TL05X datasheet(44/68 Pages) TI

44 / 68 page

TL05x, TL05xA

ENHANCED-JFET LOW-OFFSET

OPERATIONAL AMPLIFIERS

SLOS178A – FEBRUARY 1997 - REVISED FEBRUARY 2003

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

high input impedance log amplifier

The low input offset voltage and high input impedance of the TL05xA creates a precision log amplifier (see

Figure 87). IC1 is a 2.5-V, low-current precision, shunt regulator. Transistors Q1 and Q2 must be a closely

matched npn pair. For best performance over temperature, R4 should be a metal-film resistor with a low

temperature coefficient.

In this circuit, U1A serves as a high-impedance unity-gain buffer. Amplifier U1B converts the input voltage to

a current through R1 and Q1. Amplifier U1C, IC1, and R4 form a 1-

µA temperature-stable current source that

sets the base-emitter voltage of Q2. U1D amplifies the difference between the base-emitter voltage of Q1 and

Q2 (see Figure 88). The output voltage is given by the following equation:

O +

– 1

) R6

10–6

where k

+ 1.38

10–23,q

+ 1.602

10–19,

and T is Kelvin temperature

U1A

U1B

U1C

U1D

10 k

Ω

2N2484

15 V

10 k

Ω

2.5 M

Ω

150 pF

IC1

270 k

Ω

–15V

10 k

Ω

10 k

Ω

(see equation above)

NOTE A: U1A through U1D = TL05xA. IC1 = LM385, LT1004, or LT1009 voltage reference

Figure 87. Log Amplifier

2345

f – Frequency – Hz

–0.4

–0.35

–0.3

–0.25

–0.2

–0.15

–0.1

Figure 88. Output Voltage vs Input Voltage for Log Amplifier

TL05X Datasheet(PDF) 44 Page - Texas Instruments

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