JPS6044850B2 - Frequency converter gain control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a front end for an FM receiver, a VHF for TV.
The present invention relates to a gain control circuit for a frequency converter that compensates for gain fluctuations due to temperature changes in tuners and the like.

Generally, a semiconductor amplifying element in a frequency converter is biased to increase its gain, and as a result, the gain fluctuates with temperature.

Conventionally, therefore, compensation for the gain fluctuation has been made using the circuit configuration shown in FIG. That is, Fig. 1 shows the front-end circuit block of an FM receiver, where 1 is a high frequency amplifier, 2 is a local oscillator, and 3 is a dual-gate DOET (
This FET 3 is used as a conversion amplification element of a frequency converter.

The gate of this FET3 is connected to the high frequency amplifier 1.
and the output of the local oscillator 2. The drain of FET3 is connected to IF amplifier 4, and its source is connected to bypass capacitor Cl, thermistor 5, source resistor R,
grounded through a parallel connection of Further, R29R3 is a bias resistor, and C2 is a bypass capacitor. To explain the operation of such a circuit configuration, the input radio wave is selectively amplified by the high frequency amplifier 1 when it matches the resonance frequency determined by the coil relay and the capacitor Cs, and is applied to the gate of the FET 3 that constitutes the frequency converter. At the same time, the output of the local oscillation circuit 2 is also applied to another gate of the FET 3,
The heat output frequencies are mixed together and applied to the IF amplifier 4 as an intermediate frequency output.

Further, the source of the FET 3 is grounded by the bypass capacitor Cl at least for frequencies higher than the intermediate frequency, and the FET 3 has a conversion gain as a source-grounded frequency conversion amplifier. When the temperature fluctuates, the internal impedance of the thermistor 5, which is provided to prevent the FET 3 bias from fluctuating and gain fluctuations, fluctuates, causing the source DC potential to fluctuate, causing the FET 3 bias to fluctuate.
It works to prevent the bias between the source and gate of the circuit from changing. However, such a gain control method that reduces bias fluctuations not only makes the bias conditions difficult, but also makes it difficult to apply DC bias to the gate connected to the output of high-frequency amplification device 1. Since the provided bypass capacitor Co is connected in series with the LC resonant circuit in the high frequency amplifier 1, this capacitor C2 has drawbacks such as requiring a capacitor C2 with low inductance and very good high frequency characteristics at high frequencies. Was. Therefore, the present invention was made to eliminate the above-mentioned drawbacks. Instead of preventing gain fluctuations due to temperature changes in the frequency converter by suppressing bias fluctuations, the present invention compensates for them by changing the source impedance according to temperature changes. This is what I am trying to do. Hereinafter, the present invention will be explained in detail using FIG. 2. The output of high frequency amplifier 1 is a coupling capacitor C
4 to the gate of FET3.

A bias resistor R4 is connected between the gate and ground. Further, the source of the FET 3 is grounded via a series circuit consisting of a bypass capacitor C1 and a thermistor 5, which are connected in parallel with the source resistor R1. With such a circuit configuration, the thermistor 5 operates as a current feedback resistor, and the source impedance becomes a parallel impedance of the source resistor and the impedance of the thermistor 5. When the temperature changes, the bias value of the FET 3 changes, and the internal impedance of the thermistor 5 also changes. As a result, the source impedance also changes, which prevents the gain from changing. For example, if the bias setting is such that the gain decreases due to fluctuations in the bias of FET 3 when the temperature rises, using the thermistor 5 with a negative temperature coefficient will reduce the source impedance, resulting in a decrease in the gain. Fluctuations are prevented.

If the gain is designed to increase as the temperature increases depending on the vias or setting conditions of the FET 3, the purpose can be achieved by using the thermistor 5 having a positive temperature coefficient. Although the embodiments of the present invention have been described using FETs, ordinary transistors may also be used, in which case the emitter corresponds to the source of the FET.

As explained above, in the present invention, the source impedance is changed according to temperature changes in order to compensate for gain fluctuations due to temperature changes. This has great industrial effects, such as the circuit configuration being simpler than the one in which it is not used, design is easier, and costs are reduced.

[Brief explanation of drawings]

FIG. 1 is a front-end circuit diagram of a conventional FM receiver, and FIG. 2 is a front-end circuit diagram of an FM receiver according to an embodiment of the present invention. 1...High frequency amplifier, 2...Local oscillator, 3...FET14...F amplifier, 5
...Thermistor.

Claims (1)

[Claims] 1. A series circuit of a bypass capacitor and a thermistor is connected in parallel to the emitter resistor of a frequency conversion transistor,
A gain control circuit for a frequency converter, characterized in that it compensates for gain fluctuations due to temperature changes. 2. A gain control circuit for a frequency converter according to claim 1, wherein the frequency conversion transistor is an FET (field effect transistor).