JPH0227612Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an oscillation circuit using a differential amplifier, and in particular to an oscillation circuit suitable for use in local oscillation circuits such as television tuners and FM tuners. .

[Background technology and its problems]

Generally, when extracting the oscillation output from an oscillation circuit using a differential amplifier and sending it to a buffer amplifier, etc., it is necessary to improve the isolation between the oscillation circuit and the buffer amplifier, etc. to ensure stable oscillation operation. is important.

For example, TV Jiyeon Chiyuna and radio FM
Alternatively, when using a so-called frequency synthesizer tuner as an AM tuner, in addition to the main oscillation output from the local oscillation circuit to the mixer,
It is necessary to extract the pickup output to the PLL system. Conventionally, such a pickup output for a PLL is often obtained by loosely coupling a pickup coil to a resonant coil of a local oscillation circuit, and a resistor or capacitor is generally connected to such a pickup coil. This not only increases the number of parts, complicates the configuration, and increases the mounting space, but also causes loose coupling to the resonant coil, which may reduce the Q value of the resonant circuit, shift the oscillation frequency, or cause oscillation. There is a risk that adverse effects such as destabilization may occur.

[Purpose of invention]

In view of the above-mentioned points, the present invention can improve the isolation from the buffer amplifier etc. when extracting the oscillation output from the oscillation circuit, and is particularly suitable for mixed In addition to the main oscillation output to devices etc.
The purpose of the present invention is to provide an oscillation circuit that can take out the pick-up output to a PLL system or the like while maintaining good isolation with a simple configuration.

[Summary of the idea]

That is, the feature of the oscillation circuit according to the present invention is that the first,
a second transistor; a variable frequency resonant circuit connected to the collector of the first transistor; a capacitor for positively feeding back a signal from the collector to the base of the second transistor; A third transistor with a common base and an emitter connected to the collector, and a resistive load connected to the collector of the third transistor, and oscillation occurs from the connection point between the collector of the third transistor and the load. While taking out the output,
The purpose is to extract a differential oscillation output from each base of the first and second transistors.

〔Example〕

Hereinafter, as an embodiment of the oscillation circuit according to the present invention, an oscillation circuit suitable for use in a local oscillation circuit of a tuner section of a television receiver will be described with reference to FIG.

In FIG. 1, a voltage from a base bias voltage source circuit consisting of voltage dividing resistors 3, 4, a transistor 5, and a current source 6 is applied to each base of a pair of NPN type transistors 1 and 2 whose emitters are commonly connected. , are supplied via resistors 7 and 8, respectively. A constant current source 9 is connected to the commonly connected emitters of transistors 1 and 2.
The base and collector of the transistor 2 and the base of the transistor 2 are connected to a resonant circuit (so-called tank circuit) 14 via terminals 11, 12, and 13, respectively. Here, the main parts of the oscillator other than the resonant circuit 14 are configured, for example, inside a tuner IC (integrated circuit), and the above-mentioned terminals 11, 12, 13 are
Compatible with external connection pins of IC.

The main part of the resonant circuit 14 is a parallel resonant circuit consisting of a capacitor 15 and a coil 16 connected to the collector of the transistor 1 via the terminal 12, and a capacitor 17 connected between the terminals 12 and 13.
, and by positively feeding back the resonant frequency component approximately determined by the parallel resonant circuit of the collector output of one transistor 1 of the transistor differential pair to the base of the other transistor 2, It performs oscillation operation. Further, a capacitor 18 is connected to a terminal 11 led out from the base of the transistor 1. moreover,
When used as a local oscillator for a television tuner, it is necessary for the oscillation frequency to change depending on the channel selection frequency, and for this reason, a capacitor 19 is connected in parallel to the capacitor 15 of the parallel resonant circuit. A series circuit with the varicap diode 20 is connected, and this varicap diode 20
A control voltage for tuning is applied through a terminal 21 and a resistor 22.

The oscillation output from such an oscillation circuit is generally transmitted from a resonant system or from a differential pair of transistors 1 and 1.
In this embodiment, a pair of transistors 25 and 26, whose emitters are commonly connected, is used to output the output to the mixer of the tuner, which will be described later, from the bases of transistors 1 and 2, which will serve as a buffer. I am taking it out. That is, the bases of transistors 25 and 26 are connected to the bases of transistors 1 and 2, and the constant current source 27 is connected to the common emitters of these transistors 25 and 26. The output from the collector is sent via a buffer amplifier 28 to a tuner mixer, which will be described later.

Furthermore, as a characteristic configuration of the present invention, a resistor 32 serving as a load is connected to the collector of the other transistor 2 of the oscillation differential amplifier via a transistor 31 whose base is grounded.
The oscillation output is taken out from the connection point between the collector of the load resistor 32 and the load resistor 32. This oscillation output is
The signal is sent via a capacitor 33 to a PLL-based buffer amplifier, which will be described later. Note that the base of the transistor 31 is applied with a DC bias voltage from voltage dividing resistors 34 and 35, and is grounded in an AC manner by a capacitor 36.

By the way, in a differential oscillation circuit with such a configuration, the collector of transistor 2 of the differential amplifier is a part unrelated to the oscillation operation, and a load is directly connected to this collector, and the oscillation output is output from the connection point. In other words, the transistor 31
Although it is possible to omit , in this case, the base input capacitance of transistor 2 increases due to the so-called Miller effect when the oscillation output increases,
There is a risk that the oscillation operation will be adversely affected due to a decrease in the amount of feedback, a change in frequency characteristics, etc.
On the other hand, by connecting the base-grounded transistor 31 to the collector of the transistor 2 to form a so-called cascode amplifier configuration as in the present invention, the mirror effect of the transistor 2 is significantly reduced, and the adverse effect on the oscillation operation is prevented. Along with being
By improving the isolation between the oscillation circuit and the PLL buffer amplifier, stable oscillation operation can be achieved. This is extremely useful when applied to a tuner with a wide receiving frequency band that can receive not only VHF but also CATV, since the oscillation operation can be kept stable even when the level fluctuates greatly.

An example of a television tuner circuit constructed using such an oscillation circuit as a local oscillation circuit will now be described with reference to FIG.

That is, FIG. 2 shows a schematic configuration of a frequency synthesizer type television tuner circuit, and in this FIG.
Several tens of MHz such as VHF signals and so-called CATV signals
An RF signal of approximately 470MHz is supplied. this input
The RF signal is sent to a mixer (so-called mixer) 43 via a buffer amplifier 42 in a tuner IC (integrated circuit) 40, where it is mixed with the oscillation output from the local oscillation circuit 44 having the configuration shown in FIG. is converted into an intermediate frequency signal (so-called IF signal). The IF signal from the mixer 43 is sent to the output terminal 4 via the IF amplifier 45 and the IF output circuit 46.
Extracted from 7. Here, when receiving UHF,
The so-called UIF signal, which is the UHF signal from the UHF tuner section etc. converted into an intermediate frequency signal, is input to the input terminal 48.
This UIF signal is sent to, for example, a load resistor in the mixer 43 via the UIF amplifier 49, so that the mixer 43 outputs a UIF signal, which is output via the IF amplifier 45 and the IF output circuit 46. It is taken out from the terminal 47.

Next, the local oscillation circuit 44 in the tuner IC 40
The above-mentioned IC external connection terminals 11, 12, 1
3, the resonant circuit (so-called tank circuit) 14 having a coil, a capacitor, etc. is connected. A part of the oscillation output of the local oscillation circuit 44 is
The signal is taken out from a terminal 52 via a buffer amplifier 51 as a pickup output for configuring a so-called PLL circuit. This pickup output is sent to a programmable divider 54 via a prescaler 53. The prescaler 53 connects the pickup output to the programmable divider 5.
4 pre-divides the frequency to a divisible frequency, and the frequency division ratio is fixed, whereas the programmable divider 54 changes the frequency division ratio in accordance with the channel selection signal. This programmable divider 5
The output from 4 is sent to a phase comparator 55 where the phase is compared with a reference signal of a constant frequency. The phase-compared output is sent to a tuning control voltage generation circuit 57 via an LPF (low pass filter) 56, and the output voltage is sent to a variable capacitance element of the resonant circuit 14, for example, the aforementioned varicap diode 20. By this, the local oscillation frequency is controlled.

Note that the terminals 61 and 62 led out from the mixer 43 to the outside of the IC are connected to the LC resonant circuit 6 as necessary.
3 may be connected to provide a bandpass type selection characteristic such that only the frequency component of the IF signal is outputted from the mixer 43.

According to the embodiment of the present invention described above, in an oscillation circuit using a differential amplifier, when the oscillation output is taken out from the collector of the transistor 2, which is basically unrelated to the oscillation operation, to the buffer amplifier 51 of the PLL system. , transistor 3 is connected to the collector.
1 is connected to form a cascode system, and a resistor 32 serving as a load is connected to the collector of this transistor 31 to take out the oscillation output from this connection point. separation of
This results in better oscillation and more stable oscillation operation.
This is because transistors 2 and 31 form a so-called cascode amplifier, so the mirror effect of transistor 2 is significantly reduced, the input capacitance is kept low, and feedback is not adversely affected.

Furthermore, in the above embodiment, the oscillation output to the mixer 43 of the tuner is taken out from the bases of the transistors 1 and 2 of the differential amplifier via the differential pair of transistors 25 and 26. The isolation between the oscillation circuit and the buffer amplifier 28 is also good, and does not have any adverse effect on the oscillation operation.

It should be noted that the present invention is not limited to the above-mentioned embodiments, and may be applied to, for example, a configuration in which oscillation output is extracted from the bases of transistors 1 and 2 via a load.

[Effect of idea]

According to the oscillation circuit according to the present invention, a differential oscillation output is extracted from each base of the first and second transistors constituting the differential amplifier, and the collector of the second transistor is provided with positive feedback to the bases. is connected to the load via a cascode transistor, and the oscillation output is taken out from this connection point, so the mirror effect of the second transistor is suppressed, the oscillation operation is stabilized, and the oscillation circuit and buffer amplifier Not only is isolation improved, but also the differential operation by the first and second transistors is performed in a well-balanced manner, and good differential outputs can be taken out from each base. Furthermore, when the present invention is applied to a local oscillation circuit such as a frequency synthesizer tuner, the part for extracting the pickup output to the PLL system can also be configured inside the IC circuit, so compared to the conventional method for extracting the pick-up output by a loose coupling of the coil. It is easy to configure, does not take up much space, and
Even when applied to a broadband tuner that can receive CATV, it has good isolation from the oscillation circuit, and the PLL pickup output can be taken out while maintaining stable oscillation operation.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
The figure is a block circuit diagram showing an example of a television tuner circuit using the oscillation circuit according to the present invention as a local oscillation circuit. DESCRIPTION OF SYMBOLS 1...First transistor, 2...Second transistor, 14...Resonance circuit, 31...Third transistor.

Claims (1)

[Claims for Utility Model Registration] First and second transistors constituting a common-emitter differential pair of transistors; a variable frequency resonant circuit connected to the collector of the first transistor; a capacitor that provides positive feedback to the base of the second transistor; a third transistor with a common base whose emitter is connected to the collector of the second transistor; and a resistive load connected to the collector of the third transistor. An oscillation output is extracted from a connection point between the collector of the third transistor and the load, and a differential oscillation output is extracted from each base of the first and second transistors. Oscillation circuit.