Datasheet
TL05x, TL05xA
ENHANCED-JFET LOW-OFFSET
OPERATIONAL AMPLIFIERS
SLOS178A – FEBRUARY 1997 - REVISED FEBRUARY 2003
46
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, 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-kΩ 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
V
A
V
B
20
In V
A
– V
B
In (10)
XdB 8.686 In V
A
– In V
B
V
BE(Q1)
kT
q
In
V
A
R I
S
V
BE(Q2)
kT
q
In
V
B
R I
S
V
BE
V
BE(Q1)
–V
BE(Q2)
kT
q
In V
A
– In V
B
XdB
8.686
kT q
V
BE(Q1)
–V
BE(Q2)
336 V
BE(Q1)
–V
BE(Q2)
at 25°C
where
k 1.38 10
–23
,q 1.602 10
–19
, and T is Kelvin temperature
This gives a resolution of 1 V/dB. Therefore, the gain of the instrumentation amplifier is set at 33.6 to obtain
100 mV/dB.