GB2471498A

GB2471498A - A microwave push-pull amplifier using broadband hybrid stripline couplers

Info

Publication number: GB2471498A
Application number: GB0911422A
Authority: GB
Inventors: John Mclellan
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2009-07-01
Filing date: 2009-07-01
Publication date: 2011-01-05
Also published as: GB0911422D0

Abstract

A multi-octave microwave push-pull amplifier comprises two identical amplifiers between input and output broadband hybrid stripline couplers. The use of hybrid stripline couplers instead of conventional balun transformers allows a wider bandwidth. Even-order amplifier distortion products are cancelled in the output coupler. Low intermodulation is achieved over a very wide frequency band, with low passband ripple and lower power consumption than a single-ended circuit. The broadband amplifier may be used in radar systems.

Description

I

BROADBAND MICROWAVE AMPLIFIER CIRCUIT

This invention relates to a broadband microwave amplifier circuit which may be used for example in radar systems.

There is a need for a high dynamic range, low intermodulation, broadband, high power, high efficiency microwave signal amplification circuit. There are currently no broadband microwave amplifiers available with the required performance, and specifically with multi-octave low distortion at medium power levels. The best existing circuits have poor passband flatness and excessive power dissipation. High power amplifiers with acceptable distortion, such as those capable of 20 Watts or even 50 Watts, are expensive and have the capability of damaging sensitive following stages when they are driven under fault conditions.

The present invention provides a broadband microwave amplifier circuit comprising two substantially identical microwave amplifiers in push-pull configuration, an input 1800 broadband hybrid stripline coupler arranged to split an input microwave signal into out-of-phase signals and to feed those signals to inputs of respective ones of the microwave amplifiers, and an output 180° broadband hybrid stripline coupler arranged to receive and to couple the amplified outputs of the two microwave amplifiers and to provide the coupled output as an output of the circuit.

Push-pull transistor amplifiers have typically used transformers for splitting signals into out-of-phase components and for recombining the amplified components with low distortion, but multi-octave transformers are not available at microwave frequencies.

We have found that the multi-octave stripline couplers are capable of performing equivalently to a push-pull transformer at microwave frequencies. Microwave amplifiers are available with good linearity, high dynamic range and a flat frequency response for gains of typically 10 to 30B with up to +3OdBm 1dB compression, and so the invention can already be implemented practically.

Because of the push-pull configuration, significantly lower intermodulation is achieved over a very wide frequency band, with low passband ripple, and using relatively low power consumption, which may typically be 3 Watts (i-23dBm P1dB). This avoids the need for microwave amplifiers with excessive power handling that would be required in a single-ended circuit, and it reduces the potential damage that can be caused to sensitive load devices in the following stages of a radar system, for example.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 is a circuit diagram of a broadband microwave amplifier circuit embodying the invention; and Figure 2 is a graph showing the variation of suppression of unwanted harmonics in the circuit, measured in dB, with the phase error, measured in degrees, from the ideal phase difference of 1800 in the hybrid stripline couplers of Figure 1, for six different values of gain imbalance, measured in dB between the out-of-phase components in all of the components of the amplifier circuit of Figure 1.

The broadband microwave amplifier circuit of Figure 1 has two substantially identical microwave amplifiers in push-pull configuration, arranged to amplify the out-of-phase signal outputs of an input 180° broadband hybrid stripline coupler. The outputs of the amplifiers are recombined in an output 180° broadband hybrid stripline coupler, to deliver a combined amplified signal. The gain is preferably at least 6dB and preferably in the range of 10 -20dB, in each of the amplifiers, and this corresponds closely to the gain of the overall amplifier circuit of Figure 1, allowing for some loss in the couplers.

The frequency range of each stripline coupler is typically specified and optimised for the required operation band, for example 6 -18 GHz, i.e. 2.6 octaves. Stripline couplers with slightly higher loss and poorer phase match are available with 1-18 GHz coverage (more than 4 octaves), while 3+ octave coverage up to 50 GHz have higher but acceptable amplitude/phase matching and signal loss. Power handing is typically from 10mW up to 20W.

With the push-pull configuration of the amplifiers, at any given frequency the distortion produced by each amplifier is equal in amplitude and even products are in anti-phase at the final output from the output stripline coupler, apart from any amplitude and phase imbalances in the couplers and the amplifiers. Imbalance effects can be minimised by comparing the measured performance of the individual devices and selecting the closest matched sets from a batch, made at the same time. The input and output stripline couplers are preferably substantially identical and where manufacturing consistency tends to have a bias to say the 180° port, they can be configured with the input hybrid 1800 port connecting through the amplifier to the output hybrid 0° port to compensate.

A pair of amplifiers in push-pull configuration is capable of providing up to 3dB increase in P1dB and in lP3, and typically an increase of at least 13dB in 1P2, compared with a single amplifier. (P1dB is the 1dB compression point, at which power has dropped 1dB from the ideal. 1P2 and 1P3 are the intercept points for the second and third order intermodulation products produced when two sinusoidal inputs are applied.) The power dissipation is typically one tenth that of a single amplifier with the same 1 P2 performance.

Figure 2 illustrates the suppression of unwanted harmonics in the amplifier circuit of Figure 1, for six different values of the imbalance, measured in dB, between the out-of-phase signals in the amplifier circuit as a whole. The graph shows the variation of suppression against phase error measured in degrees, from the perfect phase difference of 1800. This corresponds to the simple model for suppression which equates it to: 2olog(1-RcosC), where R is the ratio of the lower gain path to the higher gain path and 0 is the phase error from 180°. From this graph it is apparent that the amplifier circuit of Figure 1 may readily be implemented using currently available components, with known amplitude and phase imbalances. A budget of less than 2dB in amplitude and less than 20° phase imbalance for the two amplifiers and the two hybrids can readily be achieved in practice.

Claims

CLAIMS: 1. A broadband microwave amplifier circuit comprising two substantially identical microwave amplifiers in push-pull configuration, an input 1800 broadband hybrid stripline coupler arranged to split an input microwave signal into out-of-phase signals and to feed those signals to inputs of respective ones of the microwave amplifiers, arid an output 1800 broadband hybrid stripline coupler arranged to receive and to couple the amplified outputs of the two microwave amplifiers and to provide the coupled output as an output of the circuit.
2. A circuit according to claim 1, in which the stripline couplers are configured to operate over a frequency range of plural octaves.
3. A circuit according to claim 2, in which the frequency range coverage is within the range ito 50 GHz.
4. A circuit according to any preceding claim, in which each of the microwave amplifiers has an amplification of at least 6dB.
5. A circuit according to any preceding claim, in which the output power of the circuit is in the range 10mW to 20W.
6. A circuit according to any preceding claim, in which the input and output hybrid stripline couplers are substantially identical.
7. A broadband microwave amplifier circuit substantially as described herein with reference to the accompanying drawings.