25-10-2012, 02:14 PM
DESIGN OF TELESCOPIC CASCODE OP-AMP WITH P-TYPE INPUTS
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ABSTRACT
Operational amplifiers are integral part of any analog circuit. The fundamental definition of an operational amplifier can be defined as a differential amplifier with a large gain. There is a need for differential amplifier in almost all analog circuits in order to weed out the common mode noise signal which is present in the environment around the system.In this paper we discuss the design of a single stage telescopic cascade amplifier according to the specifications provided. The design process involves determining the aspect ratios of the various transistors and the topology of the circuit.The design procedure for a Single Stage Telescopic op amp is developed using design equations .The designed circuit is simulated using Spectre Tools. On the basis of
simulation results the performance analysis has been done. The designed specifications are validated and the graphical analysis has been done. The simulated results are validating
our designed values.The Telescopic op-amp has the inherent disadvantage of low output swing. A CMOS Telescopic operational amplifier is analyzed and the results are presented in the form of design equations and procedure.
INTRODUCTION TO OPERATIONAL AMPLIFIER:
A operational amplifier is a high gain differential amplifier where there a greater emphasis laid on eliminating the noise by having a differential input which eliminates the common noise present in both the inverting and the non-inverting terminal of the amplifier.Operational amplifiers (usually referred to as op-amps) are key elements in analogue processing systems. Ideally they perform the function of a voltage controlled current source, with an infinite voltage gain. Operational amplifiers are an integral part of many analog and mixed-signal systems .Op amps with vastly different levels of complexity are used to realize functions ranging from dc bias generation to high-speed amplification or filtering. The design of op amps continues to pose a challenge as the supply voltage and transistor channel lengths scale down with each generation of CMOS technologies
CIRCUIT SYMBOL:
The circuit symbol of an op amp is a triangle shown in figure. It has two input terminals and one output terminal. The terminal (-) sign is called inverting terminal and the terminal (+) sign is called as non – inverting terminal.
OP AMP CHARACTERISTICS
The basic schematic of op-amp is shown in Figure. It is a four terminal block with two inputs and two outputs. One of the outputs is the analog ground. The key function of the op-amp is to generate at the output an amplified replica of the voltage across the input terminals. Ideally, the voltage gain is infinite. Moreover, the input impedance is infinite as well and the output impedance is zero.
We normally define an op amp as a “ high-gain differential amplifier” . By “ high” , we mean a value in the range of 101 to 105 . Since op amp are usually employed to implement a feedback system, their open-loop gain is chosen according to the precision required of the closed-loop circuit.
Up to two decades ago, most op amps were designed to serve as “ general – purpose” building blocks, satisfying the requirements of many different applications. Such efforts sought to create an “ ideal” op amp, e.g., with very high voltage gain (several hundred thousand), high input impedance and low output impedance, but at the cost of many other aspects of the performance,
e.g., speed, output voltage swing and power dissipation.
By contrast, today’s op amp design proceeds with the recognition that the trade-offs between the parameters eventually require a multidimensional compromise in the overall implementation, making it necessary to know the adequate value that must be achieved for each parameter.
PARAMETERS ASSOCIATED WITH AN OP AMP PERFORMANCE:
The optimal selection of op-amp to be used in a particular application is often the key factor, which determines the success or failure of the circuit. There is a wide variety of op-amps available, from those requiring only 1 Volt supply with bias currents of the 10-15 Amp range, to those that will output hundreds of Volts at tens of Amps.
DIFFERENTIAL GAIN (Ad)
This is the open loop differential gain measured as a function of frequency. To estimate the differential gain the offset must be compensated. The small signal input generator is connected between the two input terminals through a big capacitor C, while a T network made of two resistors
and capacitor establishes a feedback path around the op amp. A typical value of the differential gain, Ad, ranges from 70 to 90 dB. For very precise functions (like high-resolution data converters), the designer need higher gains in the 100 to 140 dB range.
POWER SUPPLY REJECTION RATIO (PSRR)
If we apply a small signal in series with the positive or the negative power supply we obtain a corresponding signal at the output with a given amplification (APS+ or APS-). The ratio between the differential gain and the power supply gain leads to two PSRRs. These are two merit factors showing the ability of the op-amp to reject spur signals coming from the power supply.
Having a good PSRR is an important merit. Unfortunately, especially at high frequencies, the PSRR achieved is quite poor .A typical value of PSRR is 60dB at low frequencies that decreases to 20-40dB at high frequencies.
OFFSET VOLTAGE (VOS)
If the differential input voltage of an ideal op-amp is zero the output voltage is also zero. This is not true in real circuits: various reasons determine some imbalance that in turn lead to a nonzero output. In order to bring the output to zero it is therefore required to apply a proper voltage at the input terminals. Such a voltage is the offset.
INPUT COMMON MODE RANGE (ICMR)
This is the voltage range that we can use at input terminal without producing a significant degradation in op-amp performance. Since the typical input stage of an op-amp is a differential pair, the voltage required for the proper operation of the current source and the input transistors limit the input swing. A large input common mode range is important when the op-amp is used in the unity gain configuration. In this case the input must follow the output.
OUTPUT VOLTAGE SWING
This is the maximum swing of the output node without producing a significant degradation of op-amp performance. Since we have to leave some room for the operation of the devices connected between the output node and the supply nodes, the output swing is only a fraction of (VDDVSS). Typically it ranges between 60% and 80% of (VDD-VSS). Within the output swing range the response of the op-amp should conform to given specifications and in particular the harmonic distortion should remain below the required level.
UNITY GAIN BANDWIDTH
The speed performance of the op-amp is described by small and large signal parameters .The small signal analysis determines the frequency response sketched by a set of zeros and poles. Since we have to ensure stability, one of the poles (f1) must be dominant. The amplitude Bode diagram will display a 20dB/decade roll-off until the gain reaches 0dB .The frequency at which the gain becomes 0dB is called unity gain frequency fT . With a constant roll-off 20dB/decade of the achieved unity gain frequency equal to the product of gain and bandwidth f1A0 . Therefore, fT is also known as the gain-bandwidth product, GBW. Other poles, f2, f3…exceeding fT are named non-dominant poles.
PHASE MARGIN
This is the phase shift of the small –signal differential gain measured at the unity gain frequency. In order to ensure stability when using the unity gain configuration it is necessary to achieve a phase margin better than 60 degree. A lower phase margin (like 45 degree or less) will cause ringing in the output response. However, for integrated implementation it is not strictly necessary to ensure absolute stability.
SLEW RATE
This is the maximum achievable time derivative of the output voltage. It is measured using the op-amp in the open loop or the unity gain configuration. A large input step voltage fully imbalances the input differential stage and brings the op-amp output response into the slewing conditions .The positive slew rate can be different from the negative slew rate, depending on the specific design.
Typical values ranges between 40 and 80V/micro second.
POWER CONSUMPTION
This is the power consumed under standby conditions. The power used in the presence of a large signal can significantly exceed the one required in the quiescent conditions. Moreover, the consumed power depends on the speed specifications. Typically, higher bandwidth leads to higher power consumption .Low power operation is a very important quality factor: batteries that should supply the system for hours or days power more and more electronic systems. Thus, a key design task is to achieve the minimum power consumption for a given required speed.
FOLDED CASCODE AMPLIFIER
Although only Vds, sat is needed to saturate the bottom-most load transistors and the top –most current source transistors in order to allow for process variation, a small safety margin Vmargin is often added to Vds to ensure saturation. Accounting for these, and the Vds, sat the differential output swing is 2 Vsup – 8 Vds, sat – 4 Vmargin.
With a voltage margin of 100mV, this is estimated to be 2 Vsup – 2V. Although the currents in the output stage can be much smaller than that flowing through the input devices, in practice, the output stage current is picked to be the same or almost the same as the current in the input stage
TELESCOPIC CASCODE AMPLIFIER
Although Telescopic operational - amplifier has smaller swing, which means reduced dynamic range, this is offset somewhat by the lower noise factor. The above reason implies that the Telescopic op-amp is a better candidate for low power, low noise single stage Operational Trans conductance Amplifier. The single stage architecture normally suggests low power consumption.
Disadvantage of a Telescopic op-amp is severely limited output swing. It is smaller than that of Folded Cascode because the tail transistor directly cuts into output swing from both sides of the operational - amplifier.
The Telescopic operational - amplifier shown in fig 2.3(a), all transistors are biased in saturation region. Transistors M1 - M2, M7 – M8, and tail current source M9 must have at least Vds, sat to offer good common – mode rejection, frequency response and gain.
The maximum differential output swing of a telescopic op-amp is 2Vsup - 10Vds,sat - 6Vmargin . Under identical conditions, the output swing of this design is limited to 2Vsup-2.6V.In a 3V supply system; this represents a 45% reduction of the available output swing. At large supply voltages, the
telescopic architecture becomes the natural choice for systems requiring moderate gain for the op amp. Reducing supply voltages, on the other hand, forces reconsideration in favor of the Folded Cascode, or in the extreme case, the two-stage design.
Although a Telescopic op-amp without the tail current source improves the differential swing by 2Vds,sat + 2Vmargin (600 mV), the common – mode rejection and power-supply rejection of such a circuit is greatly compromised. Moreover, the performance parameters (such as unity gain frequency) of the op-amp with no tail or with a tail transistor in the linear region is sensitive to input common- mode and supply voltage variation, which is undesirable in most analog cases.