SA2159

HANGZHOU SILAN MICROELECTRONICS CO.,LTD

Rev: 1.0

2005.04.22

Http: www.silan.com.cn

Page 6 of 11

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 100

Ω 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 50

Ω 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