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Bharath Vasan

Jan 22, 2016

opampportal

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Related

Amp up your cans: Is your op amp stable? (Part 2)

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This post is co-authored by John Caldwell.

In the first post in our “Amp up your cans” blog series, my
colleague John Caldwell and I used a nominal value of headphone impedance to
calculate the output voltage and current requirements of headphone amplifiers.
The impedance of most headphones is not purely resistive, however. Headphones
are notorious for being a difficult load for amplifiers, necessitating proper
compensation to avoid stability issues, such as ringing and oscillations.

Numerous compensation techniques exist to address this issue using external
components, but wouldn’t it be nice if the op amp didn’t need these additional
components? During the development of the OPA1622, a high-performance, low THD+N, bipolar-input audio operational amplifier (op amp), we took on this
challenge. Our goal was to minimize, if not eliminate, the need for additional
components to maintain stability.

Figures 1 and 2 show the impedance magnitude and phase of several popular
headphones. The magnitude plot clearly reveals a resonant peak around 1MHz. This
peak results from the cable capacitance and inductance. Beyond this point, the
headphone load is dominantly capacitive, as is evident from the phase shift at
these frequencies, illustrated in Figure 2. On extracting electrical models for
the headphone impedance, the value of this capacitance can be almost
500pF!

Figure 1: Impedance magnitude of several headphone
models

Figure 2: Impedance phase of several headphone
models

Op amps are not usually designed to drive a large capacitive load. The
capacitance interacts with the open-loop output impedance, RO, of the
amplifier to introduce a pole, fpO1, in the amplifier’s open-loop
gain curve, as shown in Figure 3. This pole causes excessive phase shift in the
feedback loop, severely degrading phase margin. The reduced phase margin results
in increased ringing in the amplifier’s step response and could potentially
cause the amplifier to oscillate. The effect is worse in a unity-gain
configuration.

Figure 3: The effect of op amp open-loop output impedance,
RO,on capacitive load drive

A common solution is to use an isolation resistor, RISO, between
the amplifier and the headphone load (Figure 4), but this heavily degrades audio
performance, along with other penalties like reduced output voltage range.
John’s article, “Stabilizing
difference amplifiers for headphone applications,” examines in further
detail the negative effects of series isolation resistors in headphone
amplifiers.

Figure 4: Isolation resistor to improve capacitive load
drive

With the OPA1622, we took a different approach. We developed an
output stage with very low output impedance that pushes the second pole (created
by the capacitive load)to a higher frequency. We added additional
poles and zeros to the open-loop gain curve at frequencies fpA and
fzA (shown in Figure 5), choosing the location of the pole-zero pairs
and the pole-zero separation to:

Provide high gain bandwidth (GBW) in the audio frequency range, resulting
in a large loop gain that suppresses harmonic distortion over the audio
frequency range.

Roll off the open-loop response to cross unity-gain well below the output
pole, fpO2, resulting in sufficient phase margin with a large
capacitive load. See Figure 5.

Figure 5: OPA1622 open-loop
response vs. general-purpose amplifier for large capacitive load

Figure 6(a) shows the measured gain and phase response of the OPA1622. The pole-zeros in the response enable the gain to
roll off faster with only minor changes in the phase response, allowing adequate
phase when gain crosses 0dB. The gain bandwidth in the audio band is around
32MHz, with a unity-gain frequency (UGF) of 8MHz. Figure 6(b) shows the
open-loop output impedance of the OPA1622.

Figure 6: OPA1622 open-loop
gain and phase (a) and output impedance (b)

We designed the OPA1622 to have a low output impedance of 5.5Ω over a wide
frequency range. This allows it to drive capacitive loads over 800pF in a
unity-gain configuration, requiring no external components for compensation. See
Figure 7.

Figure 7: OPA1622 phase
margin vs. capacitive load

We demoed the device at the 2016 Consumer Electronics Show (watch John’s video of the demo) in a circuit that did not
include any series isolation resistors. The OPA1622 was stable regardless of the headphone the attendees
chose and we received overwhelmingly positive feedback regarding the sound
quality.

In part three of this series, we’ll take a look at slew rate, output drive
and how the OPA1622 achieves ultra-low distortion while driving
headphone loads.

Additional resources

View the OPA1622
data sheet.

Download the OPA1622 TINA-TI™ SPICE macromodel.

Learn more about TI’s audio operational amplifier portfolio and find technical
resources.

Watch on-demand training courses on stability and more in the TI Precision Labs – Op Amps Low-Distortion Design and Noise video series.

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