 37 is connected to an AGC circuit 38 and further connected to a feedforward mixer circuit 39.

The feedforward mixer circuit 59 includes a local oscillator 41.
More oscillation power is injected. A signal is input from the feedforward mixer circuit 39 to the feedforward amplifier 40 of the IF, and the output of the feedforward amplifier 40 is used as the output terminal 35 of the tuner. In addition, in order to detect the AGC voltage, the AGC voltage is applied to the output of the feedforward amplifier 40.
A detection circuit 42 is provided, and this detected voltage is fed back to the AGC circuit 38 for gain control, and this detected voltage is used as the switching voltage of the feedforward amplifiers 57 and 40 and the feedforward mixer 39.

Detection circuit 10.5 shown in Figures 1 to 9
A is essential for tuners, converters, etc.
A GC voltage detection circuit is used to reduce the size and simplification of the circuit.

FIG. 10 shows a second tuner configuration. This is a tuner with the same configuration as in FIG. 9. AGC detection circuit 42
The AGC voltage outputted from the switch is inputted to a switching voltage generating circuit 43, which generates a switching voltage suitable for the AGC voltage, and supplies the switching voltage to the feedforward circuits 37, 39, and 40.

By using this circuit, it is possible to absorb variations in the AGC detection voltage due to tuner variations and supply a constant switching voltage.

Above is Part 9. As shown in Section 10, a feedforward circuit is used for the amplifier, a feedforward mixer is used for the mixer, and an AG is used for the switching voltage of the distortion suppression circuit.
By using the C voltage, it is possible to simplify the filter, make the tuner a smaller IC, and stabilize the characteristics.

In the present invention, the switching circuit of the distortion suppression circuit of the feed 7 backward mixer and the feedforward amplifier has been explained using the circuit shown in FIG. 2 or 4, but as shown in FIG. 11 and FIG. The gain of the differential amplifier 7 or 28 may be controlled.

An example of this detailed circuit is shown in FIG. FIG. 13 is a diagram showing the basic circuit of a differential amplifier circuit.The basic voltage generation source 17 is set to the above-mentioned vsin, and the voltage detected by the detection circuit 10 or 31 is output between the terminals 16 and 16. applied to the base of a dynamic amplifier. The detection voltage is v, i. In the above case, the transistors 45 and 46 perform normal feedforward operation, but when the input signal level decreases and the detection voltage becomes less than vsin, the transistors 45 and 46 turn OFF and the output terminal The stable DC voltage applied from the terminal 51 is outputted to the terminal 15, and the stable DC voltage is supplied to the differential amplifier circuit 8 or 27, so that a stable signal is obtained from the terminal 25.

Further, although the present invention has shown an example in which a differential amplifier circuit is used as a signal subtracter, the present invention is not limited to this.
It is clear that similar characteristics can be obtained using other subtracters as shown in the figure.

〔Effect of the invention〕

According to the present invention, in a feedforward circuit, at an input level where an amplifier or mixer in the circuit generates distortion,
The circuit operates as a normal distortion suppression feedforward circuit, whereas when the signal input level to the circuit is lowered,
When the amplifiers and mixers in the circuit do not generate distortion, the distortion suppression feedforward circuit is not operated, and by using a configuration that operates as a normal amplifier or mixer, the feedforward circuit is activated when the signal input level is low. It is possible to prevent the addition of noise generated from the differential amplifier circuit in the circuit to the signal and to prevent malfunctions that occur in the differential amplifier circuit, etc., and the S/N is maintained with low distortion when any signal is input.
This has the effect of providing a good output signal and a stable output signal.

In addition, by applying this feedforward circuit to a tuner,
Furthermore, AGC is used as the switching voltage of the distortion suppression circuit.
By using the detected voltage, filters for suppressing unnecessary waves can be simplified, made smaller, and integrated into ICs.

Effective in stabilizing characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a feedforward circuit as an embodiment of the present invention, FIG. 2 is a circuit diagram showing an example of a switching circuit of the feedforward circuit in FIG. 1, and FIG.
The figure is a circuit diagram showing a feedforward circuit as another embodiment of the present invention, Figure g4 is a circuit diagram showing a circuit example of a switching circuit of the feedforward circuit in Figure 3, and Figure 5 is another embodiment of the present invention. 6.7.8 is a circuit diagram showing a feedforward circuit as a further embodiment of the present invention, and FIGS. FIG. 11 and FIG. 12 are circuit diagrams showing a feedforward circuit as yet another embodiment of the present invention, and FIG. 13 is a block diagram showing the main points of an embodiment of the present invention. FIG. 14 is a circuit diagram showing a feed forward circuit as still another embodiment of the present invention. Explanation of symbols 3.4...Mixer 5...A local oscillation circuit 7.8...Differential amplifier circuit 10.31...Detection circuit 11.12, 15 , 30j2, 33... Switching circuit 18.18.20... Diode 1
9.19.45,46...Transistor 17.
...Reference voltage generation source 26...Amplifier 36...Filter 57.40...Feedforward amplifier 42
...AGO detection circuit 43 ...Voltage generation circuit 39 ...Feedforward mixer 49 ...
...Subtractor

Claims (1)

[Claims] 1) In a receiving circuit including a feedforward circuit,
A level detection means for detecting the output level (or input level) of the receiving circuit, and a control means for controlling the stop and restart of the feedforward operation of the feedforward circuit depending on the detected level. A feedforward receiver circuit featuring: