OVERLOAD COMPENSATION CIRCUIT FOR ANTENNA TUNING SYSTEM

• US 3,613,008 A
• Filed: 05/15/1969
• Issued: 10/12/1971
• Est. Priority Date: 05/15/1969
• Status: Expired due to Term

• Current Assignee: Kamil Y. Jabbar
• Original Assignee: Kamil Y. Jabbar
• Inventors: Jabbar, Kamil Y.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US04/824,864
• Time in Patent Office: 880 Days
• Field of Search: 325/318,319,364,374,379,383,385,387,422,454,464,451,404 334/11,14,15 330/26,29
• US Class Current: 455/250.1
• CPC Class Codes: H03G 3/3052 in bandpass amplifiers (H.F. or I.F.) or in frequency-changers used in a (super)heterodyne receiver H03G3/32, H03G3/34 take precedence
• CPC Class Codes: H03J 3/185 with varactors, i.e. voltage variable reactive diodes

Abstract

A radio receiver is supplied with signals from a high impedance capacitive antenna coupled in series with a low impedance resistive load in the form of the emitter-base circuit of a common-base RF transistor amplifier through a series-tuned circuit, including a varactor diode connected in series with an inductor. A source of DC biasing potential is provided to vary the biasing voltage on the diode in order to change its capacitance to tune the circuit over a predetermined band of frequencies. Overload compensation for the varactor diode is provided by utilizing the automatic gain control (AGC) voltage of the receiver, with the AGC voltage being applied to the base of the RF amplifier transistor to vary the biasing potential thereon. This in turn causes a corresponding variation in the impedance of the emitter-base path of the transistor with an increasing impedance being caused by increased signal levels. As a result, increasing amounts of the signal supplied by the antenna are dropped across the emitter-base path of the transistor at high signal levels, thereby limiting the RF signal level across the varactor diode to prevent rectification of high level signals thereby.

Claims

1. In a wave signal receiving apparatus having an antenna for receiving a wave signal, an RF amplifier stage including a transistor having output, input and control electrodes, and an antenna tuning circuit means electrically coupling the antenna and the input electrode of the transistor, the tuning circuit means including a voltage-variable capacitor and circuit means connected to the voltage-variable capacitor for applying a variable bias potential thereto to selectively tune the antennatuning circuit means to predetermined frequencies, an overload compensation circuit including in combination:

connecting means for connecting said voltage-variable capacitor of said antenna tuning circuit in series circuit between said antenna and the input electrode of said transistor;

an automatic gain control circuit coupled with the output electrode of said transistor for deriving an automatic gain control voltage corresponding to the magnitude of a received wave signal; and

means for applying said automatic gain control voltage to the control electrode of said transistor to reduce the forward bias on said transistor for increased levels of output signals therefrom, thereby increasing the input impedance of said transistor to limit the voltage drop across said voltagevariable capacitor by causing an increased voltage drop to occur across the input/control electrode path of said transistor.

2. The combination according to claim 1 wherein said transistor has emitter, base and collector electrodes, corresponding to the input, control, and output electrodes, respectively, and is connected in a common base configuration, with said connecting means connecting the emitter of said transistor in a series circuit with said voltage-variable capacitor and said antenna, and with said automatic gain control voltage being applied to the base of said transistor, thereby varying the impedance of the emitter-base path of said transistor.

3. The combination according to claim 2 wherein said voltage-variable capacitor is a varactor diode connected in a series-tuned circuit, series-coupled between said antenna and the emitter of said RF amplifier transistor.