JPS62226705A - Automatic gain control amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent the increase in distortion component and the deterioration of noise figure by providing an attenuator connected to an amplifier between the lst stage common emitter amplifier circuit and a common base amplifier circuit, amplifying an input signal by the common emitter amplifier circuit and applying gain adjustment before the common base amplifier circuit of a low impedance. CONSTITUTION:The attenuator 40 using a pin diode is connected between a collector of a transistor (TR) Q1 and an emitter of a TR Q2. In supplying an automatic gain control voltage to a base of a TR Q3 via a resistor R15, the current is controlled. Thus, the impedance of the pin diode D1 and a bypass impedance of the pin diode D2 are varied to vary the attenuation of the attenuator 40. That is, after an input signal is amplified, the result is attenuated by the attenuator to apply gain adjustment at the impedance side of the common amplifier section, then distortion is suppressed.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application)
This invention relates to an automatic gain amplifier circuit used in an updown tuner (referred to as V).

(Prior Art) In tuners for CATV systems and the like, an updown tuner as shown in FIG. 3 is used. This tuner 111 selects the J9r I channel signal from among the high frequency television signals transmitted from the main unit, converts it to a high frequency, and then converts it to a channel that can be received by the tuner of the television receiver (for example, 2 channels). y channel) frequency. The purpose of temporarily converting to a higher frequency in this way is to distance the image channel after frequency conversion from the original channel and make it easier to tune since there are many transmission channels in the CATV system. In FIG. 3, the signal (g) supplied to the input terminal 10 is supplied to the first oscillator 14 via the high-pass filter 11 and the low-pass filter I2.

The first mixer) 3 mixes the oscillation output from the first oscillator 14 and the input signal to create intermediate frequency 1g.

The first oscillator 14 is a local oscillator for tuning, and its excavation frequency is varied by the control signal from the tuning circuit 15. , the intermediate frequency signal from the first mixer 13 is supplied to the second mixer 19 via a band pass filter 16, an intermediate frequency amplifier 17, and a band pass filter 18. Second
The mixer 19 mixes the intermediate frequency signal and the oscillation output from the second oscillator 20, and converts the mixed signal into, for example, the frequency of the second channel. The second channel signal thus obtained is led out to the output terminal 22 via the output circuit 21f. Note that 23 is a switching circuit used to switch the oscillation frequency of the second oscillator 20 when changing the channel frequency supplied to the television receiver.

The above-mentioned computer is housed in a 7-fold box 30 as shown in Fig. 3, for example, to ensure its storage. The same numbers are attached to each part of the shield 9 box 3θ to indicate the arrangement position of the circuit.

By the way, in the above-mentioned up-down tuner, the first mixer 13 is composed of a diode. This type of diode-based mixer has a conversion loss, so there is a limit to how much it can improve the noise figure. Therefore, in order to improve the above-mentioned noise figure, it is desired to provide a preamplifier 25 between the high-pass filter 11 and the low-pass filter 12. As this preamplifier 25, a cascade-connected amplifier circuit is generally used for the purpose of broadband amplification. However, simply providing a cascade-connected amplifier circuit causes problems such as generation of distortion components due to its nonlinear characteristics and deterioration of the noise figure.

(Problems to be Solved by the Invention) Generally, when an amplifier has nonlinear distortion, the input signal voltage e and the output signal voltage y (e) of the amplifier have the following relationship.

Kn: Coefficient indicating the linearity of the amplifier n: Distortion order In the above equation (1), in reality, the second-order distortion component (n=2),
The third-order distortion component (n=3) becomes a problem.

In order to avoid such distortion components, it is better not to install an amplifier that causes distortion, but NF
An amplifier is required for (noise figure) improvement.

Therefore, when the preamplifier 25 is provided, it becomes necessary to manage the input level of the tuner and the gain of the amplifier in order to suppress the occurrence of distortion as much as possible. First, the upper limit of the input level of the cheese is limited.

To this end, a method is generally used in which an attenuator using a bin diode is disposed at the front stage of the preamplifier to provide an attenuation effect, thereby suppressing the occurrence of distortion when a large input is applied. However, with this method, when m stages of amplifiers are connected in series, the power gain of each amplifier is G l, G 2 .・・・・・・
- Qm and noise figure is NFJ.

NF2. ...N F''m, the overall noise figure NFt is as follows: (2) As shown in (2), the noise figure deteriorates by the amount of attenuation at the input.

As mentioned above, providing an amplifier to suppress distortion components has the contradictory problem of worsening the noise figure. In order to further reduce the noise figure, it is better to increase G in equation (2) or increase the gain, but as can be seen from equation (1), the distortion component of the amplifier itself will also increase.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an automatic gain control amplifier circuit that can suppress both the distortion caused by the nonlinearity of the amplifier itself and the deterioration of the noise figure when controlling the gain of the preamplifier described above. Objective O [Structure of the Invention] (Means for Solving Problems) In this invention, by devising the arrangement position of the attenuator connected to the amplifier, in other words, the attenuator is connected to the first-stage emitter-grounded amplifier circuit and the base-grounded It is installed between the amplifier circuits.

(Function) With the above configuration, this automatic gain control circuit amplifies the input signal with the common emitter amplifier circuit and adjusts the gain before the low impedance common base amplifier circuit.
Increase in distortion components and deterioration of noise figure can be prevented.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows one embodiment of the present invention, and is an example applied to an anog down tuner of a CATV system, for example.

That is, the circuit of the present invention is provided between the high-pass filter 1 and the low-pass filter 2 of the up-down/tuner, and the other parts are the same as the circuit shown in FIG. Therefore, the same parts as in FIG. 3 will be described with the same reference numerals.

The output of the high-pass filter 1 is the combined content (Co)
is supplied to transistor Q7 via fr.

The transistor Ql constitutes an emitter follower circuit, and its emitter is grounded through a parallel circuit of a resistor (Iζ1), a resistor (R), and a conductor 7f (CJ). Further, the base of this transistor Q7 is grounded through a resistor R3. ) Transistor QJ and transistor Qz constitute a cascade-connected amplifier circuit, and the emitter of transistor Q2 is connected to the collector of transistor Q through choke coil L.

In addition, from the power supply terminal 30, after passing through the resistor R6f,
A bias is supplied to the base of transistor Q2, and this bias is also supplied to the base of transistor Q1 via resistor R,5. Historically, a feedback circuit including a resistor R4 and capacitors /fC3 and C4 is connected between the collector of the transistor Q2 and the base of the transistor Q1. Note that the capacitor C5 between the base of the transistor Q2 and the ground is a noise/arm capacitor. Further, power is supplied to the collector of the transistor Q2 via a choke coil L2'ft.

Next, the collector of transistor QJ and transistor Q2
An attenuator 40 using a pin diode is connected between the emitters. The collector of the transistor Q is connected to the cathodes of bin diodes DI and D2 via a coupling capacitor C1lf, and is grounded via a resistor l'tllf. The anode of the pin diode D horn is connected to the emitter of the transistor Q2 via the capacitor "jc12'fr" and is also connected to the emitter of the transistor Q2.
2 to the emitter of transistor Q3. In addition, the anode of P/Dio-￥012 is resistor 'i
<13 and is grounded through a resistor R14'f,
1 to the collector of transistor Q3. In history, the anode of the bin diode D)2 and the transistor Q
A capacitor (Near 3, C14
The L-column circuit is connected, and the midpoint between the capacitors C13 and C)4 is grounded.

When the automatic gain control 1E voltage is supplied to the base of the transistor 03 via the resistor R15, the amount of current is controlled. This means that the impedance of the bin diode D and the impedance of the bin diode L)2 are changed by 11 times, and the Arati-Y 40
It is equivalent to -1 degree that the amount of acreage reduction is changed. Ctb
acts as a feedback capacitor for preventing older vibration of the cascade amplifier t.

Automatic gain 14 + 'l supine terminal 4 of the above Aratita 40
1 is supplied with a control U=pressure (AGC′ pressure) from a video intermediate frequency amplification stage (not shown). This provides the advantage of a cascade amplifier.

As described above, according to the present invention, it is possible to attenuate the amplified human fff signal using an attenuator and adjust the gain on the low impedance side of the grounded amplification section, thereby suppressing the occurrence of distortion. .

Furthermore, inside the attenuator 40, the diode 012 acts as the main signal path, and the diode 012 acts as a bypass for unnecessary signals, etc., but even if there is distortion generated in each element, the polarity is reversed. They will cancel each other out.

Figure 2 shows the circuit of the present invention in which the total attenuation is increased and the gain is decreased by 4! (f: This is the result of measuring the change in the sound index. The input signal uses 250 MHz. As can be seen from this percentage, the increase in the noise index is suppressed.

〔Effect of the invention〕

As described above, it is possible to provide an automatic gain control amplifier circuit that suppresses both distortion caused by the nonlinearity of the amplifier itself and changes in the noise figure.

4. d 1Mr (Offl'+ *'ll Explanation: A circuit diagram showing an embodiment of the invention of the first part, Figure 2 is a diagram showing L in the phase diagram of the circuit according to the invention, Third
This figure is an explanatory diagram of the configuration of the γ tag gun tuner of the CA i'V system, and FIG. 4 is an explanatory diagram showing an example of the arrangement of the BL path sensor of FIG. 3.

Q7+Q2...Transistor, IC1-170, R horn 1 and 15...Resistor FC,, C1~C5, C1l~
C16... Contemp, 40... γ Tsudeneta.

Personnel fee included Suzue Takehiko Diagram 2 on patent attorney

Claims (1)

[Claims] An automatic gain control amplifier circuit characterized in that an attenuator whose attenuation is adjusted by a gain control voltage is connected between a cascade-connected emitter common transistor and a base common transistor.