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Design >> Analog Design >> Slew rate enhancement in opamps
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Message started by vivkr on Jun 29^th, 2005, 8:22am

Title: Slew rate enhancement in opamps
Post by vivkr on Jun 29^th, 2005, 8:22am

Hi,

Can anyone suggest a good scheme for enhancing slew rate of an opamp,
say a folded cascode, without recourse to class-AB output stages?

Thanks
Vivek

Title: Re: Slew rate enhancement in opamps
Post by Paul on Jul 4^th, 2005, 8:03am

Vivek,

below a couple of references that may answer your question.

A low power OTA with adaptive biasing and common-mode compensation
Zhiliang Hong; Yan Wang;
Solid-State and Integrated Circuit Technology, 1995 4th International Conference on
24-28 Oct. 1995 Page(s):188 - 192

Power-efficient super class AB OTAs
Lopez-Martin, A.; Baswa, S.; Ramirez-Angulo, J.; Carvajal, R.G.;
Solid-State Circuits Conference, 2004. ESSCIRC 2004. Proceeding of the 30th European
21-23 Sept. 2004 Page(s):231 - 234

Low-voltage low-power adaptive biased high-efficiency integrated amplifiers
Ferri, G.;
Electronics, Circuits and Systems, 2001. ICECS 2001. The 8th IEEE International Conference on
Volume 3, 2-5 Sept. 2001 Page(s):1529 - 1532 vol.3

A 900-mV low-power ΔΣ A/D converter with 77-dB dynamic range
Peluso, V.; Vancorenland, P.; Marques, A.M.; Steyaert, M.S.J.; Sansen, W.;
Solid-State Circuits, IEEE Journal of
Volume 33, Issue 12, Dec. 1998 Page(s):1887 - 1897

900 mV differential class AB OTA for switched opamp applications
Peluso, V.; Vancorenland, P.; Steyaert, M.; Sansen, W.;
Electronics Letters
Volume 33, Issue 17, 14 Aug. 1997 Page(s):1455 - 1456

Be careful with the adaptivity ratio and related bias stability issues.

Good luck,

Paul

Title: Re: Slew rate enhancement in opamps
Post by ethan on Jul 5^th, 2005, 1:46am

vivkr wrote on Jun 29^th, 2005, 8:22am:

Hi,

Can anyone suggest a good scheme for enhancing slew rate of an opamp,
say a folded cascode, without recourse to class-AB output stages?

Thanks
Vivek

I have read one paper, saying "in SC integrator, settling behavior is determined by the time constant of the non-dominant pole", ..."fast settling respose" need very high frequency non-dominant pole".

In the disseration of Ronald Takeshi Kaneshiro, "Circuit and Technology Considerations for High frequency" UC berkeley, 1981 or 1982, he mentioned in the "optimization of the Transient response" that "it is recommended to inserta a left-half plane zero in the transfer function of the amplifier. This zero is incorporated in th efolded-cascode amplifier by adding a feedfoward capacitor Cff across the cascode element. The addition of Cff loads both the output and the cascode nodes, but it does not create any addtional poles. "

hope it helps.

Title: Re: Slew rate enhancement in opamps
Post by raul on Mar 19^th, 2006, 8:53pm

In the case of a two stage amplifier where the first stage is a folded cascode, you can try tying the miller compensation capacitor to the source of the common-gate devices in the folded cascode instead of tying the cap to the high impedance node of the folded-cascode. This will split the poles better and allow you to reduce the capacitive load at the high impedance node of the folded cascode which sets the slew-rate. This architecture has some gotchas; run large signal sims to be certain you have no large signal oscillations. good luck.