JPS5912205B2 - High frequency amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency amplifier circuit suitable for VHF and UHF electronic tuners of television receivers.

Figure 1 shows an example of a conventional tuner circuit for a television receiver, in which 1 is an antenna, 2 is a bandpass filter, 3 is a high-frequency amplification FET (field-effect transistor), and 4 is a tuning circuit generally designated by 6. inductance element (coil),
5 is a variable capacitance element (variable capacitor) of the tuning circuit 6, 7
8 is a coupling capacitor, 8 is a frequency conversion circuit, 9 to 11 and 15 are bias resistors, 12 and 13 are bias capacitors, 14 is a power supply terminal, and 16 is a gain control voltage terminal.

In FIG. 1, the signal received by antenna 1 is applied to the first gate electrode G1 of FET 3 via bandpass filter 2, and the signal amplified by FET 3 is applied to a tuning circuit connected as a load to the drain electrode. 6 (consisting of a parallel circuit of a coil 4 and a variable capacitor 5),
The signal is transmitted to the next stage frequency conversion circuit 8 via the coupling capacitor 7.

Note that the DC power supply voltage of the terminal 14 is applied to the first gate G1 of the FET 3 via the bias resistor 10, and
It is applied to the drain electrode via the coil 4.

Further, the gain control voltage of the terminal 16 is applied to F through the resistor 15.
It is applied to the second gate electrode G2 of ET3.

Generally, in the high frequency amplifier circuit shown in FIG. 1, the receiving frequency is selected by changing the capacitance of the variable capacitor 5 so that the resonant frequency of the tuning circuit 6 matches the target receiving frequency.

However, the output capacitance of the high frequency amplification element FET3 is usually 1
Since it reaches ~2PF, the minimum value of the equivalent variable capacitance of the tuning circuit 6 increases, and as a result, the variable capacitance range is narrowed, making it impossible to cover the entire reception frequency range.

In particular, in the VHF and UHF bands, the maximum value of the variable range of the variable capacitor of the tuning circuit 6 cannot be made very large (if it is made too large, the Q of the tuning circuit will drop and the gain will drop), so the receiving frequency range is very limited. It gets narrower.

Furthermore, it is difficult to manufacture a variable capacitor with an extremely wide variable range in an electronic tuner.

There are limits.

Furthermore, since FET 3 is directly coupled to tuning circuit 6,
The gain of the high-frequency amplifier is too large, causing deterioration of the cross-modulation interference removal characteristics and internal cross-modulation characteristics of the frequency conversion circuit 8.

In order to prevent such deterioration, damping resistors are added to the tuning circuit to reduce the gain, but this lowers the Q of the circuit, so it is necessary to improve the IF interference removal characteristics and image interference removal characteristics. make worse.

Therefore, conventionally, as shown in FIG. 2, a small-capacity coupling capacitor 17 is inserted between the FET 3 and the tuning circuit 60 in the circuit of FIG. Attempts have been made to reduce the gain by weakening the coupling with the inserted capacitor 17, and to reduce the capacitance applied in parallel to the variable capacitor 5.

However, in this case, the coupling capacitance of the capacitor 17 is
Since it must be made sufficiently smaller than the output capacitance of T3, the gain of the entire circuit is reduced due to this coupling capacitance.

In addition, the added choke coil should be one that does not affect other parts of the circuit over the entire frequency band used (ideally, it should exhibit infinite inductive reactance over the entire target frequency band). Therefore, it is manufactured by winding a conductor with a small diameter many times, but it is manufactured by manufacturing a conductor that always exhibits an extremely large inductive reactance over a wide range of frequency bands used, such as VHF and UHF television tuners. It is difficult to do this, and in reality, the more the wire is wound, the more capacitance occurs between the lines, and above the self-resonant frequency due to this capacitance, it becomes a capacitive reactance.

It would be good if it were possible to bring the self-resonance frequency far above the entire target band and to create a device with high inductive reactance over the entire band, but as mentioned above, such a thing is impossible.

Therefore, a choke coil with a self-resonance frequency of several MH2 to 100 MH2 is usually used as this choke coil, but such a choke coil already acts as a capacitive reactance of several pF in the receiving frequency band of a VHF television tuner. , this capacity is capacitor 17
As a result, the effective capacitance of the tuning circuit still cannot be lowered, which is an obstacle to expanding the variable range of the receiving frequency.

Furthermore, as a result of adding the capacitance of the choke coil in parallel with the output capacitance of the high frequency amplification element FET3,
The equivalent load impedance (its output impedance is typically quite high) is lowered, creating a larger mismatch and correspondingly lowering the gain.

Furthermore, if the capacitance of the coupling capacitor 17 is increased to compensate for this decrease in gain, the receiving frequency variable range becomes narrower as described above.

An object of the present invention is to eliminate the drawbacks of the conventional circuit described above, to have a simple circuit configuration, and to be able to easily combine a high frequency amplification element and a tuning circuit without causing mismatch, resulting in an extreme gain reduction. It is an object of the present invention to provide a high frequency amplification circuit which has a sufficiently wide variable range of reception frequency by suppressing the equivalent additional capacitance to a variable tuning capacitance element and is free from various interference characteristics.

In order to achieve these objects, the high-frequency amplification circuit of the present invention includes an active element for high-frequency amplification (for example, FET), a tuned circuit that is its output load, and a high-frequency coil that couples the active element for high-frequency amplification and the tuned circuit. It is characterized by having the following.

In other words, the output electrode of the active element for high frequency amplification (for example, F
The drain electrode of the ET) and the tuning circuit are not directly connected as shown in Figure 1, but are connected via a small capacitance 1 as shown in Figure 2.
7, but rather through a high frequency coil having a certain distributed capacitance or having a suitable capacitance connected in parallel.

With this configuration, the output capacitance of the active element (
For example, the capacitance between the drain and source electrodes of an FET (FET) is only added to the variable capacitance element of the tuning circuit via the effective capacitance of the high-frequency coil (as a series capacitance of both), so its variable range is different from that in Figure 1. It can be made large enough.

Also, the equivalent additional capacitance for the variable capacitor 5 in Fig. 2 is FE
Sum of T output capacitance and choke line capacitance and coupling capacitance 17
In contrast, in the present invention, there is only a series capacitance between a similar output capacitance and the effective capacitance of the high-frequency coil, so it can be made sufficiently small.

In the case of FIG. 2, the coupling capacitance 17 must be made sufficiently smaller than the variable capacitance element 5, but in the present invention, the additional capacitance can be made small without such a restriction on the effective capacitance reactance component of the high-frequency coil. Therefore, the gain is not reduced.

In a preferred embodiment of the present invention, the parallel resonance frequency of the high-frequency coil itself or the portion consisting of a capacitor connected in parallel with the high-frequency coil is selected to be slightly lower than the lowest frequency channel of each target receiving channel frequency band. do.

With this selection, when the low frequency channel and the high frequency channel are separated from each other and the high frequency channel gain is lower than the low frequency channel as in a VHF television tuner, for example, the difference can be compensated for.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram of an embodiment in which the present invention is applied to a two-band electronic tuner for a television receiver, in which a variable capacitance diode is used as the variable capacitor.

In FIG. 3, 20 is an antenna, which is connected to the first gate G1 of the FET 50 via a bandpass filter 21. In FIG.

The part surrounded by dotted line 85 is a double tuning circuit, and FET5
The output drain electrode of 0 and this tuning circuit 85 are connected by a parallel circuit of a coupling capacitor 90 and a coupling choke coil 91.

The output end of the tuning circuit 85 is connected to the subsequent frequency conversion circuit 80 via a coupling capacitor 48.

A gain control voltage is applied to the second gate G2 of the FET 50 from a terminal 70 via a bias resistor 22.

2324.25.26 is a bias resistor for driving the FET 50, and 40 is a bias capacitor.One double tuning circuit 85 is a variable capacitance diode 51.

52 and tracking correction capacitors 46° and 47, coils 60 and 61 for the higher receiving frequency channel band (i-band) of the two bands, and coil 62 for the lower receiving frequency channel band (low band).
, 63, 64, 32 is the terminal 73
This is a bypass resistor that holds this terminal at ground potential when the terminal is disconnected from the DC voltage source.

Next, the operation of the embodiment shown in FIG. 3 will be explained.

During low band reception, positive power supply voltage (10B) is applied to terminal 72.
) is applied and the terminal 73 is released.

As a result, the terminal 73 becomes the ground potential (charged potential), and the switching diodes 53 and 54 are both reverse biased and become non-conductive, so that the coils 60 and 62, 61 and 63 of both the low band and the biband are connected to each other. They can be connected in series and receive low band signals.

At this time, the DC voltage (10 B) of the low band terminal 72 is applied to the coils 64, 62° 60 and the high frequency choke coil 91.
is applied to the drain electrode of FET 50 as a power supply voltage, and is applied to the first voltage via bias resistor 24.25.
Bias voltages for the gate electrode G1 and source electrode S are also applied.

During biband reception, terminal 72 is disconnected and a switching voltage (10 B) is applied to terminal 73.

At this time, the switching diodes 53 and 54 become conductive, and the coils 62 and 63 are both connected to the bypass capacitor 44.
The coils 60 and 61 are short-circuited in terms of high frequency, and only the coils 60 and 61 operate to receive biband signals.

At this time as well, the voltage (10B) at the terminal 73 is applied as a power supply voltage to the drain D of the FET 50 via the switching diode 53, the coil 60, and the choke coil 91. A bias voltage is applied to the first gate G1 and source S of the FET via resistors 24 and 25.

However, the resistor 29 is connected to the diode 54 during biband reception.
This resistor is used to ensure that the diode 54 is in a conductive state by allowing a current to flow through the diode 54.

As a feature of this embodiment, the drain electrode of the FET 50 and the input terminal of the double-tuned circuit 850 are coupled through a parallel circuit of a capacitor 90 and a choke coil 91, and a bias voltage to the drain of the FET 50 is supplied through the choke coil 91. ing.

By the way, as is clear from what I said earlier, UH
A choke coil used in the F band or VHF band usually needs to have an inductance of about several 100 nH to several μH.

For this reason, choke coils are usually wound with thin diameter conductor wires that are closely connected or layered many times, and since stray capacitance occurs between the wires in the choke coil, it is difficult to avoid choke coils. A coil is represented by a parallel circuit of inductance and capacitance.

In the embodiment of FIG. 3, this choke coil 91 has a 2.2
A μH type is used, and its line stray capacitance is 2.
2 pF.

Therefore, the FET 50 and the double tuning circuit 85 are coupled by the stray capacitance of 2.2 pF plus the capacitance of the coupling capacitor 90.

In the conventional circuit shown in FIG. 2, the choke coil is directly connected between the power supply and the output drain of the FET 50, so this stray capacitance 175 - the output capacitance (drain-source capacitance) of the F''ET 50. is added in parallel to FET5
As a result of the mismatch with 0, the output voltage appearing across the choke coil becomes low and the gain decreases.

Moreover, since this voltage is further divided through the coupling capacitor 17, the gain is further reduced.

Therefore, if the capacitance of the capacitor 7 is increased to prevent mismatch and gain reduction due to the capacitor 17, the parallel capacitance of the FET output capacitance and the choke coil stray capacitance will be added to the variable capacitance 5 of the tuning circuit. This narrows the tuning range.

On the other hand, in the embodiment of the present invention shown in FIG.
When viewed from the output end (drain source) of the choke coil, the stray capacitance of the choke coil does not directly add to the output capacitance;
Via a double-tuned circuit (equivalently as a smaller capacitance)
Since it is added, no mismatch will occur and the gain will not be unduly reduced.

Also, this stray capacitance or the capacitance of the capacitor 90 is the second
Since it is not necessary to make it as small as the capacitor 17 shown in the figure, a decrease in gain can be prevented from this point as well.

Furthermore, when viewed from the double-tuned circuit side, what is added to the tuning capacitance (variable capacitance diode 51) is not the sum of the stray capacitance of the choke coil 91 (or the parallel capacitance with the capacitor 90) and the FET output capacitance t. Since these series capacitances (smaller than each capacitance) can reduce the minimum value of effective tuning capacitance, the receiving frequency range can be increased by more than 1.3 times compared to the one in Figure 2. Can be expanded.

Furthermore, when compared with FIG. 1, the embodiment of FIG.
Since the damping resistance can also be made lighter, it is possible to maintain good interference characteristics without reducing the Q of the circuit.

Now, in general, in a high frequency amplification circuit using a vacuum tube or a transistor, especially an FET, the lower the frequency, the higher the gain.

When the reception frequencies of the low band and high band are more than doubled, as in a television tuner, gain deviation between the low band and high band becomes a problem.

Therefore, damping of the tuned circuit is usually performed only in the low band to lower the Q of the circuit, or the degree of coupling between the high band and low band double tuned circuits is changed with a small capacitance to reduce the gain in the no band. Although it has been attempted to make the gains of both bands the same, this is not preferable because it worsens the interference characteristics and sacrifices the gain as described above.

Therefore, in the embodiment of the present invention shown in FIG. 3, the resonant frequency of the high frequency choke coil 91 and the capacitor 90 (including the stray capacitance of the coil 91) is set to a frequency lower to some extent than the lowest receiving frequency of the low band.

With this setting, the parallel circuit (coupling circuit) of the high-frequency choke coil 91 and capacitor 90 becomes a capacitive reactance over the entire receiving frequency band starting slightly above this resonance frequency, and when receiving low-band signals, the coupling circuit with the The impedance is high, meaning it has the equivalent effect of coupling with a small capacitor, and the resonance point is much lower than the reception frequency during high-band reception, so the inductance of the choke coil 91 The reactance component is ignored, and the effect is equivalent to that of coupling with a relatively large capacitance, which is the sum of the stray capacitance and the capacitance of the capacitor 90.

Thus, according to this embodiment, the gain deviation between the low band and the high band is reduced, and the interference characteristics seen in the conventional example are also improved.

In the embodiment described above, the output electrode of the high-frequency amplification element and the double-tuned circuit are coupled by a parallel circuit of the high-frequency choke coil 91 and the capacitor 90, but the capacitor 90 is omitted. It goes without saying that the same effect can be achieved even if the high frequency coil or choke coil 91 is used alone for coupling.

In addition, active elements for high frequency amplification are not limited to FETs.
It is clear that the same effect can be achieved with transistors and vacuum tubes.

As described above in detail through the embodiments, the high-frequency amplification circuit of the present invention connects the output electrode of the high-frequency amplification active element and the tuning circuit that serves as its load by using a high-frequency coil or a capacitor connected in parallel with the high-frequency coil. With this configuration, the reception frequency range of the tuned circuit is expanded, and the active element and the tuned circuit are easily matched to obtain appropriate gain and good interference rejection. This has effects such as being able to obtain certain characteristics.

Further, since the high frequency coil also serves as a supply path for the DC power supply voltage to the high frequency amplification active element, the number of circuit components does not increase compared to the conventional circuit, and the characteristics can be improved.

Incidentally, when the resonance frequency of the high-frequency coil is set slightly below the target receiving frequency band, the effect of reducing the gain deviation in the entire frequency band is also produced.

[Brief explanation of drawings]

1 and 2 are electric circuit diagrams showing each side of a conventional tuner for a television receiver, and FIG. 3 is an electric circuit diagram of an embodiment in which the high frequency amplification circuit of the present invention is applied to a tuner for a television receiver. It is an electrical circuit diagram. 50... Active element for high frequency amplification, 85... Tuning circuit, 90... Coupling capacitor, 91... High frequency coil.

Claims (1)

[Claims] 1. A high frequency amplification circuit in which an output electrode of an active element for high frequency amplification having an output capacitance and an input electrode of a parallel variable tuning circuit having a variable capacitance are electrically connected, the output electrode of the active element for high frequency amplification having an output capacitance. and an input electrode of the parallel variable tuning circuit, a high frequency amplifier circuit comprising a high frequency coil resonating at a frequency lower than the resonant frequency of the parallel variable tuning circuit and a coupling capacitor is connected in parallel.