OPAX170 datasheet(28/60 Pages) TI2

28 / 60 page

VCC

VEE

VCC

VEE

VCC

LSK489

3.9 k

15 V

OPA172

VOUT

1.13 k

11.5

MMBT4401

300

+ +

27.4 k

OPA172, OPA2172, OPA4172

SBOS618I – DECEMBER 2013 – REVISED MAY 2018

www.ti.com

Product Folder Links: OPA172 OPA2172 OPA4172

Submit Documentation Feedback

9.2.3 JFET-Input Low-Noise Amplifier

Figure 51 shows a low-noise composite amplifier built by adding a low noise JFET pair (Q1 and Q2) as an input

preamplifier for the OPA172. Transistors Q3 and Q4 form a 2-mA current sink that biases each JFET with 1 mA

of drain current. Using 3.9-kΩ drain resistors produces a gain of approximately 10 in the input amplifier, making

the extremely-low, broadband-noise spectral density of the JFET pair, Q1 and Q2, the dominant noise source of

the amplifier. The output impedance of the input differential amplifier is large enough that a FET-input amplifier

such as the OPA172 provides superior noise performance over bipolar-input amplifiers.

The gain of the composite amplifier is given by Equation 3:

AV = (1 + R3 / R4)

(3)

The resistances shown are standard 1% resistor values that produce a gain of approximately 100 (99.26) with

68° of phase margin. Gains less than 10 may require additional compensation methods to provide stability.

Select low resistor values to minimize the resistor thermal noise contribution to the total output noise.

Figure 51. JFET-Input Low-Noise Amplifier

10 Power-Supply Recommendations

The OPA172 is specified for operation from 4.5 V to 36 V (±2.25 V to ±18 V); many specifications apply from

–40°C to +125°C. Parameters that can exhibit significant variance with regard to operating voltage or

temperature are presented in the Typical Characteristics.

CAUTION

Supply voltages larger than 40 V can permanently damage the device; see the

Absolute Maximum Ratings.

Place 0.1-μF bypass capacitors close to the power-supply terminals to reduce errors coupling in from noisy or

high-impedance power supplies. For more detailed information on bypass capacitor placement, see the Layout

section.

OPAX170 Datasheet(PDF) 28 Page - Texas Instruments

OPAX170 Datasheet(HTML) 28 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Datasheet Download

Link URL