GB2276288A

GB2276288A - Stabilisation circuit for microwave amplifiers and active networks using butterfly stub

Info

Publication number: GB2276288A
Application number: GB9305343A
Authority: GB
Inventors: Esen Bayar
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-03-16
Filing date: 1993-03-16
Publication date: 1994-09-21
Also published as: GB9305343D0

Abstract

The invention describes a microwave stabilisation network which demonstrates a solution to instability and conditional stability problems associated with high frequency solid state devices for microwave circuits. The circuit can be constructed as a stand alone stabilisation network or can be arranged to form part of a dc biasing network. It comprises a transmission line of around a quarter wavelength leading to a butterfly stub. The arrangement includes a set of stabilisation resistors configured to ensure stability across the entire frequency spectrum. <IMAGE>

Description

STABILISATION CIRCUIT FOR MICROWAVE AMPLIFIERS AND NETWORKS CONTENTS ABBREVIATIONS 1.0 ABSTRACT 2.0 BACKGROUND OF THE INVENTION 3.0 DESCRIPTION OF THE INVENTION 3.1 Theoretical Description 3.2 Qualitative Description of the Circuit Function 4.0 TECHNICAL RESULTS 4.1 Application Notes 4.2 Typical Measured Performance 5.0 CLAIMS ABBREVIATIONS dc direct current rf : radio frequency HEMT High Electron Mobility Transistor HBT : Heterojunction Bipolar Transistor MESFET Metal Semiconductor Field Effective Transistor LNA : Low Noise Amplifier SSPA Solid State Power Amplifier 2.0 BACKGROUND OF THE INVENTION Solid state microwave amplifiers are widely used in most communication networks.The components used to realise these networks are largely based on two-port solid state microwave components which could have drastic affects on equipment performance if a failure such as instability occurs. It is of extreme importance to avoid any instability behaviour, as the effects of it would result in equipment failure and could result in destruction or degradation of other functional equipment.

The conventional solution to the problem has been to design impedance matching networks which ensure unconditional stability at and around the operation bandwidth and to use nag/4 long bias lines with short circuit reference points, where n is an odd integer number. In addition to these, where instability is restricted to narrow bands, shunt resonance circuits are used. The commonly used features of the nag/4 bias lines include stabilisation resistors and rf decoupling capacitors. The non-ideal characteristics of the circuit elements and narrow band characteristics of keg/4 often leads to in-band performance degradation. These commonly used design techniques will often only yield conditional stability.Where high frequency devices such as HEMTs, HBTs and small gate periphery, high frequency MESFETs are used at lower bands, such as L- and Cbands, the achievement of good stability margins or unconditional stability conditions is much more difficult. The problem is often solved using two separate networks; the rf matching network which defines the in-band characteristics and the stabilisation network which can be designed to effect out-of-band characteristics. The invention presented describes a stabilisation circuit which provides an effective tool for ensuring unconditional stability, shaping the out-of-band characteristics without degrading in-band performance.

3.0 DESCRIPTION OF THE INVENTION The circuit performs the function of stabilisation by effecting the out-of band characteristics of a network in a predetermined manner. The performance can be optimised using CAD techniques. The circuit provides a design tool which can be built in microwave and higher frequency circuits to achieve unconditional stability characteristics.

3.1 Oualitative Description of the Circuit Function The schematic diagram of the circuit is shown in Figure 1. The construction is based on the open circuit characteristics defined by the butterfly-stub, element number (4), which connects to the main transmission line via the resistor labelled number (3) and a bias transmission line, labelled number (2). The butterfly-stub serves the prime function of defining a virtual open circuit over 5% bandwidth about a centre frequency fO. The total electrical length from the T-junction to the open circuit reference point of the butterfly-stub IT is half a wavelength at f0,i .e. 1800.

IT = lel + 1,2 + 81 = 1800 where: l,j is the effective electrical length of the butterfly-stub 1,2 is the effective electrical length of the transmission line labelled (2) in figure 1.

81 is the added electrical length of the stabilisation resistor labelled (3).

An important feature of the network concerns the two stabilisation resistors (3) and (5), used on either side of the butterfly-stub. As a result of this arrangement any in-band signal sees more of (3) and virtually none of (5), and any out-of-band signal over selected frequency bands sees both of the resistors and is hence significantly attenuated which reduces the magnitudes of S21 and S12. The elements labelled (2), (3), (4) and (5) can be carefully chosen to attenuate selected out-of-band frequencies with very little or no in-band insertion loss increase. Depending on circuit performance requirements, a small degradation of in-band insertion loss can be traded off for improved stability performance.The circuit offers a versatile stabilisation network with two separate stabilisation resistors; one for in band effects and the other for out-of-band effects. Where the in-band stability is very good and further improvement is not needed, the first resistor (element number (3)) can be shorted. These features are of extreme importance in LNA and SSPA designs. In the case of the former the input circuit is mismatched to optimise NF performance. This often results in a need for a small improvement in the K-factor across the operation band and a substantial improvement in the out-of-band K-factor.

Where high gain, high frequency, low noise devices are used, the instability problem becomes more difficult to solve and the described design network could provide the ultimate solution. The preferred dc bias line in LNA applications would be via a high impedance line, labelled (7).

In the latter case, i.e. in SSPAs, it is more suitable to integrate this circuit to its output network, and to apply dc bias via the high impedance line, labelled (8). Where efficiency of the unit is very critical the bias point can be arranged to connect at the junction between the butterfly-stub, element (5) and the low value stabilisation resistor, element (3).

3.2 Theoretical Description The theory of unconditional stability is defined by Rollett Stability Factor, K [1] K = 1 +lDI-2- IS11i2 152212 21S12.S211 The condition for unconditional stability is: K > 1 S11 < I S22 < 1 where Sij defines scattering parameters of a two port network, i = I,2andj = 1,2 D = Sll.S22-S12.S21 4.0 TECHNICAL RESULTS To assess the effectiveness of the presented circuit a C-band LNA was designed and built. The described effects of the stabilisation network were analysed using CAD techniques and the practical performance was tested.

Computer analysis of the modelled network was observed to be in good agreement with the measured data.

4.1 Application Notes The presented invention has a wide range of applications in various microwave and higher frequency active networks. The prime application area encompasses different amplifier types ranging from LNAs to SSPAs.

Some of the other applications may extend into transistor based switching networks. Different configuration of the circuit are shown in Figure 2.

The dc block shown in Figure 2a can be used in fast switching networks to eliminate or alleviate time constraints introduced by decoupling capacitors.

An application example of the circuit used in a LNA stage is shown in Figure 3.

4.2 Typical Measured Performance VSWR and insertion loss characteristics of the stabilisation circuit is shown in Figure 4. The simulated and measured characteristics of the LNA are shown in figure 5 which demonstrates the effectiveness of the stabilisation network in circuit applications.

[1] S.Y. Lioa, "Microwave Circuit Analysis and Amplifier Design", Prentice Hall, Inc., 1987, pp. 96-101 [2] F. Giannini, R. Sorrention, J. Vrba, "Planar Circuit Analysis of Microstrip Radial Stub", IEEE Trans. on Microwave Theory and Tech., Vol. MTT-32, No. 12 Dec. 1984, pp. 1652 - 1654 [3] F. Giannini, M. Ruggieri, J. Vrba, "Shunt-Connection Microstrip Radial Stubs", IEEE Trans. on Microwave Theory and Tech., Vol. MTT-34, No. 3, March 1986, pp. 363 - 366.

Claims

5.0 CLAIMS

5.1 A Microwave stabilisation network comprising a transmission line of typically a quarter wavelength leading to a butterfly stub with a set of stabilisation resistors used to improve circuit stability or to ensure unconditional stability.

5.2 A stabilisation network as in Claim 1 but used in conjunction with high impedance lines and/or lump elements to provide a dc bias network as well as providing improved stability.

5.3 A stabilisation network as in Claim 1 or Claim 2 used in non-linear networks to provide improved stability over the operating dynamic range and environmental conditions.

5.4 As in Claim 1 or Claim 2 with dc de-coupling and rf coupling to provide stabilisation and/or biasing to dc or pulsed controlled switching networks.

5.5 As in Claim 1 or Claim 2 but used as a spurii suppression network.