CN1346176A - Bias circuit for RF power amplifier - Google Patents

Abstract

Bias circuit for RF power amplifier in the battery supply unit of the radio communication system. The bias circuit comprises an operational amplifier providing the bias for RF power amplifier and at the same time the operational amplifier is exposed in the input RF signals. The operatial amplifier has little influence on the RF signals since its approximate frequency compensation.

Description

The biasing circuit that is used for radio-frequency power amplifier

The present invention relates generally to radio-frequency power amplifier, particularly, relates to the biasing circuit of the radio-frequency power amplifier that the powered battery mobile phone unit that is used to be suitable for wireless communication system uses.

In the prior art, be used for the biasing circuit of radio-frequency power amplifier,, disclose in 648, use structure shown in Figure 1 such as United States Patent (USP) 5,629.Such biasing circuit comprises electric capacity 10 and blocks inductance 12.Blocking inductance 12 stops the biasing circuit by radio frequency amplifier to load radiofrequency signal.And, block inductance 12 and stop radiofrequency signals to enter biasing circuit, wherein radiofrequency signal may by rectification or cause biasing undesired, that depend on the radiofrequency signal amplitude move.

Biasing circuit, all biasing circuits as shown in Figure 1 provide quite high bias plasma resistance.Be in the adjacent channel frequency difference (for example, 980kHz in the U.S.) frequency, also promptly in J-CDMA the impedance of the biasing circuit on the difference frequency of interference channel can significantly be reduced by using big discrete capacitor in parallel 10 (such as 4.7 μ F).In this respect, know, near the difference frequency of adjacent channel frequency, can worsen the intermodulation of radio-frequency power amplifier and adjacent channel power than (ACPR) performance much larger than several ohm biasing impedances.Intermodulation and ACPR relate to the linearity of radio-frequency power amplifier, and particularly, ACPR is considerable in linear power amplifier is used.

Nearest progress is the radio-frequency power amplifier that is used for battery powered mobile phone, and it has two signal path, and every paths operates on the different power levels, and signal is directed into antenna by this path, so that send to the base station that this mobile phone is communicated by letter with it.The position of mobile phone with respect to the base station depended in the selection that is used for the signal path of specific transmission.When mobile phone is positioned at apart from the base station predetermined distance, selects lower power level signal path, and when mobile phone is positioned at apart from the base station predetermined distance in addition, select higher power level signal path.Like this, mobile phone only consumes necessary quantity of power, and battery can be worked the longer time interval between twice charging.

For this radio-frequency power amplifier that is operated on one of two power levels, another requirement of biasing circuit is, biasing circuit provides low-resistance on the difference frequency of adjacent channel frequency, when being biased in when switching between two radio-frequency power amplifiers in the power level signal path, also provide switching time less than 2 microseconds.This need be a progression of seamlessly adjusting radio-frequency power amplifier owing to wish the change of exporting in response to the radio-frequency power of needs.If radio-frequency power amplifier needs long switching time, then the radio-frequency power amplifier from a signal path is transferred to the losing of information that another radio-frequency power amplifier in another signal path can cause being sent to by mobile phone the base station.In order in 5 microseconds, to switch the about 800 millivolts voltage of 4.7 μ F electric capacity, need to surpass 0.5 ampere electric current.Under the environment to power and electromagnetic interference sensitivity, this is unacceptable.

Therefore, the purpose of this invention is to provide and be used for radio-frequency power amplifier, novel and improved biasing circuit.

Another object of the present invention provides the biasing circuit that is used for radio-frequency power amplifier, and it has overcome the defective that is used for the biasing circuit of radio-frequency power amplifier in the prior art.

In order to reach these and other objects, make up according to the present invention, be used for the powered battery mobile phone unit of wireless communication system, the biasing circuit of radio-frequency power amplifier, comprise the operational amplifier that is added with the input radio frequency signal on it, and the transistor source follower circuit that is connected to the output of operational amplifier.Operational amplifier has a dominant pole frequency, it causes the output at biasing circuit, impedance on the difference frequency of adjacent channel is not more than 2 ohm, and this frequency ratio to play the frequency of radio frequency signal much lower so that the loop gain less than zero dB is arranged on the frequency of radiofrequency signal.This radio frequency loading of guaranteeing biasing circuit minimizes.

Can understand the present invention best during detailed description below reading in conjunction with the accompanying drawings.

Fig. 1 is the circuit diagram of traditional, as to be used for radio-frequency power amplifier biasing circuit.

Fig. 2 makes up according to the present invention, is used for the powered battery mobile phone unit of wireless communication system, the circuit diagram of radio-frequency power amplifier biasing circuit.

Fig. 3 is the circuit diagram that can be used in the operational amplifier of biasing circuit shown in Figure 2.

With reference to Fig. 2, make up according to the present invention, be used for the powered battery mobile phone unit of wireless communication system, the biasing circuit of radio-frequency power amplifier, comprise the operational amplifier 20 that is added with the input radio frequency signal on it, and the transistor source follower output circuit 22 that is connected to the output of operational amplifier.

The transistor of transistor source follower output circuit preferably has maximum, non-wasting drain current limited characteristic.This transistor can be, and is for example shown, field-effect transistor (FET, JFET, NFET, or MOSFET).Field-effect transistor has the maximum drain current characteristic, and will transship so that source voltage descends when overload condition.Avoid when radio-frequency power amplifier transships, continuing to provide the transistor of bias voltage to the radio-frequency power amplifier base stage.The correct biasing circuit transistor of selecting will remove (debias) partially for radio-frequency power amplifier, rather than allow radio-frequency power amplifier to damage under overload condition.

Operational amplifier 20 has a dominant pole frequency, and it causes being not more than 2 ohm on the difference frequency of impedance in adjacent channel of the output of biasing circuit.In exemplary embodiment of the present invention, operational amplifier 20 is used in linear power amplifier is used.Should be taken into account that if be used for the application of nonlinear power amplifier, impedance can be greater than 2 ohm, to have improved the robustness of amplifier.At the biasing circuit of prior art shown in Figure 1,4.7 μ F electric capacity are used for providing Low ESR under the difference frequency of the adjacent channel frequency 980kHz of the U.S. (for example).By using, the needs of 4.7 μ F electric capacity have been avoided according to biasing circuit of the present invention, the transistor source follower output circuit 22 that comprises the operational amplifier 20 that adds the input radio frequency signal on it and be connected to operational amplifier output terminal.

The dominant pole frequency of operational amplifier is much lower compared with the radio frequency signal frequency of input, so that the loop gain less than zero dB is arranged on the input radio frequency signal frequency.This has reduced the loading of biasing circuit to RF signal path.Beyond the loop bandwidth of operational amplifier 20, according to improved biasing circuit of the present invention, with about 1/Gm of transistor source follower output circuit as the load of giving RF signal path.In JCDMA used, this was 50 ohm of rf bias that are loaded into the SiGe power HBT with about 1 ohm to 2 ohm 900MHz impedance of fundamental frequency, and it is slight load.

On the frequency (for example 900MHz) of radiofrequency signal, be rendered as open circuit according to biasing circuit of the present invention basically.

The analysis showed that of operational amplifier 20, more than the dominant pole frequency of operational amplifier, the output impedance of transistor source follower output circuit is rendered as emotional resistance, and is as described below.

More generally, the open-loop gain of operational amplifier can be with having a dominant pole, depends on frequency, and the expression approximate representation is: $\mathrm{Av}\left(f\right)=\frac{\mathrm{Av}\left(\mathrm{DC}\right)}{1+2\mathrm{\πfRoCo}}$ $\mathrm{Ro}=\frac{1}{{(\mathrm{Av}+1)}^{*}\mathrm{Gm}}$ $\mathrm{Zo}\left(f\right)=\frac{1}{{(1+\_\mathrm{Av}/(1+2\mathrm{\πfRoCo}))}^{*}\mathrm{Gm}}$ $\mathrm{Zo}\left(f\right)=\frac{1+2\mathrm{\πfRoCo}}{G{m}^{*}(\mathrm{Av}+1+2\mathrm{\πfRoCo})}$

Wherein Ro and Co are the operational amplifier parasitic parameters, and they determine dominant pole frequency.

For f＜≈ Av/ (2 π fRoCo), molecule is preponderated, and circuit is perceptual.

Fig. 3 is the circuit diagram that can be used in the operational amplifier of biasing circuit shown in Figure 2.It will be apparent to those skilled in the art that this operational amplifier is traditional design and structure.Therefore, the detail of this operational amplifier is optional.As long as say the operational amplifier that this operational amplifier is preferably very lower powered, required impedance operator is provided.Though in the biasing circuit that other many structures can be used to make up according to the present invention, the importantly setting of the pole and zero in the operational amplifier response function or do not have pole and zero, and the response of the k factor of combined bias circuit and rf gain level.Should see, dominant pole frequency, for example 60kHz is established by flow through low-down current density (for example about altogether 20 microamperes) in operational amplifier.Though this is convenient to the emulation convergence, higher bias level and bigger output capacitance also should be suitable embodiments.Because operational amplifier 20 is not subjected to the influence of radiofrequency signal, the radio-frequency power amplifier that is operated on the relative higher power level (for example 500 milliwatts) can be adjusted by the biasing circuit that is operated on the relatively low power level (for example 60 microwatts).

In a word, operational amplifier provides biasing for radio-frequency power amplifier, and operational amplifier is exposed in the radiofrequency signal of input simultaneously.The operational amplifier quilt is frequency compensation approx, so that to almost not influence of RF signal path.

Though show and described the present invention with reference to some exemplary embodiments, in any case, the invention is not restricted to details shown and that describe.But can in the full scope of equivalents of claim, make various modifications to those exemplary embodiments, and do not deviate from the present invention.

Claims (3)

1. be used for the biasing circuit of the radio-frequency power amplifier of wireless communication system powered battery mobile phone unit, comprise:

Be added with the operational amplifier of input radio frequency signal on it with dominant pole frequency:

(a) cause at the biasing circuit output, the impedance on the difference frequency of adjacent channel is not more than 2 ohm; And

(b) more much lower than radio frequency signal frequency, so that loop gain less than zero dB is arranged on the frequency of radiofrequency signal; And

Be connected to the transistor source follower output circuit at operational amplifier output terminal place.

2. according to the biasing circuit of claim 1, it is characterized in that the transistor of wherein said transistor source follower output circuit has maximum, non-destructive drain current limited characteristic.

3. according to the biasing circuit of claim 1, it is characterized in that the transistor of wherein said transistor source follower output circuit is a field-effect transistor.

Images