THAT2252 Datasheet, PDF (3/10 Pages) List of Unclassifed Manufacturers – IC RMS-Level Detector

600032 Rev 01
Page 3
Theory of Operation
The THAT 2252 RMS-Level Detector is designed
for high performance in audio-frequency applica-
tions requiring logarithmic output, rms response,
and wide dynamic range. The parts compute rms
level by rectifying input current signals, converting
the resulting current waveform to a logarithmic
voltage, and applying this voltage to a log-domain
filter.
Current Rectification
Figure 3 presents a simplified internal circuit dia-
gram of the 2252. The input signal current, I_in,
flows in pin 1, the input pin. OA1 drives the base
of Q3 and the emitter of Q1 (through V1) to main-
tain pin 1 at virtual ground potential. A negative
input current (flowing out of pin 1) will tend to
drive the inverting input of OA1 negative, driving
OA1’s output positive, turning on Q3. V1 is de-
signed to cut off Q1 while Q3 is on. Therefore, neg-
ative input currents are forced to flow through the
collector-emitter of Q3.
Positive Iin will drive OA1’s output negative, cutting
off Q3 and turning on diode-connected transistor
Q1. Positive input current is thereby forced to flow
through the collector-emitter of Q1. Pin 4 is nor-
mally connected through a 20 W resistor to ground
(see Figure 4, Typical Application Circuit, Page 4,
and Symmetry Adjustment, Page 6), so the
base-emitter potential of Q2 is the same as that of
Q1. Therefore, the current in the collector of Q2
(IC2) will mirror that in the collector of Q1 (IC1),
which equals the positive input current.
Since the input impedance of OA2 is high, the cur-
rent in the emitter of Q4 (I_C4), is the sum of the
currents IC2 and IC3. The mirror action of Q1/Q2
reverses the positive input currents so that they
add to the negative input currents in Q4. The cur-
rent in Q4, therefore, is equal to the absolute value
of the input current.
Mathematically,
{ { ^IC3 ⁼
- I_in , I_in
0, I_in >
£
0
0_,
and
I_C1 = I_C2 =
0, I_{in £}
I_in , I_in
0
>0
.
But, I_C4 = I_C3 + I_C2 = I_C3 + I_C1
{⁼
- I_in , I_in < 0
I_in , I_in > 0
= I_in .
See Figure 3 for definitions of these currents.
Logging Action
OA2, together with Q4 and Q5, forms a log ampli-
fier. By using two diode-connected transistors in
the feedback loop of OA2, the 2252 produces a
voltage proportional to twice the log of IC4 at the
output of OA2. This voltage, Vlog, is therefore pro-
portional to the log of the square of the input cur-
rent, plus a bias voltage (V2).
Mathematically,
V_log
=
2
V_T1n^{æèçç}
I_C4
I_S
öø÷ ₊
V₂
= 2 V_T 1n (I_C4 ) - 2 V_T 1n (I_S ) + V₂
= V_T 1n (I_C4 )² - 2 V_T 1n (I_S ) + V₂
= V_T 1n I_in ² - 2 V_T 1n (I_S ) + V₂,
Where
V_T
is
the
thermal
voltage,
kT
q
,
and
I_S
is
the
reverse-saturation current of Q4 and Q5 (assumed
to be the same in each).
1
Iin
-
+
OA1
-
+
+
OA2
-V2
IC3
Q3
Q4
Q5
IC1
20
IC2 _IC4
+
V1
Q1 - Q2
4
Vlog
V3
-
-
Q6
+
OA3
IT
6 V6
7
Vout
Figure 3. Simplified Internal Schematic
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