that2155 ETC-unknow, that2155 Datasheet - Page 5

• Manufacturer Part Number: that2155
• Description: Voltage-controlled Amplifiers
• Manufacturer: ETC-unknow

When pin 3 is at +122 mV, the output current (Q4) is
10 times (20 dB) less than the input current. At
–122 mV, the output current is 10 times (20 dB)
greater than the input current. Another way of ex-
pressing this relationship is:
Negative Input Currents
Q2/D2 to mirror the lower-half-core behavior. Pin 2 is
normally at or very near ground (see the section below
on Symmetry Adjustment for more detail), so the same
gain scaling applied to the base of Q3 is applied to the
base of Q2. The polarity (positive/negative, in dB) of
the gain is the same for the top pair versus the bottom
pair of the four “core” transistors because their sexes
(NPN/PNP) are inverted in the top versus the bottom,
while the bases are cross-connected between the input
(left) half and the output (right) half of each pair.
tent from unit to unit, since it derives from the physics
of semiconductors. Figure 7 shows actual data from a
typical 2150 Series VCA, taken at 25˚C.
Core Bias Currents
established by the Voltage Bias Generator shown in
Figure 6. This current acts like crossover bias in the
output stage of a complementary class AB power am-
plifier, smoothing the transition between turning on
the top (PNP) pair and the bottom (NPN) pair of transis-
tors in the core. This lowers distortion greatly at some
cost to noise performance, as the current noise of the
core transistors (which run at approximately 20 A) is
the dominant noise source in the 2150 Series VCAs.
Figure 7. Gain Versus Control Voltage (Pin 3) at 25˚C
Rev. 10/25/96
A
Gain
For negative input currents, Q1/D1 operate with
The resulting control over gain is extremely consis-
A quiescent bias current in the core transistors is
When pin 3 is at O V, the current ratio is unity.
V
10
0.0061
0.122
E C
E
.
C
, where Gain is the gain in decibels.
THAT Corporation; 734 Forest Street; Marlborough, Massachusetts 01752; USA
Tel: [phone]; Fax: [phone]; Web: http://www.thatcorp.com
Transistor Matching
Q1 to Q3, and from Q2 to Q4) so long as there is good
matching between all four compound transistors (tran-
sistors plus diodes). Mismatches will cause a dc output
current to flow in pin 8, which will ultimately manifest
itself as a dc offset voltage. Static offsets are of little
consequence in most audio applications, but any mis-
match-caused dc output current will be modulated by
gain commands, and may become audible as “thumps”
if large, fast gain changes are commanded in the pres-
ence of significant mismatches.
top half of the gain cell is perfectly matched, while the
bottom half is slightly off, then the gain commanded by
the voltage at pin 3 will affect the two halves of the core
differently. Since positive and negative halves of ac
input signals are handled by separate parts of the core,
this gives rise to even-order distortion products.
Symmetry Adjustment
relatively good matching between the paired transis-
tors, but even small V
ceptable asymmetries in the output. For this reason,
the bases of Q1 and Q4 are brought out separately to
pin 2 and pin 4, respectively. This allows a small static
voltage differential to be applied to the two bases. The
applied voltage must be set to equal the sum of the V
mismatches around the core (which varies from sample
to sample). Figure 3 (Page 3) includes a typical circuit
to apply this symmetry voltage. R
even-order harmonic distortion, and is usually ad-
justed for minimum THD at the output. Figure 8 plots
THD vs. the voltage between pins 2 and 4 (the two E
ports) for various gain settings of a typical part.
Opposite Polarity Control
Q1 and Q4 can also be used as an additional control
The bias current flows downwards in the core (from
Transistor matching also affects distortion. If the
The monolithic construction of the devices assures
As may be seen from the mathematics, the bases of
Figure 8. Typical THD Versus Symmetry Voltage
BE
mismatches can cause unac-
SYM
controls primarily
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BE
C+