TL441-EP_14 Datasheet, PDF (8/19 Pages) Texas Instruments – LOGARITHMIC AMPLIFIER

TL441ĆEP
LOGARITHMIC AMPLIFIER
SGLS268 − OCTOBER 2004
APPLICATION INFORMATION
Although designed for high-performance applications such as infrared detection, this device has a wide range
of applications in data compression and analog computation.
Basic Logarithmic Function
Functional Block Diagram
The basic logarithmic response is derived from
the exponential current-voltage relationship of
collector current and base-emitter voltage. This
relationship is given in the equation:
m • V_BE = In [(I_C + I_CES)/I_CES]
where:
I_C =
I_CES =
m=
collector current
collector current at V_BE = 0
q/kT (in V ^{− 1})
V_BE = base-emitter voltage
The differential input amplifier allows dual-polarity
inputs, is self-compensating for temperature
variations, and is relatively insensitive to
common-mode noise.
INPUT
A1
CA2
INPUT
A2
CA2’
Log
Log
−15 dB
Log
Log
Log
Log
−15 dB
Log
Log
Σ
Σ
YY Z Z
Outputs
Figure 10
−15 dB
−15 dB
INPUT
B1
CB2
INPUT
B2
CB2’
Logarithmic Sections
As can be seen from the schematic, there are eight differential pairs. Each pair is a 15-dB log subsection, and
each input feeds two pairs, for a range of 30-dB per stage.
Four compensation points are available to allow slight variations in the gain (slope) of the two individual 15-dB
stages of input A2 and B2. By slightly changing the voltage on any of the compensation pins from their quiescent
values, the gain of that particular 15-dB stage can be adjusted to match the other 15-dB stage in the pair. The
compensation pins also can be used to match the transfer characteristics of input A2 to A1 or B2 to B1.
The log stages in each half of the circuit are summed by directly connecting their collectors together and
summing through a common-base output stage. The two sets of output collectors are used to give two log
outputs, Y and Y (or Z and Z), which are equal in amplitude, but opposite in polarity. This increases the versatility
of the device.
By proper choice of external connections, linear amplification, and linear attenuation many different applications
requiring logarithmic signal processing are possible.
Input Levels
The recommended input voltage range of any one stage is given as 0.01 V to 1 V. Input levels in excess of
1 V may result in a distorted output. When several log sections are summed together, the distorted area of one
section overlaps with the next section and the resulting distortion is insignificant. However, there is a limit to the
amount of overdrive that can be applied. As the input drive reaches ± 3.5 V, saturation occurs, clamping the
collector-summing line and severely distorting the output. Therefore, the signal to any input must be limited to
approximately ± 3 V to ensure a clean output.
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