Gain stage control method and related control circuit

The present invention relates to a gain stage control method. The invention relates particularly, but not exclusively, to a power transistor control method and the following description is made with reference to this field of application for convenience of illustration only.

As it is well known, components realized with integrated technologies are presently the most used to realize the power amplifiers required, for example, by mobile phones and particularly, in the field of wireless telecommunication systems. To realize these components, it may be extremely important to control the power outputted from the power amplifier for a correct operation thereof and for the operation of the other circuits connected thereto.

This control should be usually ensured in the different circuit operating conditions, both in steady operation periods and in transient periods. It is known to control the output power from an electronic circuit by feedback or closed-loop control circuits like the one shown in FIG. 1 . In particular, a closed-loop power control system has been globally and schematically indicated by reference numeral 10 , which has an input terminal IN 1 suitable to receive an input signal RFin and an output terminal OUT 1 suitable to provide an output signal RFout. In particular, the output signal RFout is a power signal to be controlled.

The system 10 essentially comprises a power amplifier 11 , connected between the input terminal IN 1 and the output terminal OUT 1 , and having a control terminal Tc suitable to receive a control signal, particularly a power control voltage Vc. To control the output power signal RFout, the system 10 also comprises a control circuit realized by a feedback loop 12 connected between an output terminal and the control terminal Tc of the power amplifier

In particular, the feedback loop 12 comprises a directional coupler 13 inserted between the output terminal of the power amplifier 11 and the output terminal OUT 1 of the system 10 and connected to an envelope detector 14 , connected, in turn, to an input terminal of an error amplifier 15 in the feedback loop 12 . This error amplifier 15 has a further input terminal Tr connected to a reference signal, particularly to a reference voltage Vref, as well as an output terminal suitable to provide the control voltage Vc to the control terminal Tc of the power amplifier 11 . It should be noted that this reference voltage Vref is essentially a feedback control signal set according to the control of the output power signal RFout to be obtained.

In summary, the system 10 latches, by the feedback loop 12 , the output voltage from the envelope detector 14 to the reference voltage Vref, thus controlling the output power signal RFout as this reference voltage Vref varies. In particular, the feedback loop 12 forces the output voltage from the envelope detector 14 so that it is equal to the reference voltage Vref.

Although advantageous under several aspects, this first feedback approach has several drawbacks including:

a power loss, a high complexity and a cost increase of the system as a whole linked to the presence of the directional coupler 13 ;

a dynamic control range limitation due to the presence of the envelope detector 14 , which comprises, as it is well known, at least a diode; and

during the power signal regulation transients, a considerable variation of the feedback system loop gain within the control dynamics, with the subsequent possible instability problems.

A second approach described, for example, by D. Brunel et al. in: “Power control with the MRFIC0913 GaAs IPA and MC33169 support IC”, Motorola Semiconductors application note AN1599, 2000, provides the use of a voltage regulator connected to an external control voltage, as shown in FIG. 2 . In particular, this FIG. 2 globally shows a system 20 comprising, as for the example of FIG. 1 , a power amplifier 11 , connected between an input terminal IN 1 and an output terminal OUT 1 and having a control terminal Tc suitable to receive a control signal, in particular a power control voltage Vc. This control terminal Tc is connected to an output terminal OUT 2 of a voltage regulator 21 , inserted, in turn, between a first and second voltage reference, in particular a power supply V BATT and a ground GND, and having an input terminal IN 2 receiving an external control voltage Vext.

It should be noted that this external control voltage Vext is essentially a regulation signal of the amplifier 11 regulated according to the control of the output power signal RFout to be obtained. The voltage regulator 21 essentially comprises an error amplifier 22 having a first input terminal, in particular the inverting input terminal (−), connected to the input terminal IN 2 of the regulator 21 , as well as an output terminal connected to a control terminal of a power transistor M 20 , inserted, in turn, between the supply voltage terminal V BATT and the output terminal OUT 2 of the regulator 21 . It is known to use a P-channel MOS transistor as a power transistor. The error amplifier 22 has also a further input terminal, in particular the non-inverting input terminal (+), connected, by a first resistive element Ra 1 , to the ground GND, as well as, by a second resistive element Ra 2 , to the output terminal OUT 2 of the regulator 21 .

Essentially, the signal being outputted from the error amplifier 22 , applied to the control terminal of the power transistor M 20 , allows the voltage at the drain terminal D 20 to be linearly varied when the external control voltage Vext varies and, subsequently, a variation of the range allowed when the signal is present is obtained, when the control terminal Tc is used as a supply terminal of the amplifier 11 . The linear regulator 21 thus allows the required power control voltage Vc to be obtained and the power output signal RFout of the amplifier 11 to be regulated.

It should be noted that the use of the linear regulator 21 allows the output power to be set without using a feedback loop, avoiding the losses due to the presence of the directional coupler being required with the feedback control, as previously seen with reference to the example of FIG. 1 .

This approach has some drawbacks including:

it is necessary to use a power transistor having a low turn-on resistance RDS(ON) to reduce the efficiency loss in the highest output power condition;

when the power transistor is an external component, the presence thereof involves additional costs and a higher complexity of the system construction;

when the power transistor is integrated with the amplifier 11 , the silicon area occupation of the system as a whole is penalized; and

the linear voltage control performed by the linear regulator 21 determines a high control curve slope in correspondence with low output power levels, since a linear function results in a logarithmic function when expressed in dB/V.

An embodiment of this invention is directed to a control method of the electronic circuit output power and a corresponding control circuit, having such structural and functional features as to allow the limitations and drawbacks still affecting prior art systems to be overcome.

The gain stage control method may comprise: providing a control current signal; generating a regulation current signal related to the control current signal; transforming the regulation current signal into a biasing current, proportional to it; and biasing the gain stage by using the biasing current. The biasing current may be related to the control current signal (I CTRL ) by an exponential law.

In one embodiment, the exponential law is of the type:

I POL =K 1 exp ( K 2 I CTRL )

with:

I POL =the biasing current

I CTRL =the control current signal and

K 1 and K 2 =multiplicative constants.

Also, according to another embodiment the control current signal may be of the PTAT type. A further embodiment of this invention is directed to an output power control circuit of a gain stage inserted between a first and second voltage reference and having an input terminal suitable to receive a control voltage signal and an output terminal suitable to provide a regulation current signal to an input terminal of a biasing circuit. The biasing circuit has, in turn, an output terminal suitable to provide a biasing current to a control terminal of the gain stage. The biasing circuit generates the biasing current signal according to a linear relation with respect of the regulation current signal and the control circuit generates the regulation current signal starting from a control voltage signal according to an exponential law.

In one embodiment, the control circuit further comprises at least one operational amplifier having a first input terminal connected to a control terminal of an input transistor, a second input terminal connected to the input terminal, as well as an output terminal connected to a control terminal of an output transistor, inserted, in turn, between the output terminal and the second voltage reference.

The features and advantages of the method and control circuit according to the invention will be apparent from the following description of embodiments thereof given by way of non-limiting examples with reference to the attached drawings.

FIG. 1 schematically shows a feedback-control system realized according to the prior art;

FIG. 2 schematically shows a linear-control system realized according to the prior art;

FIG. 3 schematically shows a control circuit suitable to implement the control method according to an embodiment of the invention;

FIG. 4 shows in greater detail a particular portion of the circuit of FIG. 3 ;

FIG. 5 schematically shows an alternative embodiment of the control circuit of FIG. 3 ; and

FIGS. 6A-6C show the trend of signals characterizing the circuit of FIG. 5 obtained through experimental tests.

With reference to the figures, and particularly to FIG. 3 , a control circuit suitable to implement the control method according to an embodiment of the invention is globally and schematically indicated with 30 . The control circuit 30 is inserted between a first voltage reference, particularly a supply voltage Vcc, and a second voltage reference, particularly a ground GND, and it has an input terminal IN 3 receiving a control voltage V CTRL and an output terminal OUT 3 connected to an input terminal IN 5 of a biasing circuit 50 .

The biasing circuit 50 further has an output terminal OUT 5 connected to a control terminal in of a power transistor TX. More generally, this power transistor TX is a system gain stage whose output power is to be controlled, such as for example a power amplifier for telecommunication systems. As a whole, the combination of the control circuit 30 , the biasing circuit 50 and of the power transistor TX is indicated as the controlled system 100 .

The power transistor TX is indicated as inserted between a generic output terminal out of the controlled system 100 and ground GND. Advantageously according to an embodiment of the invention, the control circuit 30 allows a gain stage control method to be implemented by varying a biasing current I POL of a power transistor.

In particular, the control method of the power transistor output power according to an embodiment of the invention includes: providing a control voltage signal V CTRL ; converting the control voltage signal V CTRL into a control current signal I CTRL , this control current signal I CTRL having a linear relation with the control voltage signal V CTRL and being directly proportional thereto; and generating a regulation current signal I BIAS related to the control current signal I CTRL according to an exponential law; transforming the regulation current signal I BIAS into a biasing current I POL proportional to it; and biasing the gain stage by using the biasing current I POL .

Advantageously according to an embodiment of the invention, the exponential law relating the regulation current signal I BIAS to the control current signal I CTRL is of the type:

I BIAS =K 1 exp ( K 2 I CTRL )  (1)

with K 1 and K 2 being multiplicative constants linked to typical parameters of a control circuit suitable to implement the method according to an embodiment of the invention. Also the biasing current I POL of the gain stage, which is derived from the regulation current signal I BIAS will show an exponential trend with respect to the control current signal I CTRL .

It should be noted that the use of an exponential correlation law between the biasing current I POL and the control current I CTRL derived from the control voltage V CTRL advantageously allows the slope of the corresponding gain stage control curve to be reduced when expressed in dB/V. In fact, as seen with reference to prior art circuits, a linear connection between the biasing current I BIAS of a power transistor and the control current I CTRL (and thus the control current V CTRL ) is not very suitable in several applications, and particularly in telecommunication systems, since it does not allow the control curve slope expressed in dB/V to be minimized.

In a preferred embodiment of the control method, this control current signal I CTRL is also selected to be proportional to the absolute temperature value, i.e. a so-called PTAT (acronym from: “Proportional To Absolute Temperature”) current. In particular, a first steady-temperature current signal I 1 and a second PTAT current signal I 2 are used and a control current I CTRL is obtained according to the law:

with K 3 being a multiplicative constant linked to typical parameters of a voltage-current converter in a control circuit suitable to implement this preferred embodiment of the control method according to an embodiment of the invention.

The control circuit 30 shown in FIG. 3 allows the just described control method to be implemented. To this purpose, this control circuit 30 comprises an operational amplifier A having a first input terminal, particularly a non-inverting input terminal (+) connected, by a first resistor R 1 , to a control terminal of an input transistor Q 1 . In particular, the input transistor Q 1 is diode-configured and biased by a generator Gr of a steady-temperature current I REF .

The operational amplifier A has a second input terminal, particularly the inverting input terminal (−), connected, by a voltage-current converter 40 , to the input terminal IN of the control circuit 30 . In particular, the voltage-current converter 40 turns the input control voltage value V CTRL into an output control current value I CTRL .

Advantageously according to an embodiment of the invention, the control circuit 30 also comprises an output transistor Q 2 , inserted between the output terminal OUT 3 thereof and ground GND, and having a control terminal connected to an output terminal of the operational amplifier A. This output terminal of the operational amplifier A is also feedback-connected to the non-inverting input terminal (−) by a second resistor or feedback-resistor R 2 . Advantageously, the value of the first resistor R 1 is selected to be equal to the value of the second resistor R 2 to compensate the bias current effect at the operational amplifier A input.

The current value I BIAS at the output terminal OUT 3 of the control circuit 30 is thus given by:

with

A E1 and A E2 =the emitter areas of the transistors Q 1 and Q 2 , respectively;

I REF =the reference current value provided by the biasing generator to the input transistor Q 1 ;

R 2 =the value of the feedback resistor of the operational amplifier A;

I CTRL =the control current value applied to the inverting input terminal of the operational amplifier A; and

V T =the thermal voltage (equal to KT/q with K being the Bolzmann constant, T the absolute temperature and q t he electron charge).

Advantageously according to an embodiment of the invention, the biasing circuit 50 sets a biasing current I POL for the power transistor TX, which current is proportional to the input received current I BIAS , according to the law:

I POL =K 4 I BIAS   (4)

with K 4 being a steady-temperature proportional and thus:

So, a biasing current value I POL of the power transistor TX is obtained which current is linked to the control current value I CRTL , and thus to the control voltage value V CTRL , by an exponential law. Advantageously according to an embodiment of the invention, the control current I CTRL is obtained as linked to the V CTRL according to a linear relation by using the voltage-current converter 40 . In a preferred embodiment of the invention, this control current I CTRL is also selected of the PTAT type and obtained by a translinear voltage-current converter 40 , as shown for example in FIG. 4 .

In particular, the translinear voltage-current converter 40 allows a control current value I CTRL to be obtained, which complies with the following relation:

with

1/R 3 =a multiplicative constant depending on the structure of the translinear voltage-current converter 40 ;

I 1 =a steady-temperature current value; and

I 2 =a PTAT current value.

The translinear voltage-current converter 40 is a well-known circuit and the structure thereof will not be described hereafter in detail so as not to burden the description.

In this way, the bias current I POL so obtained complies with the following relation:

i.e. a biasing current I POL for the power transistor TX which is steady with respect to the temperature and exponentially depending on the control voltage V CTRL . The control circuit 30 thus allows the control method according to an embodiment of the invention to be implemented and it achieves several advantages including:

it has a high integration level since it may not need external components and therefore would have neither additional costs nor off-chip losses;

it realizes a power control curve having a low slope in dB/V;

it ensures high control dynamics;

in the absence of the feedback control device (the control circuit 30 performs an open-loop control) it manages power regulation transients without stability problems.

The controlled system 100 also comprises the biasing circuit 50 which is realized by a convenient mirror interface, which is capable of biasing different gain stages starting from a common current reference, as shown in FIG. 5 . In particular, the biasing circuit 50 is inserted between the supply voltage reference Vcc and ground GND and it has an input terminal IN 5 connected to the output terminal OUT 3 of the control circuit 30 and an output terminal OUT 5 connected to the control terminal in of the power transistor TX.

Advantageously according to an embodiment of the invention, in view of avoiding the complementary components usually used to realize mirror devices but having a limited in-current capacity, the biasing circuit 50 has been conveniently designed. In particular, the biasing circuit 50 comprises an input transistor Q 3 , inserted between the supply voltage reference Vcc and the input terminal IN 5 and has a control terminal connected to a current mirror realized by a first Q 4 , a second Q 5 and a third mirror transistor Q 6 .

In greater detail, the first mirror transistor Q 4 is inserted between the supply voltage reference Vcc and the control terminal of the input transistor Q 3 and it has a control terminal connected to a control terminal of the second mirror transistor Q 5 , having in turn a first conduction terminal connected to the supply voltage reference Vcc, as well as a second conduction terminal connected to the control terminal thereof as well as to a first conduction terminal of the third mirror transistor Q 6 . This third mirror transistor Q 6 has also a second conduction terminal connected to a control terminal of an output transistor Q 7 , inserted in turn between the supply voltage reference Vc and an internal circuit node X. Finally, the third mirror transistor Q 6 has a control terminal connected to the control terminal of the input transistor Q 3 and to a conduction terminal of the first mirror transistor Q 4 .

The biasing circuit 50 also comprises a further transistor TY which is diode-configured by a first resistor RY and inserted between the internal circuit node X and the ground GND. The diode transistor TY is also connected, by a second resistor RX, to the control terminal of the power transistor TX. Advantageously, the input transistor Q 3 thus receives the current I BIAS as a biasing current and it provides the mirror realized by transistors Q 4 -Q 6 with a base current I B3 corresponding to:

I B3 =I BIAS /β Q3

with β Q3 being the current gain of the input transistor Q 3 .

Therefore, the biasing current I 7 of the output transistor Q 7 is given by:

I 7 =I BIAS β Q7 /β Q3

with β Q7 being the current gain of the output transistor Q 7 .

It should be noted that, by selecting the output transistor Q 7 having an area which is n times greater than the input transistor Q 3 area and a mirror factor of transistors Q 4 -Q 6 equal to n, this results in the biasing current I 7 of the output transistor Q 7 being equal to n times the biasing current I BIAS received at the input. This current I 7 is also a current being always steady in temperature and it can be used as a reference for the diode transistor TY which biases the power transistor TX. Moreover, advantageously according to an embodiment of the invention, to obtain a lower sensitivity of the biasing current I BIAS with respect to the supply voltage Vcc, it is possible to select a Wilson mirror as a complementary mirror, like the one realized by transistors Q 4 -Q 6 . The main advantage of the biasing circuit 50 shown in FIG. 5 is to minimize the silicon area globally occupied by the mirror, since the transistors Q 4 -Q 6 use small currents, particularly the base currents of the input transistor Q 3 and the output transistor Q 7 .

A power control circuit as previously described has been realized and tested by the Applicant for a three-gain-stage amplifier for DCS applications. In particular, this amplifier has been designed to deliver a 33 dBm power at the operating frequency of 1.8 GHz.

The results of these tests are quoted in FIGS. 6A-6C . In particular, FIG. 6A shows the obtained power control curve (for Vcc=3.5V, f=1.8 GHz, Pin=0 dBm, Ta=30° C.), wherefrom it is possible to verify that the slope of this curve is lower than 80 dB/V in a power range from −20 dBm to the saturation power (33 dBm). FIG. 6B shows the output power with respect to the control voltage V CTRL as the supply voltage Vcc varies from 3 to 5 V (for f=1.8 GHz, Pin=0 dBm, Ta=30° C.). FIG. 6C shows the control feature when the temperature varies (for f=1.8 GHz, Pin=0 dBm, VCC=3.5V). It can be noted how the highest power variation is ±3 dB in a temperature range of −20 to 80° C.

In conclusion, the proposed control method and the control circuit allow the control of gain stages to be realized, for example, by power transistors by: a biasing current of these transistor gain stages varying according to a control voltage external to the system; and an exponential relation between the control current and the control voltage.

It should be noted that, although described with reference to power amplifiers for telecommunication applications, the present invention can be used in all applications requiring a variation of an output power level and it is independent both from the operating frequency and from the power levels being involved. From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.