MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 20

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Every Amplifier Is Waiting To Oscillate And
Every Oscillator Is Waiting To Amplify
By Bonnie C. Baker, Microchip Technology Inc.
What is operational amplifier (op amp) circuit stability, and how
do you know when you are on the “hairy edge”? Typically, there is
a feedback system around the op amp to stabilize the variability
and reduce the magnitude of the open-loop gain from part to
part. With this approach, the stability of your amplifier circuit
depends on the variability of the resistors in your circuit, not your
op amps . Using resistors around your op amp provides circuit
“stability”. At least you hope that a predictable gain is ensured.
But, it is possible to design an amplifier circuit that does quite
the opposite.
You can design an amplifier circuit that is extremely unstable
to the point of oscillation. In these circuits, the closed gain is
somewhat trivial, because an oscillation is “swamping out” your
results at the output of the amplifier. In a closed-loop amplifier
system, stability can be ensured if you know the phase margin
of the amplifier system. In this evaluation, the Bode stability-
analysis technique is commonly used. With this technique, the
magnitude (in dB) and phase response (in degrees) of both
the open-loop response of the amplifier and circuit-feedback
factor are included in the Bode plot. This article looks at these
concepts and makes suggestions on how to avoid the design
of a “singing” circuit when your primary feedback is frequency
stability.
The Internal Basics of the Operational-Amplifier
Block Diagram
Before getting started on the frequency analysis of an amplifier
circuit, let’s review a few amplifier topology concepts. Figure 1
shows the critical internal op-amp elements that you need to be
familiar with if you engage in a frequency analysis. This amplifier
has five terminals, as expected, but it also has parasitics, such
as input capacitance and frequency dependent open-loop gain.
18
Figure 1: The voltage-feedback operational amplifier frequency
model includes the input capacitances (C
that the interaction of the external input-source parasitics and
the feedback parasitics can be taken into account in a frequency
evaluation. This model also has the internal open-loop gain over
frequency (A
parasitics of the output stage are in the analysis.
Analog and Interface Guide – Volume 2
Operational Amplifiers
V
V
IN-
IN+
OL
(jω)). These two parameters ensure that the internal
C
C
CM-
CM+
C
DIFF
V
OS
A
OL
V
V
(jω)
DD
SS
DIFF
and C
CM
V
) to ensure
OUT
The two input terminals have a common-mode capacitances
to ground (C
the inputs. A
response of the amplifier. Figure 2 shows the frequency response
of a typical voltage-feedback amplifier using the Bode-plot
method.
Figure 2: A gain plot and a phase plot illustrate the frequency
behavior of a voltage-feedback amplifier. In this simple
representation, the amplifier has two dominant poles. The first
pole occurs at lower frequencies, typically between 10 Hz to
1 kHz (depending on the gain-bandwidth product of the amplifier).
The second pole resides at higher frequencies. This pole occurs
at a higher frequency than the zero decibels (dB) frequency. If it is
lower, the amplifier is usually unstable in a unity-gain circuit.
The amplifier gain response over frequency (A
voltage-feedback amplifier is usually modeled with a simple
second order transfer function. This second-order transfer
function has two poles. The two plots in Figure 2 illustrate the
gain (top) and phase response (bottom) of a typical op amp. The
units of the y-axis of the gain curve are dB.
Ideally, the open-loop gain of an amplifier is equal to the
magnitude of the ratio of the voltage at the output terminal,
divided by the differ ence of the voltages applied between the two
input ter minals.
It would be nice if this open-loop gain ratio were infinite. But in
reality, the complete frequency response of the open-loop gain,
A
20 dB/decade. This attenuation starts at the frequency where
the first pole in the transfer function appears. This is illustrated
in the Bode plot in Figure 2.
Usually, the first pole of the open-loop response of an operational
amplifier occurs between 1 Hz to 1 kHz. The second pole occurs
at a higher frequency, nearer to where the open-loop gain-curve
crosses 0 dB. The gain response of an amplifier starts to fall off
at 40 dB/decade, which is at the frequency where the second
pole occurs.
OL
(jω), is less than ideal at DC, FT attenuates at a rate of
A
OL
(dB) = 20log |( V
CM
OL
) and differential capacitance (C
(jω) represents the open-loop gain, frequency
OUT
/ (V
IN
+ – V
IN
- ))|
OL
DIFF
(jω)) for a
) between