Dear,
We want to use OPA1688 as headset output amplifier. Depending type of headset, it is possible sometime G shall be < 1.
Is this amplifier a good election. Can it work with G<1?.
Can it be stability (gain is lower than 0 and I can not apply the rule when the gain croos 0, the phase shall be higher than 45º)?
Thank you in advance.
Thank you in advance.OPA1622 -16-04-07 09_51.TSC
Hello Fernando,
The OPA1688 is a good op amp selection for headphone/headset output applications requiring low distortion (<-100dB) with output powers >=50mW. The reference design "Headphone amplifier for voltage-output audio DACs," TIPD189, provides the theory, component selection, simulation, PCB design, and measured results when using the OPA1688 as a stereo headphone amplifier. In TIPD189 the OPA1688 is configured for differential signal gain of 1.47k/2.0k = 0.735V/V and has >51 degrees of phase margin.
Configuring op amps for signal gains <1V/V is not an issue as long as the amplifier is unity-gain stable. This is because op amp loop-gain (Aol*B) is based on the circuit noise-gain (non-inverting gain), which is often different than the circuit signal gain. For example, while inverting op amps have a signal gain of G = -Rf/Ri, the noise gain of the inverting op amp circuit is based on the non-inverting gain which is equal to, G = 1+Rf/Ri. These concepts are covered in great detail in the bandwidth and stability videos in the TI Precision Labs video series.
The circuit in TIPD189 features a differential signal gain of 0.735V/V. The noise-gain, or non-inverting gain, is 1+ 1.47k/2k = 1.735V/V. Therefore, as long as the op amp is stable for G=1 then it will be stable for signal gains <1V/V because the noise gain of circuits with signal gains <1 is still greater than 1. The open-loop gain (Aol) of the OPA1688 is shown below. The OPA1688 features a unity-gain phase margin of approximately 55 degrees.
Be sure to check the stability of your final circuit as shown in TIPD189 to verify that the headphone load you'll be placing on the output of the amplifier does not negatively reduce the phase margin.
Thank you for your answer.
I see application note TIPD189 Design File and I think that I do not know to obtein phase margin with INA, because when I do bode analysis the curve appears is:
An in application note appears:
Do you know as I can do this analyse?
Thank you in advance
Dear,
The imagen in pdf is figure 6 of TIDUAW1 document.
In my desig I generate this figure with AC transfer Characteristic.
I think the problema is In application of note of reference design to analyse "stability" the schematic change (in application note the feedback loop od the op amp has been broken by inductor LT and CT. An d the generator id VG1. I do not why is done this.
Thank you in advance.
