JPH0540587Y2 - - Google Patents

Description

[Detailed explanation of the idea] The explanation will be given in the following order.

A. Field of industrial application B. Overview of the invention C. Conventional technology D. Problem that the invention aims to solve E. Means for solving the problem (Fig. 1) F. Effect G. Example (Fig. 1) H. Effect A Industrial Application Field This invention relates to a negative impedance conversion circuit.

B. Summary of the invention This invention improves the frequency characteristics by using a special connection in a negative impedance conversion circuit.

C. Prior Art In a superheterodyne receiver, the necessary selectivity can be obtained by lowering the intermediate frequency.

However, superheterodyne receivers are subject to image interference, and in particular, the lower the intermediate frequency, the more susceptible to image interference. Additionally, tracking errors may cause sensitivity unevenness within the reception band. Furthermore, there are many parts, and there are many adjustments to be made after assembly.

In this respect, autodyne receivers do not have the same problems as superheterodyne receivers. And as an autodyne receiver,
There is a system in which positive feedback is applied to the tuning circuit in the preceding stage of the detection circuit, thereby increasing the sharpness Q of the tuning circuit to obtain the necessary selectivity and high frequency gain.

That is, as shown in Figure 2, generally the center frequency of the tuning characteristic (pass characteristic) of the tuning circuit is set to ₀ ,
If the bandwidth of the -3 dB response is Δ, then there is a relationship between it and the sharpness Q of the tuning circuit as follows: Q= ₀ /Δ...(i). Therefore, in an autodyne receiver, if the sharpness Q of the tuning circuit is increased,
The bandwidth Δ at the tuning frequency ₀ becomes narrower, and the selectivity can be improved. Furthermore, positive feedback also increases high frequency gain.

However, when applying positive feedback to the tuning circuit in this way, there is a limit to the amount of positive feedback, and especially since positive feedback is applied to the high frequency signal line, the amount of positive feedback cannot be increased very much, and therefore it is not sufficient. Selectivity and high frequency gain cannot be obtained. Furthermore, since sufficient high frequency gain cannot be obtained, the overall gain is insufficient and sufficient receiving sensitivity cannot be obtained. moreover,
Since the gain is insufficient, sufficient AGC cannot be applied. Furthermore, since positive feedback is applied to the tuning circuit, stability is insufficient.

Therefore, as an improvement on such an autodyne receiver, an autodyne system is developed in which a Q multiplier is connected to the antenna tuning circuit to increase the sharpness Q of the antenna tuning circuit, thereby improving the selectivity. The receiver is patented in 1977-123013.
proposed by No.

However, with this autodyne receiver, the design is very critical; trying to obtain the required selectivity may result in oscillation, or preventing this oscillation may result in the required selectivity not being obtained. . That is, the same problems as the conventional autodyne receiver arise in terms of selectivity and stability.

Also, for example, the medium wave band ranges from 535kHz to 1605kHz, so from equation (), the bandwidth Δ is three times greater at the highest reception frequency than at the lowest reception frequency, which means that the selectivity changes greatly with the reception frequency. It ends up.

Therefore, an autodyne receiver as shown in FIG. 3 has been considered.

That is, in the same figure, the bar antenna coil
An antenna tuning circuit ₁ is constructed by connecting a series circuit of a capacitor C ₁ and a variable capacitor C ₂ in parallel to L 1 , and an intermediate tap is provided to the coil L ₁ to take out the tuning output of the tuning circuit 1 . In this case, the tuning circuit 1 has a sharpness such that sufficient stability can be obtained, for example, Q=100. Also,
C ₁ 》C ₂ .

This tuned output is then supplied to a high frequency amplifier 2. The output impedance of this amplifier 2 is made sufficiently large, and an interstage tuning circuit 3 is connected to its output terminal. In this case, this tuning circuit 3 is constructed by connecting a coil _L2 , a capacitor _C3 , and _{a variable capacitor C4 ,} which have the same values as the elements L1, _C1 , and _C2 , in a π configuration ( _C3 >> _C4 ), and the variable capacitor _C4 is linked to the variable capacitor _C2 . The sharpness Q of the tuning circuit 3 alone need only be large enough to provide sufficient stability.

Further, a negative impedance conversion circuit 4 is connected to the output end of the tuning circuit 3. This conversion circuit 4
An example of this will be described later, but its input impedance is set to a negative resistance value -r.
That is, since the output impedance of the amplifier 2 is sufficiently large and C ₃ >>C ₄ , the equivalent circuit of the tuned circuit 3 can be shown as shown in FIG. However, the resistance r _S is the equivalent series resistance of the coil L ₂ , and the resistance r _p is the overall equivalent parallel resistance.

And the negative resistance -r is set as |-r|=r _p regardless of the signal frequency.

Furthermore, the conversion circuit 4 is configured to function as an amplifier with a constant gain for the input signal (when viewed from the output end) regardless of the signal frequency.

Then, the output of the conversion circuit 4 is supplied to the detection circuit 6 through the high frequency amplifier 5, and the detection output is supplied to the terminal 7 as well as to the low pass filter 8 to form an AGC voltage. , 5.

According to such a configuration, the bar antenna coil
A signal is received by L ₁ , and among the received signals (high frequency signals), a signal with a desired frequency is selected to some extent by tuning circuit 1, and this selected received signal is sent to tuning circuit 3 through amplifier 2. supplied and further selected. The received signal selected twice is supplied to the detection circuit 6 through the change circuit 4 and the amplifier 5, so that an audio signal is obtained at the terminal 7. Further, at this time, AGC is performed in the amplifiers 2 and 5 using the AGC voltage from the filter 8.

In this case, even if the sharpness Q is small when the tuning circuit 3 is a single unit, the negative impedance change circuit 4 is connected to the next stage, and its negative resistance -r causes the equivalent parallel resistance r _p of the tuning circuit 3. is canceled, the composite sharpness Q of circuits 3 and 4 is n times (n
>1), for example, Q=250. At this time, since the sharpness Q of the tuning circuit 1 is Q = 100, the overall sharpness Q from the tuning circuit 1 to the conversion circuit 4 is Q1000 (Q = 100 corresponds to approximately 6 dB, and the Q =250 is almost
14dB, and the sum of both decibels, 20dB, is Q
1000), thus obtaining a selectivity comparable to that of a superheterodyne receiver.

In this case, in the tuned circuit 3, the equivalent parallel resistance r _p is canceled by the negative resistance -r, so the combined sharpness Q of the circuits 3 and 4 is Q=2π ₀ L ₂ /r _s . . . (ii), and this sharpness Q increases in proportion to the reception frequency ₀ . In equation (), if the sharpness Q and frequency ₀ are proportional, the bandwidth Δ will be constant.In other words, if equation () is substituted into equation (), the bandwidth Δ will be: Δ=r _s /(2πL ₂ ) becomes constant. Therefore, the selectivity of the combination of circuits 3 and 4 is constant regardless of the reception frequency, and the overall selectivity of the receiver is also approximately constant regardless of the reception frequency.

Furthermore, since the two tuning circuits 1 and 3 provide the necessary sharpness Q as a whole, the tuning circuits 1 and 3 provide the necessary sharpness Q as a whole.
3 does not require a very large sharpness Q, and therefore sufficient stability can be obtained. In particular, regarding the tuning circuit 3, the sharpness Q is increased by the negative impedance conversion circuit 4 without applying positive feedback as in the conventional case. Since Q=250 is not so large, sufficient stability can be obtained.

Further, the received signal is amplified by the amplifiers 2 and 5 and the conversion circuit 4, and at this time, since the stability is sufficiently high, sufficient gain can be obtained.
Can be made highly sensitive. Furthermore, the combined gain A of amplifier 2 and tuning circuit 3 is A = gmL ₂ / (C ₃ + C ₄ ) r _s gmL ₂ / C ₃ r _s (∵C ₃ >>C ₄ ) gm: mutual conductance of amplifier 2 Since it is constant regardless of the reception frequency, stable operation is performed from this point as well.

Furthermore, since sufficient gain can be obtained and stable operation can be performed, sufficient AGC can be performed.

By the way, the above-mentioned conversion circuit 4 is
According to Publication No. 31054, it can be configured as shown in FIG.

In the figure, _A1 is an op-amp,
A positive feedback resistor _R1 is connected between the non-inverting input terminal and the output terminal, and a negative feedback resistor _R2 is connected between the inverting input terminal and the output terminal.
A resistor _R3 is connected between the inverting input terminal and ground. An input terminal _T1 and an output terminal _T2 are connected to the non-inverting input terminal and the output terminal, respectively.

Therefore, v ₁ : Input signal voltage of terminal T ₁ i ₁ : Input signal current of terminal T ₁ v ₂ : Output signal voltage of terminal T ₂ i ₂ : Signal current of resistor R ₃ , then i ₁ = (v ₁ − v ₂ )/R ₁ i ₂ = (v ₂ − v ₁ )/R ₂ i ₃ = v ₁ / R ₃ holds, so from these equations, i ₁ = −R ₂ / R ₁ It becomes R ₃ v ₁ . Therefore, the input resistance Ri of the terminal T ₁ is Ri=v ₁ /i ₁ =−R ₁ ·R ₃ /R ₂ (iii), and a negative resistance is obtained.

In addition, since amplifier A ₁ is only subjected to positive feedback and negative feedback by resistors R ₁ to _{R 3} , the terminal
A gain determined by resistors R ₁ to _{R 3} is obtained between T ₁ and T ₂ .

Therefore, the circuit shown in FIG. 5 works as a negative impedance conversion circuit 4.

D. Problems to be solved by the invention However, the reason why the circuit of FIG. 5 works as the negative impedance conversion circuit 4 shown by the formula () is as follows
This is a case where the operational amplifier _A1 has a sufficiently wide band and a sufficient gain. and op amp
When the bandwidth of A ₁ is insufficient, resistor R ₂ ,
The gain Å during negative feedback due to R ₃ is Å = A ₀ /1 + jω / ω _c ... (iv) A ₀ = 1 + (R ₂ / R ₃ ) ω: Angular frequency of signal ω _c : Closed loop of amplifier A ₁ (at the time of negative feedback).

In addition, the input admittance of the conversion circuit 4 in FIG.
Yi can be expressed as Yi=(1-Å)Y ₁ ...(v) Y ₁ =1/R ₁ .

Therefore, from equations () and (), Yi=G ₁ {1−A ₀ /1+(ω/ω _c ) ² } +jA ₀ ω/ω _c /1+(ω/ω _c ) ² G ₁ G ₁ =1 /R ₁ , and the input conductance (input resistance) Gi of this conversion circuit 4 has frequency characteristics.
That is, when the cutoff frequency ω _c of the operational amplifier A ₁ cannot be ignored with respect to the frequency ω of the received signal, the negative resistance changes.

This invention attempts to solve these problems.

E Means for Solving Problems In this invention, a capacitor _C5 is connected to the conversion circuit 4 as shown in Fig. 1, and the value of this capacitor _C5 is selected to change the input conductance Gi to Let it be a constant negative resistance that is unrelated to .

F Effect According to this invention, even if the band of operational amplifier _A1 is not negligibly wide compared to the signal frequency,
A constant negative resistance can be obtained regardless of the frequency. Furthermore, the configuration for this purpose is low cost because it is only necessary to add a capacitor _C5 of a predetermined value, and there is no need to use a particularly broadband operational amplifier _A1 .

G Embodiment In FIG. 1, _A1 is an operational amplifier with a sufficiently large gain, and a parallel circuit of a capacitor _C5 and a resistor _R1 is connected between its non-inverting input terminal and output terminal. A resistor R ₂ is connected between the inverting input terminal and the output terminal, and a resistor R ₃ is connected between the inverting input terminal and ground. In addition, input terminal T ₁ and output terminal are connected to the non-inverting input terminal and output terminal.
T ₂ are connected respectively.

Therefore, the input admittance Yi seen from terminal _T1
is, Yi=G ₁ {1−A ₀ /1+(ω/ω _c ) ² (1+ω/ω _c ωC ₅ R
₁ )} +j{ωC ₅ −A ₀ /1+(ω/ω _c ) ² (ωC ₅ −ω/ω _c G
₁ )} ...(vi).

Therefore, if ω _f =1/(C ₅ R ₁ ), then the equation () becomes Yi=G ₁ {1−A ₀ /1+(ω/ω _c ) ² (1+ω ² /ω _c ω _f
)} +j{ωC ₅ −A ₀ /1+(ω/ω _c ) ² (ωC ₅ −ω/ω _c G
₁ )} ...(vii), and if ω _f = ω _f , equation () becomes Yi=G ₁ (1-A ₀ ) + jωC ₅ , and Gi=G ₁ (1-A ₀ )... (viii). And in general, since A ₀ > 1,
The input conductance Gi becomes negative and ()
The equation never includes the angular frequency ω.

Therefore, when looking at the amplifier _A1 from the terminal _T1 , a constant negative resistance is obtained regardless of the frequency. Further, the amplifier A ₁ works as a feedback amplifier with a gain A ₀ for the input signal.

Thus, according to this invention, operational amplifier A ₁
Even if the band is not negligibly wide compared to the signal frequency, a constant negative resistance can be obtained regardless of the frequency. Moreover, the configuration for this is
You only need to add a capacitor C ₅ of the given value,
Since there is no need to use a special broadband operational amplifier _A1 , the cost is low.

H. Effect of the invention According to this invention, even if the band of operational amplifier _A1 is not negligibly wide compared to the signal frequency,
A constant negative resistance can be obtained regardless of the frequency. Furthermore, the configuration for this purpose is low cost because it is only necessary to add a capacitor _C5 of a predetermined value, and there is no need to use a particularly broadband operational amplifier _A1 .

[Brief explanation of the drawing]

FIG. 1 is a connection diagram of an example of this invention, and FIGS. 2 to 5 are diagrams for explaining the same. _A1 is an operational amplifier.

Claims (1)

[Claims for Utility Model Registration] An amplifier; a first resistor connected between a non-inverting input terminal and an output terminal of the amplifier to provide positive feedback to the amplifier; and an inverting input terminal and an output terminal of the amplifier. a second resistor connected between the inverting input terminal of the amplifier and ground to provide negative feedback to the amplifier; a third resistor connected between the inverting input terminal of the amplifier and ground; and a capacitor connected thereto, such that the reciprocal of the time constant of the first resistor and the capacitor is the same as the cutoff frequency of the closed loop of the amplifier during negative feedback by the second and third resistors. A negative impedance conversion circuit having a value selected to provide a negative resistance at the non-inverting input terminal of the amplifier.