OP270GS datasheet(16/20 Pages) AD

16 / 20 page

OP270

Rev. E | Page 16 of 20

LOW PHASE ERROR AMPLIFIER

The simple amplifier depicted in Figure 41 utilizes a monolithic

dual operational amplifier and a few resistors to substantially

reduce phase error compared with conventional amplifier

designs. At a given gain, the frequency range for a specified

phase accuracy is more than a decade greater than that of a

standard single op amp amplifier.

The low phase error amplifier performs second-order fre-

quency compensation through the response of Op Amp A2 in

the feedback loop of A1. Both op amps must be extremely well

matched in frequency response. At low frequencies, the A1

feedback loop forces V2/(K1 + 1) = VIN. The A2 feedback loop

forces VO/(K1 + 1) = V2/(K1 + 1), yielding an overall transfer

function of VO/VIN = K1 + 1. The dc gain is determined by the

resistor divider at the output, VO, and is not directly affected by

the resistor divider around A2. Note that, like a conventional

single op amp amplifier, the dc gain is set by resistor ratios only.

Minimum gain for the low phase error amplifier is 10.

1/2

OP270E

1/2

OP270E

VIN

R2 = R1

ωT

VO = (K1 + 1)VIN

ASSUME A1 AND A2 ARE MATCHED.

AO(s) =

Figure 41. Low Phase Error Amplifier

Figure 42 compares the phase error performance of the low

phase error amplifier with a conventional single op amp

amplifier and a cascaded two-stage amplifier. The low phase

error amplifier shows a much lower phase error, particularly for

frequencies where ω/βωT < 0.1. For example, a phase error of

−0.1° occurs at 0.002 ω/βωT for the single op amp amplifier, but

at 0.11 ω/βωT for the low phase error amplifier.

–7

–6

–5

–4

–3

–2

–1

FREQUENCY RATIO (1/βω)(ω/ωT)

0.001

0.005

0.01

0.1

LOW PHASE ERROR

AMPLIFIER

CASCADED

(TWO STAGES)

SINGLE OP AMP.

CONVENTIONAL DESIGN

0.05

0.5

Figure 42. Phase Error Comparison

FIVE-BAND, LOW NOISE, STEREO GRAPHIC

EQUALIZER

The graphic equalizer circuit shown in Figure 43 provides 15 dB

of boost or cut over a five-band range. Signal-to-noise ratio over

a 20 kHz bandwidth is better than 100 dB and referred to a 3 V

rms input. Larger inductors can be replaced by active inductors,

but consequently reduces the signal-to-noise ratio.

1/2

OP270E

1/2

OP270E

3.3kΩ

47kΩ

1kΩ

60Hz

TANTALUM

VOUT

VIN

R14

100Ω

R13

3.3kΩ

6.8µF

0.47µF

680Ω

200Hz

1kΩ

800Hz

1kΩ

3kHz

R10

1kΩ

10kHz

R12

1kΩ

TANTALUM

1µF

600mH

680Ω

0.22µF

180mH

680Ω

0.047µF

60mH

680Ω

0.022µF

10mH

R11

680Ω

Figure 43. Five-Band, Low Noise Graphic Equalizer

OP270GS Datasheet(PDF) 16 Page - Analog Devices

OP270GS Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL