THAT2151 Datasheet, PDF (10/10 Pages) List of Unclassifed Manufacturers – IC Voltage-Controlled Amplifiers

Page 10
voltage-control circuitry. The 2150 Series VCAs act like
double-balanced multipliers: when no signal is present
at the signal input, noise at the control input is re-
jected. So, when measuring noise (in the absence of
signal — as most everyone does), even very noisy con-
trol circuitry often goes unnoticed. However, noise at
the control port of these parts will cause noise modula-
tion of the signal. This can become significant if care
is not taken to drive the control ports with quiet sig-
nals.
The 2150 Series VCAs have a small amount of in-
herent noise modulation because of its class AB bias-
ing scheme, where the shot noise in the core
transistors reaches a minimum with no signal, and in-
creases with the square root of the instantaneous sig-
nal current. However, in an optimum circuit, the noise
floor rises only to -94 dBV with a 50 µA signal at unity
gain — 4 dB of noise modulation. By contrast, if a
unity-gain connected, inverting 5534 opamp is used to
directly drive the control port, the noise floor will rise
to 92 dBV — 6 dB of noise modulation.
2150 Series IC VCAs
To avoid excessive noise, one must take care to use
quiet electronics throughout the control-voltage cir-
cuitry. One useful technique is to process control volt-
ages at a multiple of the eventual control constant
(e.g., 61 mV/dB — ten times higher than the VCA re-
quires), and then attenuate the control signal just be-
fore the final drive amplifier. With careful attention to
impedance levels, relatively noisy op amps may be
used for all but the final stage.
Closing Thoughts
The design and application of Voltage-Controlled
Amplifiers has traditionally been partly black art, in-
volving as much magic as science. We hope that the
foregoing discussion will help to de-mystify the subject.
THAT Corporation welcomes comments, questions
and suggestions regarding these devices, their design
and application. Please feel free to contact us with your
thoughts.
INPUT
10u
+
-
1k
20k
+15V
7
V+
3
2150
2150
S_Se_eri_re_ies_s
V^VC^CA^A
1
Ec-
OUT
-IN
Ec+
Ec+
GND
4
8
V-
2
6
5
51
5.1k
1k
-15V
Ec+
47p
20k
-
LF351
+
OUTPUT
+15V
Rsym
30¹02⁵k4^00k(2k155)
390k (2150A)
470k (2151)
50k
SYM
ADJ
-15V
Figure 15. Using Both Control Ports (Differential Drive)
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