JPH0246091Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a tuned amplifier circuit in which the tuning frequency and Q of the input side tuning circuit do not change due to changes in the AGC voltage.

(Prior Art) FIG. 5 shows a circuit diagram of a conventional tuned amplifier circuit whose gain is adjusted by an AGC voltage. In FIG. 5, a tuned amplifier circuit 1 has one end of the primary coil of a tuned transformer 2 grounded, and the other end connected to an input terminal 3.
It is connected to the. And 2 of tuning transformer 2
A capacitor 4 is connected in parallel with the next coil to constitute an input side tuned circuit, one end of this input side tuned circuit is grounded, and the other end is connected to the base terminal 61 of the transistor 6 via the coupling capacitor 5. There is. Furthermore, this base terminal 61 is connected to the AGC terminal 8 via the resistor 7. Further, a collector terminal 63 of the transistor 6 is connected to one end of an output-side tuned circuit in which a coil 9 and a capacitor 10 are connected in parallel. The other end of the output-side tuned circuit consisting of the coil 9 and the capacitor 10 is grounded via the capacitor 11 and connected to the power supply terminal 12. Furthermore, the emitter terminal 62 of the transistor 6 is connected to the grounded capacitor 13.
and is grounded via a bias resistor 14 in parallel. Here, in AGC control, the collector current of transistor 6 increases as the AGC voltage increases,
It is a forward AGC in which the amplification gain decreases.

The operation of the tuned amplifier circuit 1 will be explained below. From the signal applied to the input terminal 3, a signal with a frequency tuned to the input-side tuning circuit is extracted, applied to the base terminal 61 of the transistor 6, amplified, and then a signal with a frequency tuned by the output-side tuning circuit is extracted. A signal is extracted. The AGC is adjusted according to the input level of the signal given to input terminal 3.
A voltage is applied to the AGC terminal 8, and the higher the input level, the higher the AGC voltage becomes, the collector current of the transistor 6 increases, and the amplification gain decreases.
The lower the input level, the lower the AGC voltage.
The collector current of the transistor 6 decreases and the amplification gain increases, and the tuned amplifier circuit 1 is controlled so that it always outputs a signal at a constant output level.

(Problems to be Solved by the Invention) FIG. 6 shows a circuit diagram in which the transistor 6 of the conventional tuned amplifier circuit 1 shown in FIG. 5 is equivalently rewritten. In FIG. 6, one end of an equivalent base resistor 64 is connected to a base terminal 61, and the other end is connected to a collector terminal 63 via a base-collector junction capacitance 65 and an equivalent current source 66 in parallel. Further, the other end of the equivalent base resistor 64 is connected to the emitter terminal 62 via an equivalent emitter resistor 67 and a base-emitter junction capacitor 68 in parallel. Therefore, the base-emitter junction capacitor 68 is connected in parallel to the capacitor 4 of the input-side tuning circuit, and the input-side tuning circuit actually includes the base-emitter junction capacitor 68.

By the way, the base-emitter junction capacitance is 68
It is known that the capacitance changes in proportion to an increase or decrease in the emitter current of the transistor 6. For this reason, when the collector current, that is, the emitter current increases or decreases due to an increase or decrease in the AGC voltage applied to the AGC terminal 8, the capacitance of the base-emitter junction capacitance 68 changes, and the constant of the input side tuning circuit changes, causing tuning. frequency changes. Therefore, if the AGC voltage increases, the base-emitter junction capacitance6
8 capacity increases and the tuned frequency decreases,
Furthermore, if the AGC voltage decreases, the capacitance of the base-emitter junction capacitance 68 decreases and the tuned frequency increases, and there is a problem that the tuned frequency fluctuates due to fluctuations in the AGC voltage.

Further, FIG. 7 shows a circuit diagram equivalently rewritten using the h parameter of the transistor 6 of the conventional tuned amplifier circuit 1 shown in FIG. In Figure 7,
One end of an input impedance 70 is connected to the base terminal 61, and the other end is grounded via an equivalent voltage source 71. Furthermore, the emitter terminal 62 is grounded, and the emitter terminal 62 and the collector terminal 63
In between, an output admittance 72 and an equivalent current source 73 are connected in parallel. Here, if the current amplification factor is hfe, the emitter current is Ie, and the input impedance 70 is hie, it is known that hie=hfe×25×10 ⁻³ ÷Ie. Therefore, if the AGC voltage changes and the emitter current changes, the input impedance 70 also changes. Therefore,
Due to fluctuations in AGC voltage, the load impedance seen from the input side tuning circuit changes. Therefore, as the AGC voltage increases, the input impedance 70 decreases, and the Q of the input-side tuned circuit decreases, and as the AGC voltage decreases, the input impedance 70 increases, causing the Q of the input-side tuned circuit to increase.
Therefore, there is a problem in that the characteristics of the input-side tuning circuit change due to fluctuations in the AGC voltage, and in particular, the Q of the input-side tuning circuit deteriorates as the AGC voltage increases.

The purpose of the present invention was to solve the problems of the conventional tuned amplifier circuit described above, and to provide a tuned amplifier circuit in which the tuning frequency and Q of the input side tuned circuit do not change due to changes in the AGC voltage. It's about doing.

(Means for Solving the Problem) In order to achieve such an object, the tuned amplifier circuit of the present invention provides a tuned amplifier circuit in which a signal with a frequency tuned to the input side tuned circuit is applied to the base terminal of a transistor whose emitter is grounded. In a tuned amplifier circuit in which an AGC voltage is applied to the base terminal of a transistor, a capacitor and a variable resistance element are interposed between the base terminal of the transistor and ground, and the resistance value of this variable resistance element is adjusted by the AGC voltage. is configured to do so.

(Function) A capacitor and a variable resistance element are interposed in series between the base terminal of a transistor that amplifies a signal of a frequency tuned to the input side tuning circuit and the ground, and the resistance value of this variable resistance element is set as above. Since the configuration is configured to adjust using the AGC voltage, changes in the base-emitter junction capacitance of the transistor due to changes in the AGC voltage are canceled out by changes in the equivalent capacitance due to changes in the resistance value of the variable resistance element, and the input side tuning is adjusted. The circuit constants can be kept almost constant. Further, changes in the input impedance of the transistor can be canceled out by changes in the equivalent resistance due to changes in the resistance value of the variable resistance element, so that the load impedance seen from the input side tuning circuit can be made approximately constant.

(Description of Embodiments) Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 4. FIG. 1 is a circuit diagram of an embodiment of a tuned amplifier circuit whose gain is adjusted by the AGC voltage of the present invention, and FIG. 2 shows a capacitor and a variable resistance element that constitute the tuned amplifier circuit shown in FIG. FIG. 3 is an equivalent circuit diagram of the tuned amplifier circuit shown in FIG. 1, and FIG. 4 is a circuit diagram of the tuned amplifier circuit shown in FIG. It is a circuit diagram rewritten equivalently. In FIGS. 1, 3, and 4, the same circuit elements as in FIGS. 5 to 7 include:
The same reference numerals are used to omit redundant explanation.

In FIG. 1, a tuned amplifier circuit 20 has the following circuits added to the conventional tuned amplifier circuit 1 shown in FIG. 5. The anode of a pin diode 22, which is a variable resistance element, is connected to the base terminal 61 of the transistor 6 via a capacitor 21, and the cathode of the pin diode 22 is grounded. The anode of the pin diode 22 is connected via a resistor 23 to the collector terminal of a transistor 24 for supplying bias current.
The base terminal of this transistor 24 is connected to a resistor 25.
It is connected to AGC terminal 8 via. moreover,
The emitter terminal of the transistor 24 is connected to the power supply terminal 12 via a resistor 26.

In this configuration, when the AGC voltage applied to the AGC terminal 8 increases, the voltage at the base terminal of the transistor 24 increases, the impedance of the transistor 24 increases, the current flowing through the pin diode 22 decreases, and the current flowing through the pin diode 22 decreases. Resistance value increases. Also, if the AGC voltage decreases,
The voltage at the base terminal of transistor 24 decreases,
The impedance of the transistor 24 decreases, the current flowing through the pin diode 22 increases, and the resistance value of the pin diode 22 decreases. The resistance value of the pin diode 22 changes in the range of several ohms to about 1 kilohm depending on the flowing current.

Here, pin diode 22 and capacitor 21
The series circuit is equivalently rewritten into a parallel circuit of an equivalent resistance 27 and an equivalent capacitance 28 shown in FIG. Let the resistance value of the pin diode 22 be r, the capacitance of the capacitor 21 be jx, the resistance value of the equivalent resistor 27 be R, and the capacitance of the equivalent capacitance 28 be C, and if r ² ≫ x ² is satisfied, then R=r C= It can be expressed as 1/ω・r ² . Therefore, by increasing the AGC voltage,
When the resistance value r of the pin diode 22 increases, the resistance value R of the equivalent resistance 27 increases, and the capacitance C of the equivalent capacitance 28 decreases. Further, when the resistance value r of the pin diode 22 decreases due to a decrease in the AGC voltage, the resistance value R of the equivalent resistance 27 decreases, and the capacitance C of the equivalent capacitance 28 increases.

By the way, if the tuned amplifier circuit 20 shown in FIG. 1 is rewritten equivalently, it will be as shown in FIG. 3. 1st
The resistor 23 shown in the figure has a large resistance value and acts as an isolation resistor at high frequencies.
4th place is ignored. Here, when the AGC voltage increases, the base-emitter junction capacitance 68 increases, but the resistance value r of the pin diode 22 increases, the capacitance C of the equivalent capacitance 28 decreases, and the base-emitter junction capacitance 68 increases. Fluctuations in the equivalent capacitance 28 cancel each other out, making it possible to keep the constant of the input-side tuning circuit approximately constant.

Furthermore, if the transistor 6 of the tuned amplifier circuit 20 shown in FIG. 1 is equivalently rewritten using the h parameter, the result will be as shown in FIG. 4. Here, when the AGC voltage increases, the input impedance 70 of the transistor 6 decreases, but the resistance value r of the pin diode 22 increases, the resistance value R of the equivalent resistance 27 increases, and the input impedance 70 and the equivalent resistance 27 increase. The fluctuations cancel each other out, and the load impedance seen from the input side tuning circuit can be made almost constant.

Therefore, the constant of the input side tuning circuit can be kept almost constant as the AGC voltage increases,
The signal of the tuned frequency does not change,
Further, the load impedance seen from the input-side tuned circuit can be made substantially constant, and the Q of the input-side tuned circuit does not change. Note that even when the AGC voltage decreases, the frequency signal and Q that are tuned similarly do not change.

In the above embodiment, the pin diode 22 is used as the variable resistance element, but the invention is not limited to this, and an element whose resistance value can be changed electronically, such as a transistor, may also be used.
Further, the AGC control may be reverse AGC in which the collector current of the transistor 6 increases as the AGC voltage increases and the amplification gain increases.

(Effect of the invention) As explained above, the tuned amplifier circuit according to the invention includes a capacitor and a variable resistance element between the base terminal of the transistor that amplifies the signal of the frequency tuned to the input side tuned circuit and the ground. are connected in series, and the resistance value of this variable resistance element is adjusted by the AGC voltage. Therefore, it is possible to cancel the change in the junction capacitance between the base and emitter of the transistor due to the change in the AGC voltage, and the input By keeping the constant of the side tuning circuit almost constant, the frequency signal to be tuned does not change. Also, AGC
This provides an excellent effect in that changes in the input impedance of the transistor due to changes in voltage can be canceled out, and the Q of the input side tuning circuit does not change.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of an embodiment of a tuned amplifier circuit whose gain is adjusted by the AGC voltage of the present invention, and Figure 2 shows the capacitors and variable resistance elements that constitute the tuned amplifier circuit shown in Figure 1. FIG. 3 is an equivalent circuit diagram of the tuned amplifier circuit shown in FIG. 1, and FIG. 4 is an equivalent circuit diagram of the tuned amplifier circuit shown in FIG. This is an equivalently rewritten circuit diagram, and the fifth
The figure is a circuit diagram of a conventional tuned amplifier circuit whose gain is adjusted by the AGC voltage, and FIG. 6 is a circuit diagram in which the transistors of the conventional tuned amplifier circuit shown in FIG. 5 are equivalently rewritten. FIG. 7 is a circuit diagram in which the transistors of the conventional tuned amplifier circuit shown in FIG. 5 are equivalently rewritten using the h parameter. 1, 20: Tuned amplifier circuit, 2: Tuned transformer,
4, 21: Capacitor, 6: Transistor, 8:
AGC terminal, 22: Pin diode.

Claims (1)

[Claims for Utility Model Registration] (1) In a tuned amplifier circuit in which a signal with a frequency tuned to the input side tuned circuit is applied to the base terminal of a transistor whose emitter is grounded, and an AGC voltage is applied to the base terminal of this transistor. , a capacitor and a variable resistance element are interposed in series between the base terminal of the transistor and the ground, and the resistance value of the variable resistance element is set to the resistance value of the variable resistance element.
A tuned amplifier circuit characterized in that it is configured to be adjusted by an AGC voltage. (2) Utility model registration claim 1, characterized in that the variable resistance element consists of a pin diode.
The tuned amplifier circuit described in Section 1.