JPH066610Y2 - High frequency gain control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention is a circuit devised to flatten the output change due to the strength of the electric field in a high frequency amplifier such as a radio. Basically, it is in the form of a differential amplifier. When balanced, the output is canceled and almost no signal appears.The bias voltage on one side of this differential amplifier is set to an unbalanced state by using a variable resistance element such as a transistor, and the output is output. If the bias variable resistance element is made to appear, and the resistance of the variable resistance element changes when a strong input enters the AM detection output, and the differential amplifier approaches the balance point, the result is that the output of the detector is This is a circuit in which the output is suppressed and a flattened output is obtained. More specifically, as shown in FIG. 1, the input side of the differential amplifier has the same phase (polarity). The input and output sides have a circuit configuration in which the outputs are opposed to each other and canceled so that the bias voltage of 1 is low, the bias voltage of 2 (amplification factor variable transistor) is high, and the input to 3 is increased positively. If so, the internal resistance of 3 will decrease and the operating point on the 2 side will be 1
Since it approaches the operating point on the side and goes toward the balance direction, the output of the differential amplifier decreases, the detection output also decreases, and the output can be strongly flattened. [Therefore, effective gain control can be performed easily] Supplementary explanation 1) The balance of the intermediate frequency transformer 5 can be adjusted by placing the core at the center of the coil and turning it with a screwdriver or the like. It can be made difficult.

2) If the AM input noise of the circuit is also input to the base input of 3, the noise and distortion of the circuit can be reduced. or,
It is possible to increase the number of amplification stages and make it a highly sensitive circuit.

3) What is used as the AM detection is a commonly used envelope detection.

[Brief description of drawings]

FIG. 1 is a circuit showing a gain control system 1-differential amplification transistor 2-differential amplification transistor 3-bias variable transistor 4-high frequency transformer 5-balance intermediate frequency transformer 6-AM detection diode 7-oscillator Input Terminal FIG. 2 is a schematic partial view of a circuit in which an FET is used and the source bias is variable.

Claims (1)

[Scope of utility model registration request] 1. A transistor in a high-frequency mixer section composed of transistors 1 and 2 has an opposed type (balanced type) circuit configuration, inputs of the transistors are input in both common phases (polarity), and the output of the transistor is In order to cancel the output, it is connected to the transformer 5 with a balanced coil so that the opposite current flows, and the capacitor is tuned to change one transistor (amplification factor variable transistor 2) with the AM detection output. However, when a strong input is input, the circuit approaches the fixed operating state of 1 and lowers the high frequency mixer output to obtain the AGC (Automatic Gain Control) effect.