Feed-forward techniques are explored for the design of high-frequency Operational
Transconductance Amplifiers (OTAs). For single-stage amplifiers, a recycling folded-cascode OTA presents twice
the GBW (197.2 MHz versus 106.3 MHz) and more than twice the slew rate (231.1 V/

The growing demand for high-speed and high-precision analog ICs dictates stringent design specifications for the amplifiers which are the basic building blocks for numerous applications; IF switched-capacitor (SC) filters and high-resolution data converters with sampling frequencies above 100 MHz require very fast OTAs with settling times less than 4 nanosecods for good performance [

For single-stage amplifiers, the folded-cascode (FC) OTA has a higher signal swing than a telescopic OTA while still presenting a single parasitic pole and relatively large DC gain, and hence it is commonly used for high-frequency applications [

In multistage amplifiers, cascading of individual gain stages increases the overall amplifier gain, but each stage introduces a low frequency pole, which produces a negative phase shift and degrades the phase margin. Many phase compensation schemes for multi-stage amplifiers have been reported in literature [

The theoretical aspects of feed-forward techniques are discussed in Section

A macromodel of the capacitive amplifier used in switched-capacitor circuits is shown in Figure

Typical OTA-based capacitor amplifier: (a) schematic and (b) typical open and closed loop magnitude response.

Step response of a unity gain amplifier with enough phase margin.

Single-stage OTA slew rate (SR) is determined by the amount of current that can be delivered to or extracted from the output and the effective load capacitor

Typical root locus for a system with two poles and 1 zero: (a) zero is located at high frequencies, and (b) zero is located between the poles. In both cases the dominant pole is terminated by the zero.

Open and closed loop transfer function of a second-order system in presence of a zero. The zero is located after the open-loop poles.

The typical FC OTA is shown in Figure

Typical folded-cascode OTA.

To overcome some of these tradeoffs, a number of feed-forward compensation techniques have been reported [

Complementary differential pairs have been used for a long time in the design of rail-to-rail amplifiers [

Folded-cascode OTA using complementary differential pairs.

The overall current consumption is

The current-mirror cascode OTA shown in Figure

Current mirror OTA with cascode output stage.

A recycling folded-cascode (RFC) OTA built by the combination of the conventional FC and the current-mirror OTAs is depicted in Figure

Recycling folded-cascode amplifier.

Now it can be shown that the transconductance of the RFC is given by (

An aspect worth examining is the overall efficiency. If we define efficiency as the ratio of generated small-signal current to total DC current, that is,

This enhanced efficiency of the RFC can be viewed from another angle. If the RFC is able to achieve twice the transconductance and more than twice the slew rate

The setup in Figure

Amplifier characterization setup: (a) AC response and noise and (b) transient response.

Amplifiers AC response: (a) magnitude and (b) phase.

The phase response shows some degradation for both RFC1 and RFC2 with respect to the FC. This is to be expected. As discussed earlier, the addition of current mirrors in the signal path

OTAs transient response for an output step of 1Vpp: (a) output voltage and (b) output current.

As for noise, RFC1 shows better performance over the FC. Intuitively, the enhanced transconductance of the RFC1 reduces the noise when referred to the input. This, however, is counteracted by an increased output noise due to contributions by

The noise performance improvement of RFC1

A summary of the discussed results is shown in Table

Comparison of simulation results for the original folded-cascode and the recycling folded-cascode OTAs.

Parameter | Folded-cascode | RFC1 | RFC2 |
---|---|---|---|

Power [ | 796 | 782 | 394 |

DC gain [dB] | 52.63 | 60.91 | 59.32 |

GBW [MHz] | 106.3 | 197.2 | 105.9 |

Phase margin [deg] | 80.6 | 62.5 | 75.1 |

Total capacitive load [pF] | 3.6 | 3.6 | 3.6 |

Slew rate ( | 99.3 | 231.1 | 116.5 |

Input referred noise (1 Hz-100 MHz) [ | 53.16 | 48.48 | 69.71 |

21.7 | 11.6 | 21.7 |

Amplifiers with cascaded gain stages are very popular for SC applications as well [

Three-stage amplifiers with nested Miller compensation.

Two-stage Miller compensation.

Feed-forward compensation techniques have been used to boost the DC gain of OTAs, especially for low-frequency applications [

No capacitor Feed forward (NCFF) compensated two-stage amplifier.

_{m1}_{m2,}_{m3}

(a) Pulse response for the NCCF two-stage amplifier and (b) the Miller amplifier.

Although the variations in parameters are large, the

input and integrating capacitors of 0.5 pF, 1 pF, and

input and integrating capacitors of 1 pF, 2 pF, and

input and integrating capacitors of 1 pF, 2 pF, and

Notice that the NCFF approach (nominal case,

The aforementioned FC, RFC1, and RFC2 OTA prototypes have been fabricated in TSMC 0.18

Chip microphotograph of the FC, RFC1, and RFC2 OTA prototypes.

Pulse response for FC, RFC1, and RFC2 OTAs.

A two-stage OTA using NCFF compensation scheme was implemented in AMI 0.5

Single-ended amplifier with NCFF compensation scheme.

An inverting amplifier, similar to the one shown in Figure

(a) Simulation results for the single-ended NCCF amplifier and (b) experimental results.

Feed-forward techniques can improve the speed of closed loop switched-capacitor networks. It has been shown that the recycling folded-cascode OTA presents higher slew rate and superior settling performance than the conventional folded-cascode OTA for the same power consumption. The pole-zero pair present in feed-forward topologies must be placed at high frequencies to avoid slow settling components. Another important advantage of feed-forward schemes is that gain enhancement and smaller parasitic capacitor presented at the input reduce the error after settling than that obtained with the regular folded-cascode OTA. The NCFF compensation scheme enables both high gain and fast settling time, resulting in accurate and fast step response. LHP zeros are used to cancel the phase shift of poles to obtain a good phase margin. The effect of pole-zero mismatches on feed-forward amplifier’s performance was studied, and it was shown that the pole-zero cancellation should occur at high frequencies for best settling time performance. Simulation and experimental results for the amplifiers are in accordance with the theoretical derivations.