OPA604AP datasheet(12/16 Pages) TI

12 / 16 page

OPA604

SBOS019A

www.ti.com

(mA)

log

1234

2fO

3fO

4fO

5fO

BE = 1kHz + DC Bias

BE (V)

0.65

FFT

Frequency (kHz)

–I

(mA)

log

1234

2fO

3fO

4fO

5fO

GS = 1kHz + DC Bias

GS (V)

FFT

Frequency (kHz)

VGS

VBE

FIGURE 10. I-V and Spectral Response of NPN and

JFET.

THE OPA604 DESIGN

The OPA604 uses FETs throughout the signal path,

including the input stage, input-stage load, and the

important phase-splitting section of the output stage.

Bipolar transistors are used where their attributes,

such as current capability are important, and where

their transfer characteristics have minimal impact.

The topology consists of a single folded-cascode gain

stage followed by a unity-gain output stage. Differen-

tial input transistors J1 and J2 are special large-geom-

etry, P-channel JFETs. Input stage current is a rela-

tively high 800

µA, providing high transconductance

and reducing voltage noise. Laser trimming of stage

currents and careful attention to symmetry yields a

nearly symmetrical slew rate of

±25V/µs.

The JFET input stage holds input bias current to

approximately 50pA or roughly 3000 times lower than

common bipolar-input audio op amps. This dramati-

cally reduces noise with high-impedance circuitry.

The drains of J1 and J2 are cascoded by Q1 and Q2,

driving the input stage loads, FETs J3 and J4. Distor-

tion reduction circuitry (patented) linearizes the open-

loop response and increases voltage gain. The 20MHz

bandwidth of the OPA604 further reduces distortion

through the user-connected feedback loop.

The output stage consists of a JFET phase-splitter

loaded into high speed all-NPN output drivers. Output

transistors are biased by a special circuit to prevent

cutoff, even with full output swing into 600

Ω loads.

The following discussion is provided, recognizing that

not all measured performance behavior explains or

correlates with listening tests by audio experts. The

design of the OPA604 included consideration of both

objective performance measurements, as well as an

awareness of widely held theory on the success and

failure of previous op amp designs.

SOUND QUALITY

The sound quality of an op amp is often the crucial

selection criteria—even when a data sheet claims

exceptional distortion performance. By its nature, sound

quality is subjective. Furthermore, results of listening

tests can vary depending on application and circuit

configuration. Even experienced listeners in controlled

tests often reach different conclusions.

Many audio experts believe that the sound quality of a

high performance FET op amp is superior to that of

bipolar op amps. A possible reason for this is that

bipolar designs generate greater odd-order harmonics

than FETs. To the human ear, odd-order harmonics

have long been identified as sounding more unpleas-

ant than even-order harmonics. FETs, like vacuum

tubes, have a square-law I-V transfer function which is

more linear than the exponential transfer function of a

bipolar transistor. As a direct result of this square-law

characteristic, FETs produce predominantly even-or-

der harmonics. Figure 10 shows the transfer function of

a bipolar transistor and FET. Fourier transformation of

both transfer functions reveals the lower odd-order

harmonics of the FET amplifier stage.

SOUND QUALITY

Distortion

Rejection

Circuitry

(+)

(–)

Output

Stage

Ω

500

Ω

500

Ω

800µA

200µA

10k

Ω

10k

Ω

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