High frequency amplifying circuit

Abstract: In a high frequency amplifying circuit, an impedance matching circuit has an impedance matching inductance (12), and a switching diode (23) connected in parallel with the inductance. The diode is selectively opened or closed in accordance with the input level, thereby changing the impedance matching condition. When the input level is low, an amplified output is produced by impedance matching which is implemented by the inductance itself. When the input level increases up to a level nearly causing saturation to occur, the diode is opened and rendered conductive with the result that the impedance matching characteristic changes. As a result, the phase characteristic is prevented from being deteriorated by a changed in input level.

Description: The present invention relates to a high frequency amplifying circuit and, more particularly, to a high frequency amplifying circuit capable of preventing the phase characteristic of the amplifier from being deteriorated by an increase in input level. In a known high frequency amplifying circuit an increase in input level causes the output level to be saturated. As a result, the phase characteristic of a signal available with the amplifying circuit is noticeably deteriorated, compared to a condition wherein the input level is low. When the amplifying circuit deals with a QPSK (Quadrature Phase Shift Keying) or similar modulated signal, its phase characteristic influences the channel quality. Hence, the circuit is required to cause a minimum of deterioration in the phase characteristic of the signal. Moreover, when the phase characteristic of the signal is extremely changed, the circuit is liable to perform an operation other than the expected linear operation, e.g., parametric oscillation. United States Patent US-A-3,587,017 describes an RF amplifier with an input impedance matching circuit which has a Zener diode which can be arranged in parallel with an inductor. The present invention is defined in claim 1 below, to which reference should now be made. We have appreciated that a diode can be connected across an inductor in an output impedance matching circuit of a high frequency amplifier to protect the amplifier from excessively high input levels, and thereby reduce undesired phase variations. The inductor may constitute an arm of a π matching circuit. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings, in which: Figure 1 is a circuit diagram showing a transistor amplifying circuit including an impedance matching circuit embodying the present invention; Figure 2 is a circuit diagram showing a transistor amplifying circuit including an alternative embodiment of the present invention; Figure 3 is a circuit diagram showing a known transistor amplifying circuit including an impedance matching circuit; and Figure 4 is a circuit diagram showing another known transistor amplifying circuit including an impedance matching circuit. DESCRIPTION OF THE PREFERRED EMBODIMENTS To better understand the present invention, first brief reference will be made to a known transistor amplifying circuit including an impedance matching circuit, shown in Figure 3. As shown, the amplifying circuit has an input terminal 1, a power source terminal 2, an output terminal 3, a coupling capacitor 4, capacitors 5 and 6 and an inductor 11 for matching the input impedance, a transistor 13, bias resistors 20 and 21, an emitter resistor 22, a power source bypass capacitor 7, an emitter bypass capacitor 8, and an inductor 12 and capacitors 9 and 10 for matching the output impedance. The capacitors 5 and 6 and inductor 11 constitute an input impedance matching circuit for matching an impedance connected to the input terminal 1 and the input impedance of the transistor 13. Likewise, the capacitors 9 and 10 and inductor 12 constitute an output impedance matching circuit for matching an impedance connected to the output terminal 3 and the output impedance of the transistor 13. Figure 4 shows another known transistor amplifying circuit which is implemented by a so-called π matching circuit. As shown, the amplifying circuit has an output impedance matching circuit formed of inductors 14 and 15 and capacitors 31, 32 and 33. In Figure 4 the same constituents as the constituents shown in Figure 3 are designated by the same reference numerals. The problem with the above known amplifying circuits is that when the input level increases, the output level is saturated, as discussed earlier. As a result, the phase characteristic of a signal available with the circuitry is noticeably deteriorated, compared to the condition wherein the input level is low. Preferred embodiments of the impedance matching circuit in accordance with the present invention will be described with reference to Figures 1 and 2. In Figures 1 and 2 the same constituents as the constituents shown in Figures 3 and 4 are designated by the same reference numerals. Referring to Figure 1, a transistor amplifying circuit embodying the present invention includes an output impedance matching circuit in which an inductor 12 is cascaded to an amplifying transistor 13. The illustrative embodiment is characterized in that a diode 23 is connected in parallel with the inductor 12. When an input signal having a great amplitude is applied to the input stage of the circuit terminating at the transistor 13, the collector current of the transistor 13 would be saturated. However the diode 23 is rendered conductive at a voltage slightly lower than the peak voltage which appears between the opposite ends of the inductor 12 on the saturation of the collector current. Specifically, the amplifying circuit has an input terminal 1, a power source terminal 2, an output terminal 3, a coupling capacitor 4, capacitors 5 and 6 and an inductor 11 for matching the input impedance, the transistor 13, bias resistors 20 and 21, an emitter resistor 22, a power source bypass capacitor 7, an emitter bypass capacitor 8, the inductor 12 and capacitors 9 and 10 for matching the output impedance, and the diode or switching diode 23. In operation, so long as the level of the input signal is low enough to maintain the transistor 13 unsaturated, the amplifying circuit performs a linear operation. In this condition, the phase of the signal is scarcely changed by the amplifying circuit. However when the input level is high, the diode 23 is opened, with the result that the inductor 12 is short-circuited. This successfully reduces the change in the phase of the signal. Hence, the phase characteristic of the signal is not noticeably deteriorated even when the output level is saturated. Figure 2 shows an alternative embodiment of the present invention. As shown, a transistor amplifying circuit includes an output impedance matching circuit implemented as a so-called π matching circuit. As shown, the diode 23 is connected in parallel with an inductor 15. This configuration, like the configuration of Figure 1, prevents the phase characteristic of the signal from being noticeably changed despite the saturation of the output level. In the embodiments shown and described, the diode 23 may be replaced with a serial connection of a plurality of diodes. When the frequency of the input signal to be dealt with by any of the embodiments is high, use may be made of a Schottky type switching diode for the diode 23. In the embodiments shown in Figures 1 and 2, the transistor 13 is shown as being an n-p-n transistor. Alternatively, the transistor 13 may be implemented by an FET (Field Effect Transistor). These embodiments can achieve the previously stated advantage even when implemented as a mixer circuit. In summary, the present circuit prevents, in a high frequency amplifying circuit, a phase characteristic from being deteriorated by an excessive increase in input level. In addition, the present circuit realizes a simple impedance matching circuit using a switching diode.

Claims: A high-frequency amplifier comprising: an input terminal (1); an amplifying transistor (13) coupled to the input terminal and having a control electrode and two main electrodes, one of the main electrodes forming an output electrode of the transistor; and an output impedance matching circuit (9,10,12; 14,15,31,32,33) connected to the output electrode of the transistor and comprising an inductor (12;15); characterized by a diode (23) connected in parallel with the inductor (12;15) and operative to be rendered conductive at a voltage slightly lower than the voltage which arises across the inductor when the transistor is saturated. An amplifier as claimed in claim 1, wherein the transistor (13) is a junction transistor and the inductor (12) constitutes the collector load of the transistor. An amplifier as claimed in claim 1, wherein the inductor (15) constitutes an arm of π circuit (15,31,32) in the output impedance matching circuit (14,15,31,32,33).

Inventors: SASAKI YUTAKA, SASAKI, YUTAKA
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H03F1
IPC codes: H03F3, H03F1