Device and method for amplifying pulsed RF signals
The present invention relates to a device for amplifying pulsed signals.
It is used notably to amplify pulse-modulated radiofrequency (RF) signals.
It applies in general to all pulsed RF transmission chains, for example in the field of radar technology.
The solutions usually employed for amplifying the power of a pulsed RF signal use for example:
• common-base class-C bipolar transistors with DC voltage emitter
supply,
• common-source class-AB LDMOS transistors with DC drain supply
and controlled drain current.
The first solution offers simplicity of implementation and zero consumption in the absence of pulses. However, it has quite a low gain and a moderate efficiency, and also a distortion of the base of the pulse due to the prebias.
The second solution is more complex to implement, but offers a high gain and respects the integrity of the pulse. However, it has a residual consumption between pulses. Its gain varies with temperature. The efficiency of the stage is reduced by the unnecessary consumption due to the biasing of the transistor.
In IFF amplification, the gate supply voltage is adjusted for a given drain current (quiescent bias current).
This voltage is also present only during the transmission period in order to cancel out the drain current outside the transmission period.
Also known are systems for automatic control of the quiescent drain current an automatic bias system such as that described in the patent US 6 573 796. However, this system can be used in pulse mode only if the resulting drain current pulses are filtered by the system for measuring the

drain current.
The devices of the prior art have notably certain drawbacks.
For IFF (Identification Friend or Foe) amplification, there is residual consumption between pulses - the turn-on time of the transistor obtained with a pulsed gate voltage control is slow compared with the time to establish a square pulse (radar or IFF type). It is therefore essential to anticipate the control relative to the RF pulse to be amplified in order to avoid distorting it, thereby leaving a period over which the transistor consumes power.
As regards the automatic bias system, this cannot be used as it stands and has the drawback of having a residual consumption between pulses (quiescent current).
In general, the power consumption between pulses (increased heating, reduced efficiency) is greater the higher the maximum gain sought.
The amplification stage thus constituted does not contribute to reducing the residual radiation between pulses.
The invention relates to a device for amplifying pulsed RF signals, comprising one or more transistors, each comprising a drain, a gate and a source, characterized in that it comprises a device for synchronizing the supply for the drain with an RF pulse.
The device according to the invention offers notably the following advantages:
• the consumption is virtually zero between pulses, the supply for the
drain being zero or almost zero;
• between pulses, the stage thus formed adds its input/output isolation
to the depth of modulation of the incoming modulated RF signal, which
helps to reduce the residual radiation between pulses and allows a
modulator of lower performance to be used upstream; and
• since the consumption on the drain is limited to the strict duration of
the RF pulse to be amplified, there is no risk of raising the gate voltage
to its maximum value. The device therefore works at maximum gain,

without unnecessarily increasing transistor heat-up.
Other features and advantages of the invention will become more clearly apparent on reading the following description of an exemplary embodiment given by way of illustration but implying no limitation, together with appended figures which show:
• figure 1, one architecture of an amplification device; and
• figure 2, an LDMOS transistor in pulsed and RF power amplification
mode.
Figure 1 shows an example of the use of an LDMOS transistor in pulsed mode.
A source 1 sends radiofrequency (RF) signals to a modulator 2, which has notably the function of converting the signals into pulses. The pulsed RF signals are then transmitted to an amplifier 3 according to the invention. A device 4 has notably the function of controlling the modulation command signal sent to the modulator 2, but also of controlling the opening or closing of the low-loss switch of the amplifier 3. It receives a signal for controlling the transmission synchronization of the modulated signal.
Figure 2 shows an LDMOS transistor in pulsed RF power amplification mode.
The gate G of the transistor receives the frequency-modulated signal and is supplied by a gate supply. The drain D is connected to a DC supply by means of a low-loss switch 10. The switch 10 is controlled by means of the signal sync for synchronizing the transmission of the modulated signal (figure 1). The drain current is present only when the drain is supplied. A high-speed dual-polarity voltage limiter 12, with a value between the peak supply for the drain and the drain-source breakdown voltage of the LDMOS transistor, offers effective protection against overvoltages inherent in this type of operation. The capacitor 11 permits RF decoupling.
The instantaneous power needed is delivered by a reservoir capacitor 14 having a low capacitance, calculated according to the level of charge of the emitter, and a low series resistance (technological choice for

minimizing the losses).
The low-loss switch is produced for example from MOS power transistors. It is therefore easy to produce and offers all the qualities of withstanding the high peak drain current and having low losses.
The supply VG for the gate may be:
• a DC variable gain control voltage, for example a variable-gain
amplifier;
• a modulation signal for a stage used as a modulator;
• the gate voltage is amplitude-controllable; adjusting of the quiescent
drain current; adjusting of the RF gain of the stage.
The method according to the invention consists notably in controlling the opening and closing of the switch 10 according to the signal for synchronizing the transmission of the pulsed signal. For example, the supply for the drain of the transistor is controlled according to the signal transmitted. The drain is supplied according to the position of the RF pulses to be transmitted. It is also possible to control the gate voltage of the transistor by applying a variable DC voltage. According to an alternative method of implementation, the gate voltage of the transistor is controlled by applying a modulation signal of given waveform.

CLAIMS
1 - A device for amplifying pulsed RF signals, comprising one or more
transistors, each comprising a drain, a gate and a source, characterized in
that it includes a device (4) for synchronizing the supply for the drain with an
RF pulse.
2 - The device as claimed in claim 1, characterized in that the supply for the
gate of the transistor is a DC variable gain control voltage.
3 -The device as claimed in claim 1, characterized in that the supply for the
gate of the transistor is a modulation signal having a given waveform.
4 - A method for amplifying pulsed RF signals, characterized in that it
comprises at least one step in which the supply for the drain of a transistor is
controlled according to the signal transmitted.
5 - The method as claimed in claim 4, characterized in that the gate voltage
of a transistor is controlled by applying a variable DC voltage.
6 - The method as claimed in claim 4, characterized in that the gate voltage
for a transistor is controlled by applying a modulation signal of given
waveform.