CCIIOl Current Conveyor Amplifier, LTP Electronics.

2 . 5 H ig h F r e q u e n cy D esign T ech n iq u es

In the previous section (2.4) w e saw a huge variety o f circuits composed o f many transistors. RF circuits use a radically different design strategy, typically using only one or two devices. Here the individual transistors are often worked close to the maximum operating frequency. The design o f RF circuits can be divided into narrow and wideband circuits for linear and nonlinear applications. In this section we briefly describe matching networks, hybrids, baluns, distributed networks, wideband matching networks, amplifiers (including low noise and power amplifiers) and mixers.

2 . 5 . 1 N a rro w b a n d C ircu its

2 . 5 . 1 . 1 M a tch in g N etw o rk s

For narrowband circuits, the most common matching strategy is that o f resonant matching networks [1]. Any reactive impedance o f the input and output o f the active device are tuned out by a reactive LC elements. The real part is transformed to that o f the input and output impedances using a resonant LC network. At higher frequencies, X/8 transmission lines are used to realise the reactive impedance cancellation and X/4 lines are used for transforming the real part o f active device impedance to the input and load impedances [2]. The magnitude o f the values o f these components are determined by the characteristic impedances o f the lines. The two matching strategies are illustrated in Fig 2.5.1.

I I

Resonant . Cancel . Equivalent

Impedance Reactive Input

Conversion I Part o f I Impedance

Quarter Wave I Short . Equivalent

Transformer | Circuit Input

. Line I Impedance Inductor | o f FET o f FET FET X/4 X/8 B F ig 2.5.1 E x a m p les o f R F im p ed a n ce m a tch in g to th e in p u t o f a F E T u sin g (a) L u m p ed E lem en ts, (b) D istrib u ted E lem en ts.

Z 0//2

Vin V in //2 (+90")

0

Z0 / / 2

Fig 2.5.2 B ranch L ine Hybrid giving o ut put at 90" to each oth er whilst iso la tin g the o th er input. All lines are Zo u n less in d icated and all in p u ts and o u tp u ts are term inated with R o. All 4 b ran ch es are x/4 lon g (90").

2 . 5 . 1 . 2 T ra n sm issio n L ine Hyb ri ds

Another important design technique for narrowband circuits are the transmission line hybrids [3]. These include circuits offering out o f phase splitting with isolation between the input and output ports. The splitting is achieved through impedance transformation techniques (quarter wave transformers). The isolation is achieved through the existence o f two paths between two ports, one path is X/2 longer than the other, and the sum o f the two delayed signals is zero. If the path from the input port to the two output paths are o f different length, phase shifts are developed, 90" and 180" are the most important. The names o f common hybrids include, Rat-race, Lange, Branch and Wilkinson. In MMIC circuits below 5GHz, it is sometimes desirable to implement these large distributed structures with smaller lumped component equivalent circuits. We illustrate a typical transmission line hybrid in Fig 2.5.2.

2 . 5 . 2 W id eb a n d C ircu its

2 . 5 . 2 . 1 W id eb an d B aiuns

At low frequencies (10MHz-2GHz), wideband, isolating structures with phase shifts o f 180° with well defined port impedances are possible by using transformer hybrids. At low frequencies these are realised using toroids [4] and special ferrite formers (5], MMIC transformer hybrids can be designed with difficulty and have higher insertion losses. Note

V/2

R/2

V/2 V/2

V/2

R/2

0

F ig. 2.5.3 T h e T ra n sfo rm er H y b rid .

that the transformer hybrid has a 6dB loss in one direction and OdB loss in the other, where as the transmission line hybrids typically have a 3dB loss between input and output ports.

An alternative approach is to use the Coplanar Wave Guide to Slot Line which is suitable for frequencies in excess o f IGHz. This technique has been adopted for high frequency wideband balanced structures realised in MMIC form under the name o f the Line-Unified FET [6,7,8,9,10]

2 . 5 . 2 . 2 W id eb a n d M a tch in g N etw o rk s

The resonant matching technique used in narrowband designs (Section 2.5.1.1) can be extended to wider bandwidths by using multiple elements [1]. This technique requires extensive CAD support and considerable engineering time.

2 . 5 . 2 . 3 T h e D istrib u ted A m p lifier

An alternative wideband matching strategy is to convert the largely capacitve input o f the FET into a lumped pi-section equivalent circuit o f a transmission line. A similar transmission line is formed at the output with an identical velocity factor. The signal propagates along the input line generating a set o f distributed outputs that sums in phase at one end o f the output transmission line (at the other end the phase alignment is arbitrary and generally unusable). Bandwidths o f 500M Hz to lOGHz are typical [11]. A schematic diagram o f a distributed amplifier is given in Fig 2.5.4.

Unused Port Output Port

R

F ig 2 .5 .4 T h e D istrib u ted A m p lifier.

2 . 5 . 3 L in ear C ircu its

2 . 5 . 3 . 1 M a tch ed A m p lifiers

For maximum power transfer the matching network should give a perfect match to the input and output ports. This is not always desirable due to the possibility o f instability, and the need to meet gain specifications other than that o f maximum gain. By the use o f "Gain circles" either the input or the output is deliberately mismatched and only one o f the ports conjugate matched.

2 . 5 . 3 . 2 L ow N o ise A m p lifie rs

In the case o f low noise amplifiers, the input port is usually deliberately mismatched to give a higher voltage across the gate (due to its high impedance) and hence a better noise performance. This design is then aided by the idea o f constant noise circles which the constant gain circles intersect. For a given gain circle, the minimum noise figure point on that gain circle is selected. The output is then conjugate matched. W e illustrate this process in Fig 2.5.5. The process is normally iterative, since changing the output impedance w ill move the stability and gain circles, and also the output impedance must also be in a stable region o f the output impedance chart. The addition o f reactive components to the source can be used to move the centre o f the constant noise circles on the Smith chart to obtain a

Maximum Gai Circles of Constant Gain

Region of

Instabflity

Optimiun d e ^ n point

Desired nominal gain inimum noise figure Stable at operating fiequency

Fig 2.5.5 Design o f a Low N oise Ampl ifier, s ho wi ng constant g ai n circles, constant noise figure circles, input stability circle, all plotted on the input i mp ed an ce chart.

lower noise figure for a given gain [12]. Whilst the design is normally centred on a particular frequency, it is necessary to check the circuit is stable over all frequencies. This can be difficult for high speed devices where there is uncertainty about the magnitude o f various parasitic components.

2 . 5 . 3 . 3 Power Amplifiers

The design o f a Power Amplifier [13] tends to be less concerned with impedance matching, but much more concerned about the maximum ratings o f a device, particularly maximum

pow er dissipation and gate drain breakdown. Various overdrive schemes have been

proposed to maximise the Power Added Efficiency which utilise the filtering o f a pulsed waveform. Generally, specialist devices are used for power amplification which have improved heat sinking and better protection against drain gate breakdown.