SA2159 Datasheet, PDF (4/11 Pages) Silan Microelectronics Joint-stock – VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL

SA2159
Positive input currents
For positive input currents (IIN defined as flowing into pin 1), the op amp turns Q1 and D1 off, while
simultaneously turns Q3 and D3 on. Thus, the input signal current is forced to flow through Q3 and D3.
Logging & Antilogging
We know the basic properties Vbe of the transistor and the collector current Ic as:
Vbe = VT ln (Ic/Is )
It is assumed that D3/Q3 and Q4/D4 are matched, we therefore can conclude:
V3 = EC- - 2VT In (Ic3 / Is)
Where V3 is the voltage at the junction of D3 and D4, VT is the thermal voltage, kT/q, Ic3 is the current in the
collector of Q3, and IS is the reverse-saturation current of Q3.
In typical applications (see typical applications diagram.), pin 4 is normally at or very near ground. Pin 8 is
connected to a virtual ground. In this way, the voltage at the cathodes of D3 and D4 will cause an exponentially-
related current to flow in D4 and Q4, and out through pin 8. Mathematically:
V3 = EC+ - 2VT In (Ic4 / Is )
Exponential gain control
It can be derived from said two equations:
V3 = EC- - 2VT In (Ic3 / Is) = EC+ - 2VT In (Ic4 / Is)
EC+ - EC- = 2VT In (Ic4 / Is) - 2VT In (Ic3 / Is) = 2VT In (Ic4 / Ic3)
Rearranging terms,
Ic4 = ic3 e (EC+ - EC-) / 2VT
Av = Ic4 / Ic3 = e (EC+ - EC-) / 2VT
Where AV is the current gain, which is exponential with the difference in the voltage EC+ and EC-
For pin 4 at or very near ground, at room temperature, and converting to a base of 10 for the exponential, this
reduces to:
Av = 10 - EC- / 0.118
Another way of expressing this relationship is:
Gain = 20 log Av = - EC- / 0.0059, where the unit of Gain is dB.
dB Gain
+30
0
-30
-60
-90
-540
-360
-180
-EC
mV
0
+180
Figure 2. Gain versus control voltage at 25°C
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0 2005.04.22
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