jueves, 4 de febrero de 2010

The use of operational amplifiers in integrated circuits in MOS technology

 The use of operational amplifiers in integrated circuits in MOS technology has become increasingly common in recent years, owing to the need to integrate analog circuits and sub-systems in this technology.

The main requirements for an operational amplifier ("OPAMP") integrated into a monolithic MOS circuit are as follows:

  • high open-loop gain

  • short settling time

  • capacity to drive capacitive loads

  • high rejection to the supply voltage "PSRR", Power Supply Rejection Ratio and

  • low input-referred noise.

Other requirements which are particularly useful for integrated operational amplifiers are:

  • occupation of small area of silicon

  • ease of interconnection with other parts of the integrated circuit

  • large swing of output voltage with low harmonic distortion and

  • limited power dissipation.
A recently-established method of design uses operational amplifiers with a differential output ("fully-differential" or "double-ended" amplifiers) where the output voltage is taken not between the individual amplifier output and a fixed reference voltage (e.g. ground or another voltage generated inside the integrated circuit) but between the two amplifier output.

A main difference between single-ended and fully-differential operational amplifiers is that the latter do not have a reference node common to the input and the output of the operational amplifier.
An essential advantage of the fully-differential approach is that the following improvements are obtained:
(1) An improvement in the maximum swing of the effective output voltage;
(2) A reduction in the harmonic distortion in the output signal (more particularly in the harmonic distortion due to even harmonics) and
(3) An increase in the value of the PSRR.

A typical problem in this class of amplifiers is the need to design circuitry which fixes the "quiescent" voltage of the two outputs (i.e.--the voltages present at the two outputs in the absence of signals applied to the inputs) at a value which ensures a symmetrical, maximum swing of output signals (the quiescent output voltage is in general V REF).
Frequently, (typically when the circuit is supplied with a single voltage) the quiescent voltage of the two inputs is also fixed by circuitry which optimizes the input level with regard to the circuit requirements of the amplifier. (This voltage in general is V RIN).

One method used for designing "fully-differential" amplifiers uses two stages in cascade in order to meet the basic requirement of high gain, since each stage by itself has insufficient gain for the desired application.

The main and well-known disadvantage of this approach is that a two-stage amplifier requires the use of a rather higher compensation capacitor (whose size increases with increasing load), disposed between the first and second stage, to ensure the stability of the system in which the amplifier is inserted. This also appreciably increases the area occupied by the amplifier.

An alternative known method of design, which is becoming increasingly successful, uses a single amplifier stage which by itself has a sufficiently high gain to meet requirements. A main advantage of this type of amplifier is that it eliminates the need for a compensating capacitor.

Basically, a single-stage amplifier is an amplifier which has only one stage having a high transconductance gm and performing a transfer function I OUT =gmV IN
where I OUT represents the output signal current of the stage and V IN represents the input signal voltage. In the complete amplifier, voltage amplification is obtained as a result of the drop produced by I OUT across an output impedance Z OUT , as a result of which the open-loop voltage gain of the amplifier, A, is: A=gmZ OUT 



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