JP2000165141A - Oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an oscillation circuit where its oscillation loop is configured not via a buffer circuit and that is suitable for high frequency oscillation and a configuration employing high impedance components. SOLUTION: A BPF 12 consists of a current input/current output filter, and a feedback path is formed via a voltage input/current output current limiter circuit 11. An input signal to the BPF 12 in a voltage mode having been used for a conventional circuit is changed into a current mode, and an output of the BPF 12 is connected to the current limiter circuit 11, then a voltage input/ output buffer circuit having been so far required is not needed for this oscillation circuit and deterioration in the frequency characteristic having been caused by an impedance of the buffer circuit can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oscillation circuit suitable for being incorporated in an IC (integrated circuit).

[0002]

2. Description of the Related Art A conventional oscillation circuit built in an IC will be described with reference to FIG. A voltage limiter circuit 51 which receives a voltage signal, amplifies the voltage and outputs a voltage signal;
Buffer circuit 5 for outputting the output with low impedance
2 and a band-pass filter (BPF) 53. The BPF 53 is configured such that the input / output phase at the maximum gain frequency fo is 0 degree. Voltage limiter circuit 51 and B
By setting the total gain of the PF 53 to 1 or more, an oscillation circuit that oscillates at the maximum gain frequency fo can be obtained.

The BPF 53 of such an oscillator circuit has two
Various types having the following transfer functions have been proposed, and specific examples thereof will be omitted. Maximum gain frequency fo from outside
Can be varied, and it is also used for a control oscillation circuit.

In general, at an oscillation frequency on the order of several MHz, the impedance characteristics of the buffer circuit 52 (eg, an emitter follower) have little effect on the oscillation frequency. Decrease. There is a problem in that high-frequency performance is hindered and current consumption is increased, for example, it is necessary to increase the current flowing through the buffer to take measures.

[0005] In particular, when a circuit is formed using CMOS elements, the problem is remarkable. That is, the output impedance of the buffer circuit (for example, the source follower) is
Several orders of magnitude higher than bipolar devices
Even at frequencies on the order, Q and gain are reduced, and measures must be taken from lower frequencies, such as increasing the capacitance used for the filter to lower the impedance of the oscillation circuit itself.

[0006]

In the conventional oscillation circuit described above, since the oscillation loop is formed via the buffer circuit, the Q and open loop gain of the filter are reduced, and the output impedance of the buffer circuit is reduced to a low frequency. Measures were needed to lower

An object of the present invention is to provide an oscillation circuit suitable for use with high-frequency oscillation or a high-impedance element by forming an oscillation loop without using a buffer circuit.

[0008]

In order to solve the above-mentioned problems, an oscillation circuit according to the present invention includes a filter circuit for inputting a current signal and outputting a voltage signal serving as a transfer function of a secondary bandpass filter. A current limiter circuit for converting a signal in a predetermined range of an output signal of the filter circuit into a current and outputting the current; and a means for forming an oscillation loop by inputting an output current of the current limiter circuit as a current signal of the filter circuit And characterized by the following.

According to the above-described means, the input signal to the band-pass filter can be changed from the voltage mode to the current mode. No buffer circuit is required. Therefore, it is possible to prevent the frequency characteristic from deteriorating due to the impedance of the buffer circuit.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram for explaining an embodiment of the present invention. In this embodiment, a current limiter circuit 11 which receives a voltage signal, converts a voltage signal within a predetermined range into a current and outputs the current, and an output current Iin of the current limiter circuit 11
And a BPF 12 that outputs a voltage signal Vout, and outputs the output Vout of the BPF 12 to the current limiter circuit 1.
1 to form an oscillation path.

The BPF 12 has a transconductance-
gm1 and capacitor C1, and two cascaded integration stages of transconductance gm2, -gm3 and capacitor C2. That is, an example of a circuit including a transconductance amplifier is shown. The output of the current limiter circuit 11 is connected to the input terminal 12a of the BPF 12. The input terminal 12a is connected to the output of the transconductance gm2 of the cascade-connected transconductances -gm1 and gm2, and is also connected to the input / output of the transconductance -gm3 of negative polarity and the output terminal 12b. The output terminal 12b is connected to the input of the transconductance -gm1. The transconductance -gm1 is grounded via the capacitor C1, and the transconductance gm1
2 is grounded via a capacitor C2.

With this connection, the input signal becomes Iin
(Positive towards flowing into the filter), input-output characteristics when the output signal was Vout is, Vout / Iin = [S / C2] / [S 2 + S (gm3 / C2) + gm1gm2 / C1C2] ... (1) becomes BPF characteristics are obtained. Here, S is a Laplace operator, C1 and C2 are capacitance values of the capacitors C1 and C2, respectively, and gm1 to gm3 are transconductances of the transconductance amplifiers -gm1, gm2, and -gm3.

The Q and the maximum gain frequency fo obtained from this transfer function are not directly related to the effect of the present invention and will not be described, but the expression (1) at the maximum gain frequency fo is 1 / gm3. Become.

The input terminal (Vi) of the current limiter circuit 11
The open-loop gain from n) to the output Vout of the BPF 12 is Vout / Vin = Gout at the maximum gain frequency fo, where the conversion gain of the current limiter circuit 11 is gmA.
gmA / gm3. At the maximum gain frequency fo, BP
Since the phase of F12 becomes 0 degree, it is necessary to satisfy gmA> gm3 in order to satisfy the oscillation condition. In addition, if the values of the transconductances -gm1, gm2, -gm3 and the capacitors C1 and C2 are determined so that the maximum gain frequency fo becomes a desired frequency, an oscillation circuit with a built-in IC that oscillates at the maximum gain frequency fo is obtained.

Since the input of the BPF 12 is set to the current mode and a signal is supplied from the current limiter circuit 11,
The voltage input / output buffer circuit no longer exists.
It operates well even at an oscillation frequency on the order of 0 MHz. In particular, when a CMOS transistor is used, the conventional problem can be eliminated from a low frequency, so that the effect is enormous.

Next, another embodiment of the present invention will be described with reference to the circuit shown in FIG. This embodiment shows the current limiter circuit 11 more specifically, and FIG.
Are configured in the fully differential mode.

The simplest configuration of the current limiter circuit 11 is a so-called differential amplifier, which comprises a differential pair of NMOS transistors M1 and M2 and a bias current source Ib.
The BPF has two BPs having the same configuration as the BPF 12 in FIG.
F121 and F122, respectively, and the current limiter circuit 1
1 is supplied to each of the output currents of the differential amplifiers, and the outputs of the BPFs 121 and 122 are supplied to the current limiter circuit 1.
1 differential input. In this case, BP
F121 and F122 operate with signals having phases opposite to each other and oscillate differentially.

In the case where the differential processing is performed as described above, there is an additional merit thereof, which will be described. The transfer function of the signal VC1 in capacitor C1, VC1 / Iin = [- gm1 / C1C2] / [S 2 + S (gm3 / C2) + gm1gm2 / C1C2] ... (2) , and becomes a low-pass filter (LPF) characteristics. This is because all the capacitors C1 to C4 constituting the BPFs 121 and 122 can be connected to the ground by using the current input. The LPF signal is
The phase advances by 90 degrees (the sign of the numerator is-), that is, the phase becomes quadrature. The same applies to the signal VC3 of the capacitor C3. Since the signals VC1 and VC3 oscillate in opposite phases, it is possible to obtain four-phase signals orthogonal to each other in the BPFs 121 and 122 without providing any additional means.
In addition, the LPF characteristic suppresses the harmonic distortion component of the oscillation signal, and is thus very suitable for various uses such as detection and demodulation.

Therefore, if the configuration of this embodiment is a fully differential type, not only the effect of not requiring a buffer, but also the application as a four-phase signal generator with low distortion can be expanded.

In the circuit of FIG. 2, the transconductance amplifier used for the BPF is shown as a single input / output. This is because a transconductance amplifier can be easily realized when all of the transistors are configured by CMOS transistors, so that the entire circuit scale can be reduced.

FIG. 3 shows an example of a transconductance amplifier having a CMOS configuration. FIG. 3A shows a transconductance amplifier having a negative polarity.
It can be easily configured with a single transistor and a constant current source. FIG.
FIG. 3B shows a transconductance amplifier having a positive polarity, in which the output current of the transconductance amplifier having a negative polarity in FIG. By preparing these positive and negative bipolar transconductance amplifiers, the configurations shown in FIGS. 1 and 2 can be realized. Since the amplifier operates as a common-source amplifier circuit, it has a wide dynamic range and can operate at low voltage. Are suitable.

However, in general, there are many cases where the transconductance amplifier is constituted by differential inputs and outputs.
In this case, the BPF 121 and the BPF 122 can be integrated. FIG. 4 shows a specific example of this case. The transconductance amplifiers -gm1, gm2, and -gm3 of FIG. 1 correspond to the transconductance amplifiers gm7 to gm of FIG.
Corresponding to m9, the transfer function is the same as equation (1). The transconductance amplifiers gm7 to gm9 receive a differential voltage signal and output a differential current signal, and thus can be simply realized by a differential pair. Capacitors C1 to C4 are connected between the differential current output terminals instead of the ground,
Although many variations of the single transconductance amplifier are conceivable, description thereof is omitted here.

The current limiter circuit 11 shown in FIG.
Although the MOS element is used, the present invention is not limited to the limiter, and may be realized by using, for example, a bipolar element in all circuits, or a mixture of both does not impair the effects of the present invention. Of course, it is also possible to vary the maximum gain frequency fo by changing the gm of the transconductance amplifier and use it as a control oscillation circuit.

[0024]

As described above, according to the oscillation circuit of the present invention, it is possible to eliminate problems such as a frequency limit and an increase in current caused by a buffer, and to realize good oscillation up to a high frequency.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating an embodiment of the present invention.

FIG. 2 is a circuit diagram for explaining another embodiment of the present invention.

FIG. 3 is a circuit diagram for explaining a specific example of the gm amplifier in FIGS. 1 and 2;

FIG. 4 is a circuit diagram for explaining a specific example of the BPF in FIGS. 1 and 2;

FIG. 5 is a circuit diagram illustrating a conventional oscillation circuit.

[Explanation of symbols]

11 ... current limiter circuit, 12, 121, 122 ... BP
F.

Claims (4)

[Claims] 1. A filter circuit for receiving a current signal and outputting a voltage signal serving as a transfer function of a secondary bandpass filter, and a current for converting a signal in a predetermined range of an output signal of the filter circuit into a current and outputting the current An oscillation circuit comprising: a limiter circuit; and means for forming an oscillation loop by inputting an output current of the current limiter circuit as a current signal of the filter circuit. 2. The current limiter circuit is a differential amplifier including a transistor differential pair, wherein the filter circuit is configured to operate with a differential signal and includes a differential current input terminal, 2. The oscillation circuit according to claim 1, wherein a differential output current of the amplifier is supplied to a differential current input terminal of the filter circuit. 3. The filter circuit includes two cascaded integration stages and is connected to obtain a voltage signal having the band-pass filter characteristic from one side and a low-pass filter characteristic from the other side. The oscillation circuit according to claim 1, wherein both of the voltage signals of the low-pass filter are output as oscillation signals. 4. The filter circuit includes a source grounded CM.
2. The oscillation circuit according to claim 1, wherein the oscillation circuit is configured by a single input / output transconductance amplifier of an OS transistor.