JPH0276407A - Agc circuit for high frequency amplifier - Google Patents

Abstract

PURPOSE:To attain stable operation by allowing a change in the bias current of a 2nd transistor(TR) to flow through a diode and inhibiting the change in the current from flowing into a 1st TR. CONSTITUTION:When the level of a reception signal is high so that the level of an output signal generated on an intermediate frequency amplifying stage is more than a prescribed value, an AGC voltage corresponding to the signal level is impressed to a control terminal 16 and the connecting state of a control TR17 is controlled. Thereby, the collector current of a TR3 is increased by a current value of current flowing through a diode 20 in accordance with the connecting state of the TR17. Since the TR3 is set up so that its amplification factor is reduced against the increment of the collector current, the amplification factor of the TR3 is reduced following the increment of the AGC voltage impressed to the terminal 16. Thereby, the drain voltage of an FET2 is not changed and gain is controlled by forward AGC. Consequently, AGC operation can be attained without changing the bias of the 1st TR (FET 2) and an element for executing AGC is disconnected by the diode 20 when a reception signal is low because of no AGC operation, so that an AGC circuit for a high frequency amplifier stabilizing said disconnecting operation can be presented.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention is directed to an A for controlling the gain of a high frequency amplifier constituted by first and second transistors connected in cascade.
The present invention relates to a GC circuit, and particularly to an AGC circuit for forward AGC.

(b) Conventional technology The high-frequency amplifier of a tuner includes a source-grounded FET (field effect transistor) and a gate-grounded FET, or an emitter-grounded transistor and a base-grounded transistor in the front and rear stages, respectively. There are known devices in which a source-grounded FET and a base-grounded transistor are connected in cascade, and a high-frequency signal from an antenna circuit is input to the gate or base of the preceding transistor. .

This type of high-frequency amplifier has the advantages of a common source type FET1 or a common emitter type transistor, which has a large gain, good noise characteristics, and high input impedance, and the advantages of a common gate type FET or a common base type transistor. It is possible to obtain characteristics that skillfully combine the advantages of high

By the way, the above-mentioned high frequency amplifier is disclosed in, for example, Japanese Patent Laid-Open No. 6
As shown in Publication No. 2-122307, the subsequent FET
A device is known in which an AGC voltage is applied to the gate of the transistor or the base of the transistor in accordance with the level of the received signal. If the input signal level is above a predetermined value, this high frequency amplifier
The gain can be controlled by applying the C voltage and changing the current flowing to the subsequent transistor.

(c) Problems to be solved by the invention However, as mentioned above, the gate voltage of the subsequent FET,
Alternatively, if the base voltage of the transistor is changed, the source voltage of the FET or the emitter voltage of the transistor changes in accordance with the voltage change, so the drain voltage of the previous FET or the collector voltage of the transistor changes. . Therefore, even if the operating point of the FET 1 or the transistor is set to minimize distortion, there is a drawback that the operating point will fluctuate.

(d) Means for Solving the Problems The present invention provides an AG of high frequency amplifier which has been achieved in view of the above-mentioned points.
The present invention is intended to provide a high-frequency amplifier comprising first and second transistors connected in cascade, a control transistor to which an AGC voltage is applied to a control terminal, and an anode connected to the first and second transistors. and a diode connected between the control transistor and the current flowing through the control transistor is controlled, and the gain is controlled by changing the bias current of the second transistor according to the current flowing through the diode. I have to.

(*) Effect When controlling the gain by changing the bias current of the second transistor, the present invention controls the current flowing through the diode with the control transistor, and also controls the current flowing through the second transistor.
A forward AGC operation is performed by causing a change in the bias current of the transistor to flow through the diode, and preventing the change in current from flowing into the first transistor.

(f) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (1) is an input terminal into which a high frequency signal tuned by an antenna circuit (not shown) is input, and (2) is an input terminal for receiving a high frequency signal tuned by an antenna circuit (not shown). A source-grounded FET that amplifies the high-frequency signal input to the input terminal (1), (
3) The emitter is a drain resistor to prevent parasitic oscillation (4)
is connected to the drain of the FET (2) via
A common base type transistor connected in cascade to ET (2), (5) a capacitor that AC-grounds the base of the transistor (3), (6), (7) and (
8) is a resistor for applying a bias voltage to the base of the transistor (3); one end of (9) is connected to the collector of the transistor (3) via a collector resistor (10) for preventing parasitic oscillation; and the other end is a capacitor (11
) and connected to a power supply via a resistor (6), the second coil (13) and the first coil (12) are mutually inductively coupled to the first coil (9) A tuning circuit consisting of two coils (13) and a capacitor (14) connected in parallel, (15) is connected to the center tap of the second coil (13) and outputs to a subsequent mixing stage (not shown). The output terminal for deriving the signal, (16) is the intermediate frequency amplification stage (
AG obtained by rectifying the output signal generated by
The control terminal (17) to which the C voltage is input is the control terminal (
The AGC voltage input to the resistor (18) and (16)
A common emitter type control transistor (20) has an anode connected to the emitter of the transistor (3) and a cathode connected to the control transistor (17) via a resistor (21). ) is a diode connected to the collector of As the transistor (3), a forward AGC transistor with a large change in collector current versus amplification factor is used, and a bias is set in the transistor (3) so that when the collector current is amplified, the amplification factor decreases. There is. Further, as the diode (20), a pin diode with excellent high frequency characteristics is used.

When a high frequency signal selected by the antenna circuit is input to the input terminal (1), the high frequency signal is amplified by the FET (2), further amplified by the transistor (3), and then supplied to the first coil (9). Ru. A signal is generated in the second coil (13) that is tuned by a tuning circuit and corresponds to the high frequency signal flowing through the first coil (9). Therefore, the high frequency signal input to the input terminal (1) is amplified and a selected high frequency signal is derived from the output terminal (15).

By the way, when the output signal level generated in the intermediate frequency amplification stage is high and the received signal level is greater than a predetermined value, an AGC voltage corresponding to the signal level is applied to the control terminal (16),
The conduction state of the control transistor (17) is controlled. Therefore, the transistor (3) is connected to the control transistor (17).
17), the collector current increases by the amount of current flowing through the diode (20). Here, since the transistor (3) is set so that the amplification factor decreases as the collector current increases, the control terminal (16
) The amplification factor of the transistor (3) decreases as the AGC voltage applied to the transistor (3) increases. Therefore, in the circuit shown in FIG. 1, the gain is controlled by forward AGC without changing the drain voltage of FET (1).

On the other hand, when the output signal level generated in the intermediate frequency amplification stage is lower than the predetermined value and the received signal level is small, the control terminal (16)
Since no AGC voltage is applied to the control transistor (17), the control transistor (17) is in an off state. Therefore, since the diode (20) is off at this time, the control transistor (17) is used to change the collector current of the transistor (3).
> and resistors (21) are completely separated from the signal path for transmitting high frequency signals. Therefore, the AGC element does not adversely affect high frequency signals.

In the above embodiments, FETs and transistors are used as the first and second transistors in the claims, but the present invention is not limited thereto.

FIG. 2 is a circuit diagram showing another embodiment of the present invention, which is an improved version of the circuit shown in FIG. In Fig. 2, the same elements as in Fig. 1 are given the same figure numbers, and the difference from Fig. 1 is that a capacitor (2
2) is at the connected point.

By doing this, the high frequency signal generated at the drain of FET (2) flows through the diode (20) and the capacitor (22), so the transistor (
The signal input to 3) can be made small.

In other words, since the on-resistance is generated in inverse proportion to the current flowing through the diode (20), the signal input to the transistor (3) is attenuated by the voltage division between the on-resistance and the drain resistance (4). Therefore, the transistor (
Since there is no need to reduce the amplification factor of (3), there is no need to strictly set the operating point of the transistor (3), and a general-purpose transistor with a wide dynamic range is used as the transistor (3). You can.

Furthermore, since the transistor (3) can be operated with a small amplitude, the linear input voltage range of the transistor (3) can be expanded. Note that the other AGC operations are the same as the circuit shown in FIG.

(G) Effects of the Invention As described above, according to the present invention, the variation in the bias 'wL current of the second transistor is caused to flow through the diode,
Since the change in current is prevented from flowing into the first transistor, AGC operation can be performed without changing the bias of the first transistor, and when the received signal level is low, AGC operation is not performed. Since the elements for performing AGC are separated by diodes, it is possible to provide an AGC circuit for a high frequency amplifier whose operation is stable at that time.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the invention, and FIG. 2 is a circuit diagram showing another embodiment of the invention. Explanation of main drawing numbers (1)...FET, (3)...Transistor,
(17)...control transistor, (20)...
Diode, (22)...Capacitor.

Claims (1)

[Claims] (1) In a high frequency amplifier composed of first and second transistors connected in cascade, A
a control transistor to which a GC voltage is applied; and a diode having an anode connected between the first and second transistors and controlling the current flowing through the control transistor, the current flowing through the diode An AGC circuit for a high frequency amplifier, characterized in that the gain is controlled by changing the bias current of the second transistor accordingly.