JPS636938Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an IF output circuit for a television channel.

Conventional TV channel IF (intermediate frequency)
The circuit shown in FIG. 1 was used as the output circuit. In FIG. 1, TR ₁ and TR ₂ are cascode-connected amplification transistors, and terminal 1
Amplify the IF signal applied to the Furthermore, R ₁ , R ₂ ,
R ₃ and R ₄ are bias resistors, respectively, and C ₁ and C ₂
is a bypass capacitor. transistor tr ₂
A π-type tuned circuit 2 consisting of capacitors C ₃ and C ₅ and an inductance coil L ₁ is connected to the collector side of the circuit, and its output is led to the output terminal 3 via a coupling capacitor C ₆ . Lc is a chiyoke coil. The gain and bandwidth of the π-type tuned circuit 2 can be determined by a resistor _R5 connected across the inductance coil _L1 . In Fig. 1, the value of this resistor R ₅ is increased to reduce the damping, so the selectivity characteristic of the π-type tuned circuit 2 is a narrow bandwidth characteristic of approximately 1 MHz as shown in Fig. 4, and the output Together with the IF amplifier circuit on the television set side connected to terminal 3, staggered tuning is formed.

However, recently it has been pointed out that there are inconveniences in the process of adjusting the staggered tuning with the tuner and the IF amplifier circuit on the TV set side connected together as in the past.
Since surface acoustic wave filters with ideal selectivity characteristics are increasingly being used in IF amplifier circuits, the tuner's IF output circuit forms part of staggered tuning together with the IF amplifier circuit on the TV set side. There is no longer a need for it, but rather it has become desirable to have flat transfer characteristics within the IF band and low output impedance (75Ω as a specific example).

The second circuit is designed to meet these demands.
What is shown in the figure has already been proposed. In this Figure 2, where the same parts as in Figure 1 are given the same symbols, a capacitor C ₃ , an inductance coil L ₁ , and a common collector transistor are shown.
A tuned circuit is formed by the input impedance of TR ₃ , and the value of the damping resistor R ₅ ' is made very small to increase the damping effect, resulting in a single-peak tuned circuit with a bandwidth of about 10 MHz as shown in Figure 5. None, almost flat across the entire IF band.
The IF output is taken out from the emitter of transistor _TR3 as a 75Ω low impedance. In this circuit, we focused on the fact that the input impedance of the common collector (emitter follower) transistor is high and the output impedance is low.Despite the low output impedance, the high impedance on the input side allows for good matching of the tuning circuit and avoids loss of gain. Therefore, the damping effect of the resistor R ₅ ' is increased accordingly to widen the bandwidth. However, the circuit shown in Figure 2 has two problems: it requires a common-collector amplification stage, and because the selectivity is extremely loose, the gain outside the IF band is also quite large, making it easy for interference signals to pass through. It is not appropriate.

The present invention proposes a new and effective output circuit devised to eliminate these drawbacks.

In FIG. 3, in which the present invention is implemented, elements similar to those in FIGS. 1 and 2 are given the same symbols for convenience of explanation. There are no accompanying elements, and an output side capacitor _C5 of the π-type tuned circuit 2 is provided between the output side of the inductance coil and the power supply (+B), and a second inductance coil _L2 is connected in parallel with this capacitor _C5 . A parallel tuned circuit 4 is formed by the capacitor _C5 and the second inductance coil _L2 .
By separating the center frequencies of the parallel tuning circuit 4 and the π-type tuning circuit 2 from each other by several MHz, a band characteristic having bimodal selectivity as shown in FIG. 6 is obtained. In Figures 4 to 6, P is the video IF carrier and S is the audio IF carrier.
In the present invention, the center frequency of the π-type tuned circuit 2 may be selected close to P, and the center frequency of the parallel tuned circuit 4 may be selected close to S.

Incidentally, FIG. 7 is an equivalent circuit of the main part of FIG. 3. In the present invention, the value of the damping resistance R ₅ ″ is slightly smaller than that in Figure 1, but approximately the same (for example, several tens of kilohms, by the way, in Figure 2).
R ₅ ′ is several KΩ).

In the present invention, the π-type tuned circuit 2 converts the relatively high impedance of the output of the common base transistor TR ₂ to a low impedance in the vicinity of 75Ω,
The relatively narrow transmission bandwidth of 1 to 3 MHz obtained at this time is compensated for by the parallel tuning circuit 4 .
Incidentally, since the decrease in gain can be suppressed to about 4 dB compared to the gain obtained by the circuit shown in FIG. 1, there is no problem in practical use. It should be noted that the gain in the case of FIG. 2 is also lower than that in FIG. 1.

According to the present invention, a wideband output circuit with low output impedance can be obtained without adding an amplification transistor for impedance conversion. Furthermore, since the amount of attenuation outside the desired band can be increased, the effect of interference signals can be greatly reduced. Furthermore, the present invention has the advantage that the reduction in the gain of the bimodal characteristic of the IF output obtained by the present invention is smaller than that using a double-tuned circuit, and is extremely useful.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an output circuit of a conventional television tuner, and FIG. 2 is a diagram showing an output circuit of another conventional television tuner. FIG. 3 is a diagram showing an output circuit of a television tuner embodying the present invention. 4 is a diagram showing the characteristics of the circuit shown in FIG. 1, FIG. 5 is a diagram showing the characteristics of the circuit shown in FIG. 2, and FIG. 6 is a diagram showing the characteristics of the circuit shown in FIG. 3. FIG. 7 is an equivalent circuit diagram of the main part of FIG. 3. 2 ...π-type tuned circuit, _C5 ...Output side capacitor, _L2 ...Inductance coil, 4 ...Parallel tuned circuit.

Claims (1)

[Scope of utility model registration request] A common-base amplification transistor is connected to the input capacitor, and another inductance coil is connected in parallel to the output capacitor of a π-type tuned circuit, in which a relatively large damping resistor is connected in parallel with the inductance coil. A parallel tuned circuit is formed by the output capacitor and the other inductance coil, and the center frequency of the π-type tuned circuit is near the video IF carrier, and the center frequency of the parallel tuned circuit is near the audio IF carrier. An IF output circuit for a TV set.