JPS6276805A - High frequency gain control circuit - Google Patents

Abstract

PURPOSE:To obtain the AGC (automatic gain control) effect by changing the balance of a balance type high frequency amplifier in a direction to be balanced by an AM detection output. CONSTITUTION:Inputs to a differential amplifier are inputted in the same polarity, and a bias voltage of a transistor (TR) 1 canceling the output side to oppose the outputs is lowered and a bias voltage of a TR 2 is increased. In increasing the input to a TR 3 at a positive voltage, the internal voltage of the TR 3 is decreased, the operation of the TR 3 approaches the operating point of the TR 1 so as to be balanced. Thus, the output of the differential amplifier is decreased, the detection output is reduced to make the output flat.

Description

[Detailed Description of the Invention] This invention is a circuit devised to flatten changes in output due to strength of electric field for high frequency amplifiers such as radios. When the circuit is balanced, the output is canceled and no signal appears. The bias voltage of one side of this differential amplifier is set to an unbalanced state using a variable resistance element such as a transistor so that no output appears, and this bias variable resistance element is set to a strong input by the output from the AM detector. When , the resistance of the variable resistance element changes and the differential amplifier approaches the balance point.As a result, the output of the detector is suppressed and a flattened output is obtained.A more detailed explanation of this circuit is shown in Figure 1. The input side of the differential amplifier has the same polarity as the input, and the output side has the output facing each other for cancellation.The bias voltage of TRI is low, and the bias voltage of TR2 is high.The input to TR3 is set to increase with a positive voltage. In this case, the internal resistance of TR3 decreases, the operation of TR3 approaches the operating point of TR1, and the balanced detection output also decreases, flattening the output.Supplementary explanation 1) How to balance the intermediate same wave transformer is to Adjustment can be avoided by placing it in the center of the coil and turning it with a screw, etc. to balance it and tuning it with a capacitor. 2) If you input audio signals to the base input of TR3, you can eliminate circuit noise and distortion. can also be reduced

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the gain control system. TR1 - Differential amplification transistor TR2 - Differential amplification transistor TR3 - Bias movable transistor RFT - High frequency transformer IFT - Balance intermediate frequency transformer D - AM detection diode

Claims (1)

[Claims] Regarding a device that has the circuit configuration shown in Figure 1 and attempts to obtain an AGC (automatic gain control) effect by changing the balance of the balanced high-frequency amplifier to balance with the AM detection output.