CN1510831A - Bias circuit for RF power amplifier - Google Patents

Abstract

A bias circuit for an RF power amplifier includes a bias transistor having a collector, an emitter, and a base, wherein the collector is connected to a DC voltage source, the emitter is connected to an RF transistor, and the base is connected to a bias voltage source. An inductor is connected between the base of the RF transistor and the emitter of the bias transistor for isolating the portion of the RF input signal coupled to the bias transistor. A capacitor is connected between the emitter of the bias transistor and ground or between the base of the bias transistor and ground to direct the portion of the RF input signal coupled to the bias transistor to ground, thereby preventing the bias transistor from being driven to saturation.

Description

Bias circuit for RF power amplifier

technical field

The invention relates to a bias circuit of a radio frequency power amplifier, in particular to a bias circuit for improving the linearity of the radio frequency power amplifier.

Background technique

FIG. 1 shows a schematic diagram of an example of a conventional bias circuit for an RF power amplifier. Referring to FIG. 1 , in the conventional resistive bias circuit 100 , a bias voltage source Vbias is supplied to the base of the RF transistor 102 via a bias resistor 104 to provide base current of the RF transistor 102 . The capacitor 106 is connected between the RF input port of the amplifier and the base of the RF transistor 102 to couple the RF input signal (not the DC signal) to the base of the RF transistor 102 . The collector of the RF transistor 102 serves as the output port of the amplifier via the output matching circuit 108 . A disadvantage of the existing resistive bias circuit 100 is that it can only provide limited bias current control. For example, if the bias resistor 104 has a small resistance, unless the bias voltage source Vbias changes with temperature, temperature variations can cause unacceptable variations in the quiescent current associated with the RF transistor 102 . On the other hand, if the bias resistor 104 has a large resistance value, the RF transistor 102 may be under-biased or have an undesirably large quiescent bias current at high drive levels.

FIG. 2 shows a schematic diagram of another example of a conventional bias circuit for an RF power amplifier. The conventional active bias circuit 200 shown in FIG. 2 is an improvement of the conventional resistive bias circuit 100 shown in FIG. 1 . Referring to FIG. 2 , the conventional active bias circuit 200 includes a bias transistor 202 to allow the RF transistor 102 to draw an appropriate amount of bias current according to the RF drive level while still maintaining a low quiescent current. A bias voltage source Vbias is applied to the base of the bias transistor 202 via the bias resistor 104 . The bias transistor 202 is an emitter follower transistor. The collector of bias transistor 202 is connected to Vcc. The existing active bias circuit 200 has the advantage of low impedance.

However, the active bias circuit 200 shown in FIG. 2 has the disadvantage that the bias transistor 202 may enter a saturated state. Specifically, when the RF transistor 102 is driven into a state of high power output, a part of the RF input signal will be reversely coupled from the collector of the RF transistor 102 to the base of the RF transistor 102, and then may enter an active bias circuit 200. As a result, bias transistor 202 is driven into saturation by the portion of the radio frequency input signal coupled to bias transistor 202 , making its own operating behavior more nonlinear. In this case, the active bias circuit 200 cannot follow the RF input signal to provide a linear bias current to the RF transistor 102 .

Contents of the invention

In order to overcome the above-mentioned deficiencies of the prior art, the object of the present invention is to provide a bias circuit for a radio frequency power amplifier, which can prevent the bias transistor from being affected by radio frequency input signals, thereby improving the linearity of the radio frequency power amplifier.

To achieve the above object, the present invention provides a bias circuit for a radio frequency power amplifier, the radio frequency power amplifier includes a radio frequency transistor and a first capacitor, wherein the radio frequency transistor has a collector, an emitter and a base pole, and one end of the first capacitor is connected to the collector of the radio frequency transistor, and the other end is used to receive a radio frequency input signal, the bias circuit includes: a bias transistor having a collector, an emitter and a base, wherein the collector is connected to a DC voltage source, and the base is connected to a bias voltage source; and a second capacitor is connected between the emitter of the bias transistor and the ground for making The portion of the RF input signal coupled to the bias transistor is directed to ground, preventing the bias transistor from being driven into saturation.

The bias circuit for the radio frequency power amplifier provided by the present invention may also include a third capacitor connected between the base of the bias transistor and the ground, so as to couple the radio frequency input signal to the bias transistor A portion of this is directed to ground, preventing the bias transistor from being driven into saturation.

The bias circuit for the RF power amplifier provided by the present invention may also include an inductor connected between the base of the RF transistor and the emitter of the bias transistor to isolate the RF input signal coupled to part of the bias transistor.

Description of drawings

Figure 1 shows a schematic diagram of an example of a conventional bias circuit for an RF power amplifier;

Figure 2 shows a schematic diagram of another example of a conventional bias circuit for a radio frequency power amplifier; and

3A and 3B are schematic diagrams of bias circuits for RF power amplifiers according to the present invention.

Explanation of symbols in the figure

100 Resistive bias circuit

102 RF transistors

104 Bias resistor

106 Capacitance

108 output matching circuit

200 Active bias circuit

202 Bias transistor

301, 302 Diode-connected transistors

303 resistor

304 inductance

305, 306 capacitance

Detailed ways

The purpose, features and advantages of the bias circuit for radio frequency power amplifier of the present invention will be described in detail below with reference to the accompanying drawings.

3A and 3B are schematic diagrams of bias circuits for RF power amplifiers according to the present invention. Referring to FIG. 3A , in the bias circuit for an RF power amplifier according to the present invention, a bias voltage source Vbias supplies current to diode-connected transistors 301 and 302 connected in series via a resistor 303 . Specifically, each of the diode-connected transistors 301 and 302 has a form in which its base is connected to its collector to form a diode. The voltage at the collector of diode-connected transistor 301 is twice VBE. This voltage is applied to the base of bias transistor 202, which is an emitter follower transistor. The collector of the bias transistor 202 is connected to a DC voltage source Vcc. Since the emitter voltage is the base voltage minus VBE, the emitter voltage of bias transistor 202 is equal to VBE (2VBE−VBE=VBE). This is the bias voltage applied to the RF transistor 102 .

In order to prevent the RF input signal from being coupled back from the RF transistor 102 to the bias transistor 202, causing the bias transistor 202 to be driven to a saturated state, an inductor 304 is arranged between the emitter of the bias transistor 202 and the base of the RF transistor 102 . The inductor 304 reduces the portion of the RF input signal coupled to the bias transistor 202 to prevent the bias transistor 202 from being driven into saturation. Therefore, the linearity of the RF power amplifier is improved.

Although the inductor 304 can effectively reduce the portion of the RF input signal coupled to the bias transistor 202 , it cannot completely isolate it. Therefore, the bias circuit for the RF power amplifier according to the present invention further includes a capacitor 305 connected between the emitter of the bias transistor 202 and the ground. Since the capacitor 305 acts as a short circuit for the RF input signal, the portion of the RF input signal coupled to the bias transistor 202 can be directed to ground. In this way, the bias transistor 202 is prevented from being driven into saturation by the RF input signal, and thus the linearity of the RF power amplifier is improved.

FIG. 3B is a schematic diagram showing another embodiment of a bias circuit for an RF power amplifier according to the present invention. Referring to FIG. 3B, a capacitor 306 is connected between the base of the bias transistor 202 and ground. Since the capacitor 306 acts as a short circuit for the RF input signal, the portion of the RF input signal coupled to the bias transistor 202 can be directed to ground. In this way, the bias transistor 202 is prevented from being driven into saturation by the RF input signal, and thus the linearity of the RF power amplifier is improved.

The above description is only a preferred embodiment of the bias circuit for radio frequency power amplifier of the present invention, and does not constitute a limit to the scope of the substantive technical content of the present invention. The essential technical content of the bias circuit for radio frequency power amplifier of the present invention is broadly defined in the claims of the present invention, any technical entity or method completed by others, if completely defined in the claims of the present invention The same, or its equivalent changes, are considered to be covered in the scope of this patent.

Claims (9)

1. bias circuit that is used for radio-frequency power amplifier, this radio-frequency power amplifier comprises a radio-frequency (RF) transistors and one first electric capacity, it is characterized in that, this radio-frequency (RF) transistors has the collection utmost point, an emitter-base bandgap grading and a base stage, and an end of this first electric capacity is connected in this collection utmost point of this radio-frequency (RF) transistors and the other end in order to receive a radio-frequency input signals, and this bias circuit comprises:

A bias transistor has the collection utmost point, an emitter-base bandgap grading and a base stage, and wherein this collection utmost point is connected to a direct voltage source and this base stage is connected to a biasing voltage source; And

One second electric capacity is connected between this emitter-base bandgap grading and ground of this bias transistor, uses so that the part that is coupled to this bias transistor in this radio-frequency input signals is directly imported ground, thereby prevents that this bias transistor is driven to saturation condition.

2. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 1, it is characterized in that, more comprise one the 3rd electric capacity, be connected between this base stage and ground of this bias transistor, with so that the part that is coupled to this bias transistor in this radio-frequency input signals is directly imported ground, thereby prevent that this bias transistor is driven to saturation condition.

3. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 1, it is characterized in that, more comprise an inductance, be connected between this emitter-base bandgap grading of this base stage of this radio-frequency (RF) transistors and this bias transistor, in order to the part that is coupled to this bias transistor in isolated this radio-frequency input signals.

4. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 1 is characterized in that, this biasing voltage source comprises:

A resistance is connected between this base stage of a supply voltage and this bias transistor;

A plurality of diodes are series between this base stage and ground of this bias transistor, in order to predetermined voltage this base stage to this bias transistor to be provided.

5. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 4 is characterized in that, each in these a plurality of diodes is formed in the mode that its base stage is connected in its collection utmost point by a transistor.

6. bias circuit that is used for radio-frequency power amplifier, this radio-frequency power amplifier comprises a radio-frequency (RF) transistors and one first electric capacity, it is characterized in that, this radio-frequency (RF) transistors has the collection utmost point, an emitter-base bandgap grading and a base stage, and an end of this first electric capacity is connected in this collection utmost point of this radio-frequency (RF) transistors and the other end in order to receive a radio-frequency input signals, and this bias circuit comprises:

A bias transistor has the collection utmost point, an emitter-base bandgap grading and a base stage, and wherein this collection utmost point is connected to a direct voltage source and this base stage is connected to a biasing voltage source; And

One second electric capacity is connected between this base stage and ground of this bias transistor, uses so that the part that is coupled to this bias transistor in this radio-frequency input signals is directly imported ground, thereby prevents that this bias transistor is driven to saturation condition.

7. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 6, it is characterized in that, more comprise an inductance, be connected between this emitter-base bandgap grading of this base stage of this radio-frequency (RF) transistors and this bias transistor, in order to the part that is coupled to this bias transistor in isolated this radio-frequency input signals.

8. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 6 is characterized in that, this biasing voltage source comprises:

A resistance is connected between this base stage of supply voltage and this bias transistor;

A plurality of diodes are series between this base stage and ground of this bias transistor, in order to predetermined voltage this base stage to this bias transistor to be provided.

9. the bias circuit that is used for radio-frequency power amplifier as claimed in claim 8 is characterized in that, each in these a plurality of diodes is formed in the mode that its base stage is connected in its collection utmost point by a transistor.

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