How to Choose CMOS
Operational Amplifiers
Kent Chon
Corporate Account Manager
Elantec Semicondcutor Inc.
Introduction
Explosive growth in the high-speed digital market has provided both an opportunity and a challenge to analog manufacturers to develop high speed and low cost
analog circuits. As analog market grows,
it requires more of the high speed, low
cost, and low power analog circuits and
CMOS operational amplifiers are one of
the solutions. This article discusses the
unique features and the applications using CMOS operational amplifiers.
Features of CMOS operational
amplifiers
The MOS technology has both advantages and disadvantages as compared
with the bipolar one. An MOS device
has an extremely high impedance at its
input (gate) terminal, which enables it to
sense the voltage across a capacitor without discharging it. Also, there is no inherent offset voltage across the MOS device when it is used as a conduction
switch. Furthermore, high-quality capacitors can be fabricated reliably on an
MOS chip. These features make the realization of such circuits as precision
sample-and-hold stages feasible on an
MOS chip. This is usually not possible
in bipolar technology.
On the negative side, the
transconductance of MOS transistors is
inherently lower than that of bipolar ones.
A typical transconductance value for a
moderate-size MOS device is around
0.5mA/V; for a bipolar transistor, it may
be about a hundred times larger. This leads
to a higher offset voltage for an MOS amplifier than for a bipolar one. At the same
time, the input capacitance of the MOS
transistor is typically much smaller than
that of a bipolar one. Also, the noise generated in an MOS device is much higher,
especially at low frequencies, than in a
bipolar transistor. The behavior of an
amplifier realized on an MOS chip tends
to be inferior to an equivalent bipolar realization in terms of offset voltage, noise,
and dynamic range. However, it can have
much higher input impedance than its bipolar counterpart.
As a result of these properties,
switched capacitor circuits are especially
suitable for linear applications, where element-value accuracy is important, but the
signal frequency is not too high, and the
dynamic range required is not excessive.
Voice and audio frequency filtering and
data conversion are in this category, and
they represent the bulk of past applications.
Ideally, the op-amp is a voltage -controlled voltage source with infinite voltage gain and with zero input admittance
as well as zero output impedance. It is
free of frequency and temperature dependence, distortion, and noise. Practical opamps can only approximate such an ideal
device. The main differences between the
ideal op-amp and the real device are the
following:
1. Finite Gain: For practical op-amps,
the voltage gain is finite. Typical values for low frequencies and small signals are A = 102 ~ 105, corresponding
to 40 ~100dB gain.
2. Finite Linear Range: The linear relation Vo = A (Va-Vb) between the input and output voltage is valid only
for a limited range of Vo. Normally,
the maximum value of vo for linear
operation is somewhat smaller than
the positive dc supply voltage; the
minimum value of Vo is somewhat
positive with respect to the negative
supply.
3. Offset Voltage: For an ideal op-amp,
if Va = Vb (which is easily obtained
by short circuiting the input terminals), then Vo = 0. In real devices,
this is not exactly true, and a voltage
Vo,off≠ 0 will occur at the output for
shorted inputs. Since Vo,off is usually directly proportional to the gain,
the effect can be more conveniently
described in terms of the input offset
voltage Vin,off, defined as the differential input voltage needed to restore
Vo = 0 in the real device. For MOS
op-amps, Vin,off is about 5 - 15mV.
4. Common-Mode Rejection Ratio
(CMRR): The common-mode input
voltage is defined by
Vin,c = (Va + Vb)/2
As contrasted with the differentialmode input voltage
Vin,d = Va - Vb
Accordingly, we can define the differential gain AD (which is the same as
the gain A discussed earlier). The CMRR
is now defined as AD/AC or CMRR =
20log10 (AD/AC) in dB. Typical CMRR
values for MOS amplifiers are in the 60 80 dB range. The CMRR measures how
much the op-amp can suppress commonmode signals at its inputs. These normally represent undesirable noise, and
hence a large CMRR is an important requirement.
5. Frequency Response: Because of
stray capacitances, finite carrier mobilities, and so on, the gain A decreases at high frequencies. It is usual
to describe this effect in terms of the
unity-gain bandwidth, this is, the frequency fo at which /A(fo)/ = 1. For
MOS op-amps, fo is usually in the
range of 1 -10MHz. It can be measured with the op-amp connected in a
voltage-follower configuration.
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6. Slew Rate: For a large input step voltage, some transistors in the op-amp
may be driven out of their saturation
regions or completely cut off. Therefore, the output will follow the input
at a slower finite rate. The maximum
rate of change dVo/dt is called the slew
rate. It is not directly related to the
frequency. For typical MOS op-amps,
slew rates of 1- 20 V/µs can be obtained.
7. Nonzero Output Resistance: For a real
MOS op-amp, the open-loop output
impedance is nonzero. It is usually
resistive, and is of the order of 0.1 5kΩ for op-amps with an output
buffer; it can be much higher for opamps with unbuffered output. This
affects the speed with which the opamp can charge a capacitor connected
to its output, and hence the highest
signal frequency.
8. Noise: The MOS transistor generates
noise, which can be described in terms
of an equivalent current source in parallel with the channel of the device.
The noisy transistors in an op-amp
give rise to a noise voltage Von at the
output of the op-amp; this can be
again modeled by an equivalent voltage source Vn = Von/A at the op-amp
input. Unfortunately, the magnitude
of this noise is relatively high, especially in the low-frequency band
where the flicker noise of the input
devices is high; it is about 10 times
the noise occurring in an op-amp fabricated in bipolar technology. In a
wide band, the equivalent input noise
source is usually of the order of 10 50 µV RMS, in contrast to the 3 - 5µV
achievable for low-noise bipolar opamps.
9. Power-Supply Rejection Ratio
(PSRR): if a power-supply voltage
contains an incremental component v
due to noise, hum, and so on, then a
corresponding voltage Apv will appear
at the op-amp output. The PSRR is
defined as AD/Ap, where AD = A is the
differential gain. It is common to express the PSRR in dB; then PSRR =
20log10(AD/Ap). Usual PSRR values
range from 60 to 80 dB for the opamp alone; for the complete filter, 30
- 50 dB can be achieved.
10. DC Power Dissipation: Ideal op-amps
require no dc power dissipated in the
circuit: real ones do. Typical values
for an MOS op-amp range from 0.25
to 10mW dc power drain.
The EL5144 amplifiers is fabricated
in CMOS technology voltage feedback,
high speed, rail to rail amplifier designed
to operate on a single +5V supply. The
EL5144 offers unity gain stability with an
unloaded -3dB bandwidth of 100 MHz.
The input common mode voltage range
extends from the negative rail to within
1.5V of the positive rail. Driving a 75Ω
double terminated coaxial cable, the
EL5144 amplifier drives to within 150mV
of either rail. The 200V/µsec slew rate and
0.1% / 0.1˚ differential gain / differential
phase makes these parts ideal for composite and component video application.
With its voltage feedback architecture, this amplifier can accept reactive
feedback networks, allowing them to be
used in analog filtering application. This
device also has a power-savings disable
feature. The major benefits are large
swing without saturation on single supplies, low dG/dP characteristics and the
very high input impedance. CMOS opamp also has low power and low DC error due to small input bias current. Railto-rail input and output swing significantly increases dynamic range, especially in low supply applications.
Applications
1. Comparator
Although optimized for use as operational
amplifiers, the EL5144 & EL5146 amplifiers can be used as a very fast, single
supply rail-to-rail I/O comparator. Most
op amps used as a comparator allow only
slow speed operation because of output
saturation issues. The EL5144 & EL5146
amplifiers do not suffer from output saturation issues. Figure 3 show the amplifier implemented as a comparator. The
EL5144 can be used to gain up the signal
with high bandwidth, and then the
EL5146 compares the output voltage of
the EL5144 and reference voltage of the
EL5146.
Figure 2. Two stage comparator
Figure 1. Basic Operational Amplifier Block Diagram
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2. Free Running Oscillator
Figure 3 is an EL5144 configured as a
free running oscillator. To first order,
Rosc and Cosc determine the frequency
of oscillation according to:
Fosc = 0.72 / (Rosc * Cosc)
For rail to rail output swing, maximum frequency of oscillation is around
15MHz. If reduced output swings are
acceptable, 25 MHz can be achieved.
Figure 3. Sine-wave Oscillator
3. Photodiode Current to Voltage
Converter
Low input bias current op-amp is ideal
for current to voltage converting applications. Since the input current of the
EL5144 from photo diode is very small,
the specification of input bias current is
important factor. Input bias current of the
EL5144 is 2nA that will help to reduce
the dc errors.
Figure 4. Photodiode current-to-voltage converter
4. Sample & Hold
This application utilizes the two features
of CMOS amplifier. During
the hold mode, Enable function
can provide high output impedance stage. The EL5246 is best
for the low leakage Sampleand-hold application because
high input impedance does not
discharge the capacitor C1.
Figure 5. Sample-and-hold
5. Instrumentation Amplifier
Combination of a precision bipolar amplifiers and an inexpensive 5V single-supply CMOS op amp to get the best performance. Using a rail-to-rail CMOS op
amps allow the output swing rail-to-rail
on a single 5V supply. With EL5144 family, output swing can be expected to be
within 150mV of the rails.
The gain of the instrumentation amplifier - EL2044 is set by the following
equation.
IB
Rin
Cin
Vo (max)
Vo (min)
BW
Tri State
EL5144 (CMOS)
2nA
1.5GΩ
1.5pF
4.97V
0.03V
100MHz
High Output Impedance
EL2044 (Bipolar)
2.8µA
15MΩ
1.0pF
3.8V
0.3V
80MHz
N/A
Table 1. Comparison chart of CMOS and bipolar op-amps.
Av =[(R2+2R1)/R1] * R4/R3,
if R1=R5, R3=R6, and R4=R7.
Conclusion
The gain of CMOS amplifier EL5144 is set by R8, R9, and R10.
With high input impedance and rail-torail specifications CMOS operational
amplifier is ideal solution for Sample-
and-Hold, Current-to-Voltage Converter,
Instrumentation Amplifier, Comparator,
and Oscillator applications.
Av = 1+ (R1*R2 + R2*R3)/(R1*R3)
The EL5144 is in the instrumentation
amplifier feedback loop, its exact gain is
unimportant. Using R8, R10 divider to
set the gain forces the instrumentation
amplifiers output swing to be centered
midway between the +5V supply and
ground for rail-to-rail output swing.
The compensation capacitor, C1, provides high-frequency feedback around
EL2044, A3, to assure loop stability.
The following table 1 is the comparison chart of Bipolar vs. CMOS operational amplifiers. CMOS realization has
high impedance I/O and rail-to-rail I/O.
Figure 6. Instrumentation Amplifier
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