JPS6062704A - Temperature compensating circuit of voltage controlled oscillator - Google Patents

Abstract

PURPOSE:To obtain a voltage controlled oscillator with high temperature stability without components having less temperature coefficient by providing a temperature compensating circuit giving a temperature compensating characteristic to an oscillated amplitude deciding an oscillated output frequency of the voltage controlled oscillator. CONSTITUTION:An area ratio of emitters of transistors (TR) Q3, Q4 is selected nearly as Q3:Q3=10:1, and a voltage DELTAFBE produced across a resistor R3 has a relation of DELTAVBE=VTln(10 I 2/ I 1) (II) is obtained from equation of DELTAVBE= (VBE)Q4-(VBE)Q3 ( I ) and Q3:Q4=10:1, where Vt is a thermoelectromotive force, 300K at room temperature. Relation of I 1= I 2=VBE/R3 (III) is obtained by a current mirror circuit comprising TRs Q7-Q9. Moreover, a voltage VR4 produced across a resistor R4 is expressed as equation (IV). Relation of equation (V) is established, where VRR is a band gap reference voltage and a current I G flowing to a TRQ14 is given by equation (VI). That is a positive on negative optional temperature coefficient is given to the reference voltage VR by selecting properly the ratio of the resistors R4 and R3.

Description

Detailed Description of the Invention Technical Field The present invention relates to a voltage controlled oscillator (hereinafter abbreviated as VCO) that can be adopted, for example, to an audio IC circuit, and particularly relates to an oscillation amplitude that determines the oscillation output frequency of the VCO. The present invention relates to a temperature compensation circuit for a voltage controlled oscillator that can have an arbitrary temperature coefficient and select changes in oscillation output frequency due to temperature changes.

<Prior Art> Generally, when a VCO constituting a PLL loop is used in an FM modulator or the like, quite strict requirements may be placed on the temperature stability of the oscillation output frequency.

Conventionally, for example, an emitter-coupled VCO has been constructed by commonly connecting the emitters shown in FIG. 1 and connecting a temperature compensation circuit THC including a resistor to this emitter. The output from, for example, a phase comparator (or passed through a low-pass filter) is input to the temperature compensation circuit THC as the CO input, and the output terminal V is input. The oscillation output frequency f of vCO is obtained at ut. D, *D b is a diode, the forward voltage drop across this diode is vl, and the current flowing from the emitter to the temperature compensation circuit THC is 1.
, then the output terminal V. The oscillation output frequency j obtained at ut. is given by the following equation.

In this way, conventionally, the oscillation output frequency S. Assuming that the temperature coefficient of the external constant of the VCO (capacitor Co) can be ignored, the temperature dependence of J is determined by the forward voltage of the diode c~1 or ),
The emitter current has a temperature coefficient equal to the temperature coefficient due to the forward voltage drop of this diode. Temperature compensation was performed by selecting .

However, according to the above temperature compensation, the oscillation output frequency J. It is only possible to obtain a temperature coefficient of about 400 ppm/''C, and if there are stricter requirements for temperature stability, external components such as capacitors will be expensive as parts. Therefore, the conventional VCO temperature compensation circuit described above has the disadvantage that it cannot meet the requirements in cases where components without temperature coefficients cannot be used.

<Purpose> The present invention has been made in order to eliminate the drawbacks of the conventional temperature compensation circuit of a VCO. The purpose is to select changes in oscillation output frequency due to temperature changes.

<Embodiment> The present invention can be adopted, for example, as an FM audio IC for a video tape recorder.

In Figure 2, Qa and Qb are transistors, and a capacitor C6 is connected between the collectors of both transistors.
It can be externally connected to the C circuit, and a resistor Re can be externally connected to the common emitter I through the transistor Qd. These external capacitors and resistors are used at the oscillation output frequency J.

It can be selected as appropriate. An input vim from a phase comparator is applied to the input of transistor Qd. R
b+Rc is a load resistance, and the oscillation amplitude of the VCO is designed to be given by the product of IGRa as described later. That is, the voltage drop across the resistor Ra (point A) is the voltage drop across the transistor Qc.
, Qd, and the voltage value obtained by subtracting the base-emitter voltage of these transistors Qc or Qd is applied to both ends of the capacitor Co as an oscillation amplitude. That is, the amplitude appearing on the capacitor co is controlled by changing the current IQ. In this case, the output terminal V. The Uth oscillation output frequency obtained is given by the following equation.

In this case, the external capacitor is used as a CO.

A styrene capacitor can be used, and a metal film resistor can be used as the resistor Re. As can be seen from equation (2), the oscillation output frequency J. The temperature dependence of is determined by the oscillation amplitude of IGIIR8, and by appropriately setting the temperature coefficient of current IP' to that of IG@Ra, it is possible to select and control changes in the oscillation output frequency due to temperature changes. .

THC indicates a circuit for performing such temperature compensation, and the third
A specific circuit configuration is shown in the figure.

The temperature compensation circuit in FIG. 3 includes transistors Q7 to Q.

R3-R has a current mirror circuit and transistors Qa and Qd for selecting the emitter area ratio.
Reference numeral 4 represents a resistor that allows the current IQ to have an arbitrary temperature coefficient by selecting a ratio of these, and Q13 to Q15 represent transistors forming a current mirror circuit.

Next, the operation of the temperature compensation circuit will be explained in more detail.

First, if the area ratio of the emitters of transistors Q3 and Qd is chosen as Qs:Qa=10:17, and the voltage generated across R and g is △VBE, then △VBE: (VBE) Qd - (vBE) Q3 ° -1l
From lQ3:Q4-1oll, where vT is the thermal voltage, which is 300°C Kelvin at room temperature.

Due to the current mirror circuit of the transistors Q7 to Q, the voltage VR4 generated across the resistor R4 is the voltage across the resistor R5, that is, if the bandgap reference voltage is vR.

becomes. Diodes D2 to D4 and transistor Q12
~Q]4 cancel each other's voltage changes. Transistors QI3 to Q15 constitute a current mirror circuit, and the current flowing through transistor Q14 is equal to the IQ lightning current.

Formula +51 in Oite, first term (7) (VBF,
) Qa has a temperature coefficient of -2m V/''C,
The bandgap reference voltage vR of the second term can have any positive or negative temperature coefficient. here.

The oscillation output frequency of the VCO is expressed by the following equation.

The voltage generated across resistor R14 in FIG. 2 is IGR14. That is, the temperature dependence of SC can be controlled by giving an appropriate temperature coefficient to IGR]4.

Effect> As explained below, according to the temperature compensation circuit of the VCO of the present invention, there is a bandgap reference voltage source for selecting an arbitrary temperature coefficient, and oscillation is performed based on the voltage generated by this reference voltage source. Since the oscillation amplitude forming means for forming the oscillation amplitude that determines the output frequency is provided, the oscillation amplitude that determines the oscillation output frequency of the VCO can have an arbitrary temperature coefficient, which prevents changes in the oscillation output frequency due to temperature changes. It has the advantage of being able to choose.

(8)

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a conventional VCO, Fig. 2 is a circuit diagram of a VCO according to an embodiment of the present invention, and Fig. 3 is a circuit diagram of a VCO according to an embodiment of the present invention.
It is a block diagram of the temperature compensation circuit of O circuit. Explanation of symbols Q a + Q b') transistor, co = capacitor, Ra% Ro: resistor, VHC': temperature compensation circuit 1, f, two oscillation output frequency, VR: band gear yp reference voltage. Agent Patent attorney Aihiko Fukushi (and 2 others) (9) Tea 1 diagram □ Unexamined Japanese Patent Application Sho GO-62704 (4) CC Ma 2wI Φ-prisoner

Claims (1)

[Claims:] A bandgap reference voltage source for selecting an arbitrary temperature coefficient in a voltage controlled oscillator. 1. A temperature compensation circuit for a voltage controlled oscillator, comprising an oscillation amplitude forming means for forming an oscillation amplitude that determines an oscillation output frequency based on the voltage generated by the reference voltage source.