Abstract: The current advances in
Composite Right/Left Handed Transmission lines (CRLH TL) has led to its uses in distributed active circuits which can achieve wide bandwidth of operation with smaller circuit sizes. The design of Distributed Amplifiers (DA) is discussed with the use of conventional Right
Handed Transmission lines (RH TL) and CRLH
TL. The designs are implemented using simple transistor model based on voltage controlled current source (VCCS) and also by using a
BSIM3 model of 130nm RFCMOS process. Some design options are also discussed. The intended frequency band of operation is the UWB range in the 5-12 GHz.
Index Terms: CRLH Transmission lines,
Distributed Amplifiers, Ultra wideband.
Introduction
The main components of Distributed
Amplifiers (DA) are active devices (transistors) which act as amplifying modules which are coupled by transmission lines. The presence of the transmission lines results in an intrinsic high bandwidth response. Thus we use the transmission line to couple the propagating signal between successive amplifying modules which results in the power coupling without the parasitic capacitances of the devices getting added together. If a single amplifier is used to get reasonable gain we have to use wide devices which increase the parasitic capacitances, especially at the output node, which results in narrow bandwidth. As a result in case of RF amplifiers more use is made of frequency selective resonant or tuning elements to get high gain for some particular centre frequency. In case of DA we use the intrinsic ultra wideband nature of transmission lines to our advantage.
In recent times much advances have been made in the use of double negative or left handed media to make innovative transmission line based circuits which result in smaller circuits with lower losses. These circuits will also have different frequency response compared to conventional Right Handed transmission lines
(RH-TL).
RH TL based Distributed Amplifiers
Distributed Amplifiers made of conventional (right handed) transmission lines consists of two actively coupled transmission lines; called the Gate line and the Drain line. The
RF signal is fed into the input of the gate line from where at uniform distances (equal to some designed electrical length) individual signals are fed into each of the active devices, which act as the amplifying modules. The outputs of the amplifying modules are fed into the drain line at the designed distances from each other.
Fig 1: Distributed Amplifier Structure

In our design we have used only a single RF transistor as the amplifying module. The RF input power (P in
) is fed into one end of the gate line. From the gate line at certain distances the signal is fed into the gate of the transistors. The drains of the transistors are connected to the drain line. Outputs are taken from both ends of the drain line and are designated as forward and reverse gain (G fwd
and G rev
). The frequency response of the output ports are controlled by the phase constant of the transmission lines.
Ultra wideband response is obtained when both the gate and drain lines have identical phase constants. Thus they should have same characteristic impedances (Z
0
) and identical electrical lengths in between each transistor feed.
In this case the forward gain becomes frequency independent in an ideal analysis [1].
Fig 2 :Ideal gain responses for RH TL distributed amplifiers (G fwd
is flat)
When the insertion phase, i.e electrical length (β), in between the transistors is odd multiples of 180° the load lines are identical for all the transistors thereby leading to identical drain voltages and currents. Thus with a minimum β=180° we can get identical output powers from each module which are then coupled together [2]. Thus when using RH TLs, we need larger transmission lines.
The main advantages of transmission line based distributed amplifiers are
1.
Easy to implement in microstrip technology.
2.
In normal RF amplifier design, we need resonant or tuning circuits to get high gain at some frequency, thereby giving a narrow bandwidth system. Also parasitics hinder wide bandwidth systems.
However in distributed amplifiers it is possible to get wide band response.
CRLH TL based Distributed Amplifiers
Double negative or left handed media have negative permittivity and permeability simultaneously. Currently many designs are being investigated which have transmission lines having left handed characteristics [4] and these also include active device based distributed circuits like distributed amplifiers and mixers
[5].
Conventional (right handed) transmission lines can be implemented by lumped parameters using a low pass LC structure (series L and shunt C). Left handed transmission lines are implemented as high pass LC structures (series
C and shunt L). However in practical implementations due to presence of parasitics even in inductors and capacitors we do not get pure left handed characteristics but composite
CRLH characteristics which is left handed at low frequency and right handed at high frequency.
Fig 3: Lumped element representation of RH and CRLH TLs.

frequency.
Fig 6:Ideal gain responses for distributed amplifiers
Implementation of Distributed Amplifiers
Distributed Amplifiers w using both RH and CRLH TLs. line segments were designed with
LH TL C=1pF and L = 10 we need a transistor mode design we first used a simple voltage current-source (VCCS) model fo with 100mS transconductance, with C
C ds
= 30fF and C gd
= 20fF.
response. element LH TL
Fig 7: VCCS model of NMOS
The simulation results for this are given below:
Fig 8: Gain responses with RH TL

Fig 9: Gain responses with CRLH TL with different value of L (Z
0
)
We can see that we have a multiband response and we can still have some frequency selectivity by choosing the L (or C) values of the LH TL appropriately.
Next we tested the design with a BSIM3 model of a 130nm RFCMOS process NMOS transistor which we have from the UMC foundry. The simulation results for that are given below:
Fig 10: Gain responses with RH TL
Fig 11: Gain responses with CRLH TL with different values of β
Here also we notice the wide bandwidth operation of distributed amplifiers. There are some gain scaling issues when using the BSIM model but that could be due to the fact that the
130nm model may have many process driven parameters which needs to be modeled carefully at such high frequencies. The plots and simulations are almost bordering on the cut-off frequencies (f t
) of the technology.
Future Scope
One issue of implementing distributed amplifier using CRLH TL is that the series capacitor in the LH TL model causes biasing problems as it decouples the DC voltages between successive stages of the DA. Hence we can try and implement interdigital capacitors and short/open circuited stubs as inductors can be studied as an alternative.
We can incorporate other standard gain boosting techniques in the amplifying modules.
Instead of just using a single transistor in

common source configuration we can go for constant current source loads using PMOS transistors or using regulated cascode sort of configurations to increase the gain of the amplifying modules. These circuits can also be investigated however at the cost of increased complexity of the structure and lower ease of implementation in a standard microstrip technology.
Also other active distributed structures like distributed mixers can be investigated.
Conclusion
We have studied the modeling and simulation of distributed amplifiers using both conventional right handed transmission lines and the composite CRLH transmission lines. The simulation results show that these structures can be used in ultra wide band systems for RF applications.
References:
[1] J. Mata-Contreras, T.M. Martín-Guerrero, and C. Camacho-Peñalosa, “Assessment of a
Composite Right/Left-Handed Transmission Linebased Distributed Amplifier implemented in microstrip technology”; Proceedings of the 36th
European Microwave Conference
[2] K. W. Eccleston, “Application Of Left-
Handed Media In Distributed Amplifiers”;
Microwave And Optical Technology Letters / Vol. 44,
No. 6, March 20 2005
[3] J. Mata-Contreras, T.M. Martín-Guerrero, and C. Camacho-Peñalosa, “Distributed Amplifiers
With Composite Left/Right-Handed Transmission
Lines” Microwave And Optical Technology Letters /
Vol. 48, No. 3, March 2006
[4] A. Lai, C. Caloz, and T. Itoh, Composite right/left-handed transmission line metamaterials,
IEEE Microwave Mag 5 (2004), 34–50.
[5] J. Mata-Contreras, T.M. Martín-Guerrero, and C. Camacho-Peñalosa, “Active Composite
Right/Left-Handed Transmission Line-Based
Mixers”; IEEE MELECON 2006, May 16-19,
Benalmádena (Málaga), Spain