MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 23

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

The point of interest in Figure 6 is where the A
intersects the 1/β curve. The rate 20 dB/decade of closure
between the two curves suggests the phase margin of the
system and in turn predicts the stability. In this situation,
the amplifier is contributing a -90 degree phase shift and the
feedback factor is con tributing a zero-degree phase shift. The
stability of the system is deter mined at this intersection point.
The system phase shift is calculated by subtracting the 1/β(jω)
phase shift from the A
phase shift is -90 degrees. Theoretically, a system is stable if the
phase shift is between zero and -180 degrees. In practice, you
should design to a phase shift of -135 degrees or smaller.
Figure 7: This system is marginally stable with a -135 degree
phase shift at the intersection of the two gain curves.
In the case presented in Figure 7, the point of inter section
between the A
marginally stable system. At that point the A
changing -20 dB/decade. The 1/β(jω) curve is changing from
a +20 dB/decade to a 0 dB/decade slope. The phase shift of
the A
curve is +45 degrees. The system phase shift is equal to -135
degrees.
Although this system appears to be stable, i.e., the phase shift
is between zero and -180 degrees, the circuit implementation
is not be as clean as calculations or simulations would imply.
Parasitic capacitance and inductance on the board can contribute
additional phase errors. Consequently, this system is “marginally
stable” with this magnitude of phase shift. This closed-loop
circuit has a signifi cant overshoot and ringing with a step
response.
In Figure 8, the A
The 1/β(jω) is changing at a rate of +20 dB/decade. The rate
of closure of these two curves is 40 dB/decade and the system
phase shift is -168 degrees. The stability of this system is very
ques tionable.
In Figure 9, A
1/β(jω) is changing at a rate of 0 dB/decade. The rate of closure
in these two curves is 40 dB/decade, indicating a phase shift of
-170 degrees. The stability of this system is also questionable.
Operational Amplifiers
OL
(jω) curve is -90 degrees. The phase shift of the 1/β(jω)
OL
OL
(jω) is changing at a rate of -40 dB/ decade. The
(jω) curve and the 1/β(jω) curve suggests a
OL
(jω) is changing at a rate of -20 dB/decade.
OL
(jω) phase shift. In this case, the system
OL
OL
(jω) curve is
(jω) curve
Figure 8: In a practical circuit implementation, given layout
parasitics, this system is unstable.
Figure 9: In a practical circuit implementation, given layout
parasitics, this system is also unstable.
Conclusion
At the beginning of this article you were asked, “What is
operational-amplifier circuit stability and how do you know
when you are on the “hairy edge?” There are many definitions
of stability in analog, such as, unchanging over temperature,
unchanging from lot to lot, noisy signals, etc. But, an analog
circuit becomes critically unstable when output unintentionally
oscillates without excitation. This kind of stability problem stops
the progress of circuit design until you can track it down.
You can only evaluate this kind of stability in the frequency
domain. A quick paper-and-pencil examination of your circuit
readily provides insight into your oscillation problem. The
relationship between the open-loop gain of your amplifier and the
feedback system over frequency quickly identify the source of the
problem. If you use gain and phase Bode plots, you can estimate
where these problems reside. If you keep the closed-loop phase
shift below -135 degrees your circuit oscillations do not occur
and ringing will be minimized. If you do this work up front with
amplifiers, you can avoid those dreadful designs that kick into an
unwanted song, a.k.a “The Amplifier Circuit Blues”.
Analog and Interface Guide – Volume 2
21