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in the feedback of a buffer: Ask why!

Resistors in the feedback of a buffer: Ask why!

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Resistors in the feedback of a buffer: Ask why!

John Caldwell

Jul 15 2014 17:27 PM

Comments 4

Whenever I review a schematic or a PCB layout for a younger engineer, I make
a point to pick out items and simply ask them “why?” Why did you choose this
component? Why did you place it here on the PCB? My point in asking these
questions is that every design decision should have a valid reason that the
engineer understands.

For example, a resistor in the feedback path of an op amp configured as a
buffer provokes an immediate “why?”

Figure 1: An op amp buffer circuit with a resistor included in
the feedback path

The ugly truth is that often an engineer doesn’t know why he or she included
resistor R2. They may have seen it on a previous schematic and felt obliged to
include it. Most commonly, these resistors are used in low speed applications
(<50 MHz) to cancel the DC offset produced by the input bias current of the
op amp. However, as my now-retired colleague Bruce Trump pointed out, this is rarely effective.

R2 may also provide some protection to the inverting input in the event of an
ESD strike at the output. Also, some op amps, especially JFET input types, may
produce lower distortion if both inputs have matched source impedances. But
without understanding the purpose of R2, the value is often selected randomly,
leading to instability.

To understand how R2 causes instability, let’s add the input capacitance of
the op amp to the circuit as shown in Figure 2.

Figure 2: Op amp input capacitances added to the circuit of
figure 1

R2 forms a low pass filter at the inverting input with the input common-mode
capacitance CCM2 and the input differential capacitance
CDM. The low-pass filter in the feedback path produces a zero in the
noise gain curve (1/β) of the amplifier at the frequency:

If this zero occurs far above the unity gain bandwidth of the amplifier, as
shown by fz(2) in Figure 3, it should not affect the stability
of the circuit. However, if the zero occurs at or below the unity gain bandwidth
of the op amp, like fz(1), the noise gain curve will intersect
the open loop gain curve at greater than 20dB/decade, indicating probably
instability.

Figure 3: Open loop gain (red) and noise gain (blue) curves
for a buffer amplifier

The phase shift from this zero will begin 1 decade below
fz, so a conservative design guideline is:

By substituting the equation for fz, we can
determine the maximum value for R2 to ensure stability:

To show this effect, I simulated an OPA172 in a buffer configuration and measured the phase
margin for different values of R2. The unity gain bandwidth of the OPA172 is 10MHz and
the input common-mode and differential capacitances are both 4pF. Using the
design guideline in equation 4, the maximum value for R2 is:

The TINA-TI™ simulation schematic I used to measure phase margin
is shown in figure 4. The feedback loop is broken at the op amp output by
inductor L1 and a source (VG1) is AC coupled into the feedback loop. The loop
gain is given by the probe labeled “LG” and phase margin is measured when the
loop gain = 0dB.

Figure 4: TINA-TI™ simulation schematic for an OPA172 configured as a buffer

Figure 5 is a plot of the phase margin as the value of R2 is
increased. The blue line is the maximum value of R2 we calculated in
equation 5.  Below this limit, the reduction in phase margin is minimal,
with it falling to 62° at R2 = 200 Ohms. Above this limit, the phase
margin decreases rapidly.

Figure 5: Plot of phase margin versus R2 value for
the OPA172

Remember that this analysis doesn’t include the effects of capacitive loads
or PCB parasitics, which will also degrade the phase margin of the circuit.

In some circuits there may be a valid reason to include R2, but ask yourself
what you’re trying to accomplish with this resistor before putting it in the
schematic. If the required value is large, you’re probably headed for stability
problems. Asking yourself “why” in any engineering pursuit is crucial to
improving as an engineer!

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$post.CommentCount)

TINA-TI,
OPA172,
Bruce
Trump

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Comments

Deepak
murali

Jul 16 2014 07:53 AM

Nice blog. Can you give me more information on this line from your
blog:

"Also, some op amps, especially JFET input types, may produce lower
distortion if both inputs have matched source impedances."

Soufiane
Bendaoud

Jul 16 2014 18:01 PM

Hi John:

Allow me to say that sometimes not having a feedback resistor can hurt even
with bipolar devices. In the presence of large transients the protection
diodes can short the input to the output if the feedback resistor isn't large
enough. Of course if it's too large than one should consider filtering the
additional noise generated by Rf and compensate adequately for the phase shift
and you've done an excellent job at explaining the phase lead compensation.
Thank you

John
Caldwell

Jul 16 2014 18:36 PM

Deepak,

When used in a non-inverting configuration, all op amps produce some level
of "common-mode distortion". This effect is worst with high source impedances
because in some op amps the input common-mode capacitance varies with
common-mode voltage. Capacitors with a voltage coefficient always produce
distortion when driven from a non-zero source impedance. By matching the
impedances presented to each input, the distortion produced by the varying
input capacitance can be cancelled. Douglas Self demonstrated this in his
article series on op amp common-mode distortion: http://www.eetimes.com/document.asp?doc_id=1278866&page_number=2
(see the measurements on the TL072 op amp). This effect is reduced in modern
JFET input op amps but not completely eliminated. I plan to write an article
on this topic later in the year so stay tuned!

John
Caldwell

Jul 16 2014 18:43 PM

Deepak, this effect is also mentioned in the OPA134 datasheet in the
section on source impedance and distortion.

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