JPS5946468B2 - High frequency amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in high frequency amplification circuits used in television receivers, and particularly to improvements in high frequency amplification circuits for AFC circuits used to stabilize local oscillation frequencies.

In recent years, with the adoption of ICs in television receivers, intermediate frequency video amplification circuits (hereinafter referred to as VIF amplifier circuits), video detection circuits, and automatic local oscillation frequency control circuits (hereinafter referred to as AFC
High-frequency circuits (referred to as "circuits") are being integrated into ICs and becoming smaller and more sophisticated.

Along with this, synchronous detection circuits have been adopted as video detection circuits to improve image quality and stability of circuit operation. One example of this will be explained based on the drawings. In FIG. 1, the intermediate frequency video signal amplified by the VIF amplifier circuit 1 is transferred to a VI which includes a carrier signal generation circuit 2, a synchronous detection circuit 3, a carrier signal amplification circuit 4, and a video amplification circuit 5.
The signal is input to the circuit C of the E detection circuit 6, and its output is input to the AFC circuit and the video amplifier. The carrier signal generation circuit 2 is a circuit that amplifies the intermediate frequency video signal output from the VIF amplifier 1, selects it with a tank circuit that resonates with the carrier signal, and limits the amplitude to generate a carrier signal used for synchronous detection. The synchronous detection circuit 3 performs multiplication detection of the intermediate frequency video signal and the carrier signal, and can perform detection with good linearity even if the input signal is small compared to envelope detection using a diode that has been used in the past. LLD (LowLevelDe)
It is also called a
Normally, the output is operated at 0.1 to 0.2 vppC', and the output is 1 to 2 vpp by the video amplifier 5.
is amplified. Further, the carrier signal amplifier 4 is for amplifying the carrier signal supplied to the AFC circuit, receives an input from one side of the resonant circuit T, and normally has a frequency of 200 mvr.
ms to 400 to 500 mvrms. Note that the carrier signal generation circuit 2 is connected to the AFC I which will be connected later.
It is possible to incorporate it into the VIF detector circuit 6, but if it is incorporated in the AFC circuit, the gain of the transmitter amplifier will be as large as 26 dB or more, increasing the possibility of oscillation.
It is considered that a good design method is to amplify the signal to about dB and then suppress the gain of the limiter amplifier built in the AFC circuit to about 20 dB to eliminate oscillation. Also, the second
The figure shows an example of a specific circuit of the synchronous detection circuit 3, carrier oscillation circuit 2, and carrier amplifier 4 in FIG. In the figure, the intermediate frequency video signal is
, and is input to the bases of transistors Q5 and Q7. Note that VO is applied as a bias through Rl, , Rl. The outputs from the transistors Q5, Ql are applied to the bases of the transistors Q6, Q8 of the first differential amplifier and the transistors Q9l, QlO of the second differential amplifier, and
, Q, and O have a carrier frequency (58 in Japan).
75 "MH2") is connected to the resonant tank circuit A,
Only intermediate frequency video signals are selected, and diodes Dl, D,
A carrier signal is generated by adding amplitude limitation to the signal. That is, the transistor Q, , QlOl coil L1 capacitor C
2. The diodes Dl, D, and the resistors R2, R correspond to the carrier generation circuit. Also, transistors Ql, Q
Reference numerals 2, Q, and Q4 are synchronous detection units that multiply the intermediate frequency video signal amplified by the first differential amplifier and the carrier signal. Video output is obtained from resistor R1 or resistor R4. CW is also added to the base of transistor Q,,
The signal is amplified by the transistor Ql, outputs from the emitter of the transistor Ql, and is input to the AFC circuit. The emitter of the transistor Ql2 has an IC internal capacity of 10PFf to increase the AC gain. This type of amplifier has a simple circuit, requires little current, operates well even at high frequencies, and has the advantage of not requiring many IC pins because the entire circuit can be fabricated within an IC chip. However, the connection relationship between transistor Ql, capacitor C, and resistor RlO is similar to that of a circuit used for normal transistor detection.
Similarly, there is a high possibility that envelope detection will occur with the base-emitter diode of the transistor Ql2 and the capacitor C.

The possibility of this happening increases as the resistance value of the resistor RlO increases.
When the detection is performed, high frequency waves are generated. When the high frequency level is high and the order is high, it has the disadvantage that it creates beads with the broadcast radio waves and interferes with the screen. By the way, if RlO and C3 are selected well at the design stage, it is possible to only amplify and not detect the signal, but if it is integrated into an IC, there is a possibility that the signal will be detected due to variations. (When integrated into an IC, the resistance variation is ±20%, the capacitance variation is ±50%, and the input signal size also varies.) .

Hereinafter, one embodiment of the present invention will be described based on FIG. 2.

In the figure, C4 is a capacitor inserted between the emitter of transistor Ql and the base of transistor Ql, and its capacitance is small, about 1 to 3 PF. (
Note that the components other than the capacitor C4 are the same as those described above. ) Next, its operation will be explained. First, considering the case where the base input of transistor Qll increases due to variations in the IC circuit, the high frequency signal amplitude increases at the base of transistor Ql, so a detection action occurs, and harmonics appear at the emitter of transistor Ql2.

The harmonics are the carrier, transistor Ql, and resistor R.
lO. ．． It is amplified by the emitter peaking amplification of capacitors C, and resistor Rll, and goes out from the emitters of transistors Q, , and goes to the AFC circuit. High-order harmonics use intermediate connection leads as antennas and fly into the air, causing interference with the image. However, with capacitor C, high-order high-frequency waves pass through capacitor C4, and transistor Qll
The feedback signal enters the base of transistor Ql2 and is negatively fed back, and the feedback signal acts to cancel harmonics at the emitter of transistor Ql2. Further, since negative feedback is provided by the capacitor C4, the higher the harmonic, the better the effect. This is clear from what is shown in FIGS. 4 and 5. In other words, the frequency spectrum of the output appearing at the emitter of transistor Q, when capacitor C, is attached (Figure 5) has higher harmonics than the frequency spectrum when capacitor C4 is not attached (Figure 4). It can be seen that there is a large attenuation. Although the fundamental wave is only attenuated by about 2 dB,
The fourth harmonic is attenuated by 20 dB or more. )Also,
If the resistance value of the resistor RlO and the capacitance of the capacitor C increase, detection will occur more easily and harmonics will be generated more easily, but in this case as well, a capacitor C4 is connected between the emitter of the transistor Ql3 and the base of the transistor Qll. This can be prevented by providing feedback. As is clear from the above, according to the present invention, a capacitor is inserted between the input and output of a transistor amplifier that receives a carrier signal of a synchronous detection circuit and performs emitter peaking using a capacitor so as to provide negative feedback. It has a simple structure, is suitable for IC implementation, and is capable of high-order harmonics. Since waves can be attenuated to a large extent, it is possible to obtain a harmonic amplifier that does not have any adverse effect on images.

Note that instead of inserting the capacitor C, the capacitor C shown in FIG.
Even if a series resonant circuit that resonates at the frequency of a high-order harmonic that causes interference as shown in 1 is inserted, the frequency will be fed back and attenuated as in 1 above.

Further, FIG. 3B shows a parallel resonant circuit resonating with the fundamental frequency, which prevents attenuation of 2 [DB] of the fundamental frequency as shown in FIG. 5, and only harmonics are attenuated. Furthermore, in the case of only capacitor C4 shown in Fig. 2, if the base of Qll and the emitter of Ql are placed on adjacent pins of the IC, the inter-pin capacitance (
2~3PF), and there is no need to install a capacitor outside.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a specific circuit diagram of the main part of FIG. 1, and FIG.
Modifications of the capacitor C4 shown in FIGS. 4 and 5 show frequency spectra when the capacitor C4 shown in FIG. 2 is not inserted and when the capacitor C4 is inserted.

Claims (1)

[Scope of Claims] 1. A high-frequency amplifier circuit which receives a carrier signal of a synchronous detection circuit as an input and is characterized in that a negative feedback means is inserted between the input and output of a transistor amplifier which is subjected to emitter peaking using a capacitor. 2. The high frequency amplifier circuit according to claim 1, characterized in that a capacitor is inserted between the input and output of a transistor amplifier subjected to emitter peaking using a capacitor so as to provide negative feedback. 3. The IC pin-to-pin capacitance is used as the capacitance of a capacitor inserted between the input and output of a transistor amplifier subjected to emitter peaking using a capacitor so as to provide negative feedback. High frequency amplifier circuit. 4. Claim 1, characterized in that a parallel resonant circuit that resonates with the fundamental frequency of the carrier signal is inserted between the input and output of a transistor amplifier subjected to emitter peaking using a capacitor so as to provide negative feedback. high frequency amplification circuit. 5. Claim 1, characterized in that a series resonant circuit that resonates with high-order harmonics is inserted between the input and output of a transistor amplifier with emitter peaking using a capacitor so as to provide negative feedback. high frequency amplification circuit.