JPS6133482B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In amplitude modulated broadcast receivers, the power level of the received input signal may vary over a wide range.
Traditionally, this has caused various problems in the radio frequency amplifier stage of the receiver. A high gain amplifier is required to boost very weak signals, but when a large signal pre-input occurs, this amplifier is driven into saturation. This results in signal distortion and relatively high power intermodulation or crosstalk.

The invention uses separate high-gain and low-gain amplifiers coupled by a gradual switch circuit,
The above problem is solved without reducing the sensitivity by sending the low level input signal to a high gain amplifier and the high level input signal to a low gain amplifier. Signals of intermediate strength will be amplified by both amplifiers. In operation, a broadcast signal received at the antenna terminal is applied to both a high gain and a low gain amplifier. When there is no signal or the received signal is relatively low power, the amplifier circuit is biased such that the high gain amplifier is "on" and the low gain amplifier is "off". . After the receiver,
An automatic gain control circuit monitors the output power level from the radio frequency amplifier. When the output power of the high gain amplifier increases as the input power increases and the high gain amplifier is driven to the saturation region, the automatic gain control circuit gradually decreases the gain of the high gain amplifier. on the other hand,
At the same time, switch on the parallel low gain amplifier. When the input power increases above a certain predetermined level, the AGC circuit completely switches off the high gain amplifier and all input signals are applied to the low gain amplifier.

The advantage of the switch circuit described above is that the receiver can have a very wide range of input signal levels without sacrificing sensitivity and without either radio frequency amplifier being driven into saturation. That's what it means. This prevents signal distortion and reduces the power level of intermodulation products generated within the amplifier.

Another advantage of the invention is that the circuit can be used as part of an integrated circuit, particularly an LSI for a full amplifier.

In the figure, transistors Q ₁ and Q ₂ constitute a high gain radio frequency amplifier, generally designated 1, and transistors Q ₃ and Q ₄ constitute a low gain radio frequency amplifier, generally designated 2. Configure. The above-mentioned high gain amplifier 1
The current of the low gain amplifier 2 is controlled by the transistor Q ₅ and the current of the low gain amplifier 2 is controlled by the transistors Q ₆ and Q ₇ . Said transistor Q ₅ , and therefore the high gain amplifier 1, is turned on by the transistor Q ₁₀ ,
It will be turned off. Similarly, transistors Q ₆ and Q ₇ and thus the low gain amplifier 2 are turned on and off by transistor Q ₉ . Rather than abruptly turning off one amplifier and turning on the other, this switching occurs gradually, with both amplifiers on at intermediate signal levels. You will understand what is done. A fixed bias voltage is applied to the bases of transistors _Q8 and _Q9 , while the AGC is taken out as usual.
A voltage is applied to transistor _Q10 . AGC circuits are not shown in the drawings as they are well known to those skilled in the art.

If any low power radio frequency signal appears at the antenna terminal 3, the circuit is controlled so that the high gain amplifier 1 is in the "on" mode and the low gain amplifier 2 is in the "off" mode. The amplified output of the high gain amplifier 1 is converted to an intermediate frequency, e.g. It is demodulated by (The mixers, filters and detectors mentioned above are all familiar to those skilled in the art and are not shown in the figure). In the detector, a DC voltage proportional to the signal power level is created, and this voltage is applied to the base of transistor Q ₁₀ as an automatic gain control input. As the AGC voltage increases, transistor Q ₁₀ begins to turn off.

In actual operation, the circuit is adjusted as follows. That is, as long as the power level of the radio frequency input is within the linear amplification range of the high gain amplifier 1, the level of the AGC voltage created by the detector will be
Adjusted to be insufficient to turn off Q ₁₀ . As the power of the input increases, the AGC voltage increases, transistor Q ₁₀ becomes diode Q ₁₁
Begin to gradually reduce the ground current flowing through.
Therefore, the base voltage of transistor _Q5 decreases, and the current of high gain amplifier 1 decreases. As a result of this current reduction, the gain of the high gain amplifier 1 is reduced.

When the AGC voltage begins to turn off transistor Q ₁₀ , the emitter voltage of transistor Q ₉ becomes
Begin passing the ground current through the diode _Q12 .
This current increases the base voltage of transistors Q ₆ and Q ₇ , turning low gain amplifier 2 "on". As the power of the input signal continues to increase, eventually a point is reached where the high gain amplifier 1 is completely "off" and all amplification of the signal is performed by the low gain amplifier 2. By appropriately choosing the resistor R ₁ connected between the emitter leads of transistors Q ₃ and Q ₄ , it is possible to add very large Input power level can be added.

Under certain high signal conditions, even low gain amplifiers can become overdriven. The delayed AGC voltage can be applied to bias #2, which cuts off transistors _Q3 and _Q4 , causing the low gain amplifier to be cut off and become an attenuator.

It goes without saying that the specific circuits illustrated above are only for the purpose of illustrating the present invention, and that various changes may be made without departing from the gist of the present invention.

[Brief explanation of the drawing]

The figure illustrates one embodiment of the invention and shows a simple diagram of a receiver circuit having a high gain amplifier and a low gain amplifier and switch circuit. 1...High gain amplifier, 2...Low gain amplifier, 3
...Antenna input terminal, Q ₁ , Q ₂ , Q ₃ , Q ₄ , Q ₅ ,
Q ₆ , Q ₇ , Q ₈ , Q ₉ , Q ₁₀ ...transistors.

Claims (1)

[Claims] 1. In a radio frequency amplifier having an output line to a mixer, an input line from an antenna circuit, and an input line from an AGC voltage source proportional to the signal power level of this antenna circuit, the antenna A device that applies the output of the circuit directly in parallel to both a high-gain amplifier and a low-gain amplifier, and a device that biases the high-gain amplifier on when there is no signal from the antenna circuit and converts the low-gain amplifier into a low-gain amplifier. a fixed bias device that biases the AGC voltage off such that as the AGC voltage increases, the gain of the high gain amplifier is gradually decreased and the gain of the low gain amplifier is gradually increased; A radio frequency amplifier, characterized in that it has a device for applying as an automatic gain control voltage to a switch circuit that controls a high gain amplifier and a low gain amplifier. 2. The radio frequency amplifier of claim 1 having an AGC voltage applied to the low gain amplifier to cut off the low gain amplifier so that the amplifier acts as an attenuator. 3. A radio frequency amplifier according to claim 1 or 2, comprising a high gain amplifier that is switched off at a level below the saturation level.