AbstractIn this paper, a number of tunable grounded
negative resistor circuits are presented. These new negative resistors exhibit important features such as simplicity, independent tunability and wide frequency range. One of the introduced negative resistor circuits is simulated using TSMC 0.18m process parameters and compared to a couple of other negative resistors in the literature.
KeywordsMOS, resistance simulation, negative resistor, tunable resistor, active resistor.
I. INTRODUCTION ECENTLY, there has been a growing interest in the realization of active resistors, both positive and negative, appropriate for on-chip fabrication. Not only positive-
valued tunable resistors but also negative-valued resistors are increasingly being needed as a key element for the implementation of filters, oscillators, amplifiers, mixers, artificial neural networks and control systems [1-3].
Negative resistor implementations started with the discovery of tunnel diode and continued with more and more improved circuits in the literature [4-8]. Negative resistors were mostly preferred as negative resistance loads (NRL) within these filter implementations for the compensation of parasitic resistances. In the following years, a number of floating and grounded resistance circuits have been introduced and utilized in RF bandpass filter applications [9-13]. In the last few years new techniques have been proposed for the implementation of traditional cross-coupled transistor pairs forming negative resistance [14-17]. In most of the reported topologies in the literature coupling of negative resistance to the main circuit appears as a problem, especially for floating negative resistors. As the negative loads are designed within the circuits, their bias currents and other parameters inevitably affect the main circuit parameters and additional adjustment may be needed. A number of examples concerning this issue can bee seen in [8,9,13,14]. As a classical approach for implementing a negative resistor, cross-coupled transistor pairs are arranged in grounded topology in  and main drawback here is the reusage of the current between negative resistance load and active inductor, which affects the circuit parameters. Isolation between the negative resistance load and the main circuit is important with the fact that parasitic input capacitance and biasing issues directly change the behavior of the main circuit. This need is fulfilled in  by applying ac coupling for the separation of
Manuscript received May 5, 2011 A. Sunca is MS student at the Dept. of Electrical & Electronics Engg
Bogazici University Istanbul/TURKEY (e-mail: email@example.com ) O. Cicekoglu and G.Dundar are with the Dept. of Electrical &
Electronics Engg Bogazici University Istanbul/TURKEY (e-mail: firstname.lastname@example.org, email@example.com )
the negative resistance load, which is a slight modification of the negative resistor topology used in .
In this paper, we present a number of grounded negative resistor circuits, all having different equations of resistance and these circuits are to be ac coupled to the main circuit. Different from the topology in , proposed negative resistors are easier to bias and tune. In addition they have less number of transistors including bias circuitry. Minimum dimensions can be used for the transistors to keep the power consumption and input parasitic capacitance low. One of the proposed negative resistor circuits is examined and compared to that of a grounded negative resistor in  and floating negative resistor in  by using the simulation results obtained in . In addition, a general comparison is made regarding the usage of the proposed negative resistors among other proposed negative resistor circuits in the literature.
II. CIRCUIT PRINCIPLE Negative resistors are well-investigated in the aspects of
large and small signal characteristics including noise, stability and bandwidth by . Although negative resistors are potentially unstable, their usage for compensation of parasitic resistances makes them stable in higher level circuits.
Twelve negative resistance simulators are shown in Table III. Each pair of them realizes the same input resistance as shown in the table. Note that they are topologically related in pairs.
The gate-source capacitance of the MOS transistors may introduce additional poles and zeros to the input impedance function that may deteriorate the functionality of the circuit. To illustrate this we examined the negative resistance simulator (b) in Table III considering the gate-source capacitances and drain-source conductances as a source of the non-ideality effect. The circuit is repeated in Fig.1 for convenience. Its non-ideal input impedance is given as:
( )( ) ( )
1 2 3 1 2 3 1 2 32
1 2 3 1 2 1 3 1 3 2 2 2 3 2 1 3 1 1 2 3 1
m m m o o oIN
m m m o o o o o m o o o o
g g g g g g s C C CZ
g g g g g g g g s g C g C g C g C g C sC C C+ + + + + + + +
+ + + + + + + + + + (1)
where gmi is the transconductance, Ci is the parasitic gate-source capacitance of Mi and goi is the drain-source conductance of Mi. Neglecting drain-source conductances, resistance at low frequencies is given as:
gggR ++= (2)
Note that the input impedance equation has a second order denominator and it has a zero at:
which implies one condition for desired operation as < z.
MOS Only Simulated Grounded Negative Resistors Abdullah Sunca, Oguzhan Cicekoglu, and Gunhan Dundar
978-1-4577-1411-5/11/$26.00 2011 IEEE TSP 2011328
Fig.1. Proposed negative resistor
Complete schematic of the negative resistor circuit is shown in Fig.2. Resistance can be tuned changing gm1 after adjusting gm2 and gm3.
VDD VDD VDD
Fig.2. Complete negative resistor circuit
III. SIMULATION RESULTS
Negative resistor behavior is investigated through frequency range and phase response. Magnitude and phase of the input impedance of the proposed negative resistor circuit is shown Fig.3.
In contrast to that of a grounded negative resistor in , frequency range in which the pure resistor behavior is observed (-180 degrees phase shift) shows a better transition. Table I shows a comparison about frequency and phase relations between these two negative resistors. Other large and small signal parameters for  and  are compared in Table II. It is seen that proposed negative resistor circuit serves a more appropriate solution concerning frequency response and loading effects. For tuning possibilities, negative resistor in  is the best although its operation is limited up to 4 MHz.
(b) Fig.3. (a) Magnitude of input impedance of the negative resistor, (b) phase of input impedance of the negative resistor
COMPARISON OF FREQUENCY AND PHASE RELATIONS
Design (for -10K)
Phase angle (degrees) at specified frequency 100 kHz 500 kHz 16 MHz 300 MHz 1GHz
 -180 -180 -155 -70 -60 This work -180 -180 -180 -172 -150
COMPARISON OF LARGE & SMALL SIGNAL PARAMETERS
Performance   This work Technology 0.5 1.2 0.18 Power Supply 1.5 V 3 V 1.5 V Tuning Range -8K to -250K -8K to -11K -2K to -11K Power Consumption
Constant Resistance Range 0~4 MHz 0~300 MHz 0~900 MHz