SA2159 Datasheet, PDF (6/11 Pages) Silan Microelectronics Joint-stock – VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL

SA2159
Figure 4. Using Both Control Ports Pin 2 and Pin4
Control port source impedance
The control ports (pin 2 through 4) are connected directly to the bases of the logging and /or antilogging
transistors. Since the collector currents of transistors are signal-related, the base currents will also be signal-
related. If the source impedance of the control voltage is large, the signal-related base currents will cause signal-
related voltage to appear at the control ports, which will affect precise logging and antilogging, in turn causing
distortion. At high signal levels, even 100Ω can spoil the good performance of these parts. Due to signal current
based on a transistor β of approximately 300(NPN) or 100(PNP), the distortion caused by a specific and non-zero
source impedance by determining the base voltage modulation can be estimated.
In SA2159 VCA design, the source impedance of pin 2 and pin 3 are zero, while the source impedance of pin 4
is 50Ω because pin 4 is intended for connection to the symmetry control.
DC input signals
Any dc currents in the feedback loop of the internal op amp will show up as dc terms in the output signal, and
will be modulated by gain commands. The bias current compensation circuit inside the chip offers the input bias
current to the input differential amplifier. Ac input coupling is strongly recommended to prevent an input dc
current supplied from external application circuit of the VCA.
Figure 5. DC offset vs. gain, after symmetry adjustment
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0 2005.04.22
Page 6 of 11