TL05 datasheet(61/65 Pages) TI

61 / 65 page

TL05x, TL05xA, TL05xY

ENHANCED-JFET LOW-OFFSET

OPERATIONAL AMPLIFIERS

SLOS178 – FEBRUARY 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

voltage-ratio-to-dB converter

The application in Figure 90 measures the amplitude ratio of two signals and then converts the ratio to decibels

(see Figure 91). The output voltage provides a resolution of 100 mV/dB. The two inputs can be either dc or

sinusoidal ac signals. When using ac signals, both signals should be the same frequency or output glitches will

occur. For measuring two input signals of different frequencies, extra filtering should be added after the

rectifiers.

The circuit contains three low-offset TL05xA devices. Two of these devices provide the rectification and

logarithmic conversion of the inputs. The third TL05xA forms an instrumentation amplifier. The stage performing

the logarithmic conversion also requires two well-matched npn transistors.

The input signal first passes through a high impedance unity-gain buffer U1A (U2A). Then U1B (U2B) rectifies

the input signal at a gain of 0.5, and U1C (U2C) provides a noninverting gain of 2 so that the system gain is still

one. U1D (U2D), R6 (R13), and Q1 (Q2) perform the logarithmic conversion of the rectified input signal. The

instrumentation amplifier formed by U3A, U3B, U3D scales the difference of the two logarithmic voltages by a

gain of 33.6. As a result, the output voltage equals 100 mV/dB. The 1-k

Ω potentiometer on the input of U3C

calibrates the zero dB reference level. The following equations are used to derive the relationship between the

input voltage ratio expressed in decibels and the output voltage.

XdB

+ 20 log

+ 20

In V

– V

In (10)

XdB

+ 8.686 In V

–In V

BE(Q1) +

q In

BE(Q2) +

q In

BE +

BE(Q1)

–V

BE(Q2) +

In V

–In V

XdB

+ 8.686

kT q

BE(Q1)

–V

BE(Q2) +

336

BE(Q1)

–V

BE(Q2)

at 25

°C

where

+ 1.38

10–23,q

+ 1.602

10–19, and T is in kelvins.

This would give a resolution of 1 V/dB. Therefore, the gain of the instrumentation amplifier is set at 33.6 to obtain

100 mV/dB.

TL05 Datasheet(PDF) 61 Page - Texas Instruments

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