JPH0531844B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a variable frequency oscillation circuit with good linearity.

Background technology and its problems A variable frequency oscillation circuit whose oscillation frequency can be controlled by voltage or current is called a VCO or CCO.
CCOs can be roughly divided into LC oscillation types and multivibrator types.

In general, the LC oscillation type uses a variable capacitance diode in the resonant circuit and supplies a control voltage to it to vary the oscillation frequency, while the multivibrator type controls the charging and discharging current of the capacitor, and controls the thread. The oscillation frequency is varied by controlling the shjord level.

However, in the LC oscillation type, if a hyperstep junction is used as the variable capacitance diode, the IC
If a general PN junction is used, the variable capacitance range will be small and the variable frequency range will be narrow.

In addition, multivibrator types have poor oscillation stability and poor C/N, such as VCO or CCO.
It is very difficult to hope for a performance equal to or better than that of the LC oscillation type.

Therefore, the CCO shown in Figure 1 is being considered as a CCO that eliminates these problems.

That is, in FIG. 1, transistor Q ₁
The collector of is connected to the power supply terminal _T1 , a constant current source _Q2 is connected between its emitter and ground, and a resistor _R1 and a solid-state resonator _X1 are connected in parallel between its collector and base. and a capacitor _C1 is connected between its base and emitter,
Output terminal _T2 is connected to this emitter.

Further, the emitters of transistors Q ₃ and Q ₄ are connected to each other, and a variable constant current source Q ₅ is connected between these emitters and ground, thereby forming a differential amplifier 1. Further, the emitters of transistors Q ₆ and Q ₇ are connected to terminal T ₁ , their bases are connected to each other, and the collector of transistor Q ₆ is connected to make transistor Q ₆ the input side, and the terminal T ₁ A current mirror circuit 2 is constructed with the reference potential point being . The collectors of transistors Q ₃ and Q ₄ are connected to the collectors of transistors Q ₆ and Q ₇ , respectively, to form an inverting amplifier 3.

Furthermore, a feedback capacitor C ₂ is connected between the collector and base of the transistor Q ₄ , and the base of the transistor Q ₄ is connected to the bias power supply V ₁ through a resistor R ₂ .
The base of Q ₃ is also connected to the power supply V ₁ . Also, the collector of transistor _Q4 is connected to the emitter of transistor _Q1 .

According to this configuration, circuits 1 and 2 function as an inverting amplifier 3 with the base of transistor Q ₄ as an inverting input terminal and the collectors of transistors Q ₄ and Q ₇ as output terminals, so that the circuit of FIG. The AC equivalent circuit of is shown in FIG. In other words, negative feedback is applied to amplifier 3 by capacitor C ₂ ,
The input terminal of amplifier 3 is shunted by resistor _R2 , and the output terminal of amplifier 3 is shunted by resistor R2.
It will be connected to the emitter of _Q1 .

Therefore, the output admittance Y of amplifier 3 at this time is Y=ω ² C ² / ₂ / 1 + (ωC ₂ R ₂ ) ² (q/2kTR ₂ I ₅ +1) +
jωC ² / ₂ R ₂ /1 + (ωC ₂ R ₂ ) ² (q / 2kTR ₂ I ₅ +1)...
(i) ω: Angular frequency q/kT: Boltzmann constant, etc.I ₅ : Constant current of current source Q ₅ , which is equivalent to a series circuit of a resistor and a capacitor, and the Q of the capacitor is Q=1/ It is expressed as ωC ₂ R ₂ .

Therefore, if Q≫1, the admittance Y can be regarded as only a capacitance, and its equivalent capacitance C _X is as follows: C _x =C ₂ (q/2kTR ₂ I ₅ +1) (ii). That is, when looking at the amplifier 3 side from the emitter of the transistor _Q1 , this can be regarded as the capacitance _Cx .

Therefore, the circuit of FIG.
It can be shown as shown in the figure. And this third
The circuit shown in the figure is a Colpitts oscillation circuit, which oscillates at the series resonance frequency of the inductance of the vibrator X ₁ and the capacitances C ₁ and C _x .

At this time, the equivalent capacitance C _x is expressed by equation (ii), which changes depending on the magnitude of the current I ₅ . Therefore,
The circuit shown in FIG. 1 is a Colpitts oscillation circuit, and its oscillation frequency can be varied by a constant current _I5 of a variable constant current source _Q5 .

In this way, CCO can be realized, but in this case,
In particular, the CCO shown in FIG. 1 does not require an element such as a hyperstep junction variable capacitance diode, so it can be easily integrated into an IC. Also, the equivalent capacitance C _x is (ii)
Since the capacitance C _x can be varied over a wide range by the current I ₅ , the variable frequency can be widened.

Furthermore, this CCO is basically an LC oscillation, so
It has high stability and good C/N. Also, the number of parts is small.

Furthermore, no direct current flows between the emitter of transistor Q ₁ and the collectors of transistors Q ₄ and Q ₇ , and the potential at these points _is
Therefore, the current voltage V _cc can be lowered. Furthermore, since the output end of the amplifier 3 is connected to the transistor _Q1 , the oscillation signal level at the emitter of the transistor _Q1 can be increased compared to the case where the input end is connected.

In this way, the CCO shown in Figure 1 has many advantages. However, in this CCO, the equivalent capacitance C _x
is expressed by equation (ii) for the control current _I5 , and since the capacitance _Cx changes linearly with the current _I5 , the oscillation frequency f with respect to the current _I5 is non-linear as shown by the solid line in Figure 4. It will change to. For this reason, the control sensitivity to the oscillation frequency f is high in the high range of oscillation frequency f (upper side of Figure 4), and the control sensitivity is low in the low range of oscillation frequency f (lower side of Figure 4), making it difficult to use. There is. Also, this CCO
When used in a PLL, the loop gain changes significantly depending on the oscillation frequency f, so there is a drawback that a PLL with desired characteristics cannot be obtained.

Purpose of the Invention This invention aims to eliminate these problems.

Summary of the Invention Therefore, in the present invention, the control current _I5 is corrected using the exponential characteristic of the transistor to make the control signal versus oscillation frequency characteristic linear.

Embodiment That is, in FIG. 5, the collector and emitter of a transistor _Q5 are connected as a constant current source _Q5 .

Further, the emitters of the transistors Q ₁₁ and Q ₁₂ are connected to each other, and a constant current source Q ₁₃ is connected between the emitters and _the ground.
The base of Q ₁₂ is connected to control terminals T ₁₁ and T ₁₂ to form a differential amplifier 11. Further, the transistors Q ₁₄ and Q ₁₅ constitute a current mirror circuit 12 in which the terminal T ₁ is the reference potential point and the transistor Q ₁₄ is the input side.
The collectors of Q ₁₄ and Q ₁₅ are connected to the collectors of transistors Q ₁₁ and Q ₁₂ , respectively, to form a non-inverting amplifier 13.
is configured.

The collectors of transistors Q ₁₂ and Q ₁₅ are connected to the base of transistor Q ₁₆ , the emitter of transistor Q ₁₆ is connected to terminal T ₁ , and between terminal T ₁ and the base of transistor Q ₁₆ , Diodeed transistor Q ₁₇ and resistor
R ₁₁ are connected in series, and a constant current source Q ₁₈ is connected between the midpoint between the transistor Q ₁₇ and the resistor R ₁₁ and the ground.

Further, the collector of the transistor Q ₁₆ is connected to the collector of the transistor Q ₁₉ , and the current mirror circuit 14 is formed by the transistor Q ₁₉ and the transistor Q ₅ with the transistor Q ₁₉ on the input side and the ground as a reference potential point. configured.

Note that a control voltage E is supplied between terminals T ₁₁ and T ₁₂ .

In such a configuration, if the current flowing from the amplifier 13 toward the resistor _R11 is _I13 , then the terminal
When the control voltage E supplied to T ₁₁ and T ₁₂ has a polarity such that the terminal T ₁₁ is positive, I ₁₃ >0 and the terminal
When T ₁₂ has a positive polarity, I ₁₃ <0, and this current I ₁₃ corresponds to the control voltage E, and E = 0.
Then I ₁₃ =0.

For simplicity, if we ignore the base currents of transistors Q ₁₆ and Q ₁₇ , we get V _BE17 − I ₁₃ R ₁₁ = V _BE16 V _BE16 , V _BE17 : The base currents of transistors Q ₁₆ and Q ₁₇
Since the emitter voltage is the collector current of transistor _Q16 , _I16
I ₁₆ Is [exp {q/kT (V _BE17 − I ₁₃ R ₁₁ )}] Is: saturation current of transistor Q ₁₆ .

Also, the collector current I ₁₆ of the transistor Q ₁₆ is equal to the collector radio wave of the transistor Q ₁₉ , and the transistors Q ₁₉ and Q ₅ constitute the current mirror circuit 14, so I ₅ = I ₁₆ Is [exp{q /kT(V _BE17 −I ₁₃ R ₁₁ )}]……(
iii).

Therefore, if the collector current I ₅ when I ₁₃ = 0 (E = 0) is I ₅ (0), then I ₅ (0) Is {exp (q/kTV _BE17 )} ...(iv) Therefore, from equation (iii) and (iv), I ₅ (0)/I ₅ Is{exp(q/kTV _BE17 )}/Is[exp{
q/kT(V _BE17 −I ₁₃ R ₁₁ )}=exp(q/kTI ₁₃ R ₁₁ ). That is, the current I ₅ changes exponentially from the reference I ₅ (0) with respect to the current I ₁₃ .

Therefore, the oscillation frequency f changes almost linearly with respect to the current _I13 , as shown by the broken line in FIG.

Note that when I ₁₃ = 0 (E = 0), the collector current I ₁₆ of the transistor Q ₁₆ is the same as that of the transistor Q ₁₇
This is equal to the constant current _I18 of the constant current source _Q18 , so _I5 = _I18 , and the oscillation frequency f is the reference frequency _f0 .

Thus, according to the present invention, the oscillation frequency f can be varied linearly with respect to the control voltage E or the control current _I13 . Moreover, since this linearity correction is performed using the exponential characteristic of the transistor, there is little variation or fluctuation in the control signal versus oscillation frequency characteristic.

Therefore, constant control sensitivity can be obtained regardless of the oscillation frequency f. Furthermore, in the case of a PLL, the loop gain is constant, and desired characteristics can be easily obtained.

In the example shown in FIG. 6, transistor Q ₁
This is a case where the emitter is grounded and the current mirror circuit 14 is omitted.

Note that in the above description, a capacitor may be added in parallel or in series with the capacitor C _x if necessary.

Effects of the Invention The oscillation frequency can be changed linearly with respect to the control signal. Moreover, it is suitable for IC implementation,
Stability and C/N are good, and the variable frequency range is wide. Additionally, it can operate at low voltage and has a small number of components.

[Brief explanation of the drawing]

1 to 4 are diagrams for explaining this invention, and FIGS. 5 and 6 are connection diagrams of an example of this invention. 3 is an inverting amplifier.

Claims (1)

[Claims] 1 A differential amplifier and a current mirror circuit are connected to form an inverting amplifier, a feedback capacitor is connected between the output terminal of this inverting amplifier and an inverting input terminal, and a feedback capacitor is connected to the inverting input terminal and an AC ground point. A resistor is connected, and the output terminal is connected to the resonant circuit of the oscillation circuit as its resonant capacitor, and the capacitance is changed by varying the magnitude of the constant current of the constant current source of the differential amplifier. , emitters of the first and second transistors are connected to a reference potential point, another resistor is connected between the bases of the first and second transistors, and the base and collector of the first transistor are connected to each other. and is connected to a constant current source, the collector current of the second transistor is supplied as a constant current of the constant current source of the differential amplifier, and the connection between the base of the second transistor and the other resistor is A variable frequency oscillation circuit in which the oscillation frequency is varied by supplying a control current to the connection point.