of Microprocessors

BASIC ANALOG MODULES
209
22 pF
RI
SIGNAL
50 k
INPUT SIG.IN +
-IOTO+IOV
R2
50kll
+15 V
1/2 SSM2020
",,°1
OFFSET
TRIM
-15V
10 Mil
50 kO
SIG.IN -
OUT
+IOV ref EXP. EXP. EXP.
CONY. EXP. CONY. CONY. CONY.
COlL. EMITTERS BASE- BASE+
II kll
100 kO
10 Mil
I ref
OUTPUT
-10 TO
+IOV
liNEAR
CONTROL o----'VII'v----1
INPUT Rl
10 kll
RE
10 kll
RE
EXPONENTIAL IB kll
CONTROL o----'VI/'v---------------J
INPUT
Linear = 0 Exponential = 0, Ie = 0.1 /-LA
Gain = - 80 dB
Linear = + 10, Exponential = 0, Ie = 1.0 mA,
Gain = 0 dB
Linear = 0, Exponential = + 10, Ie = 1:0 mA,
Gain = 0 dB 8 dBIV
Fig. 6-20. Practical VeA using the SSM 2020
although not precisely zero, is sufficient to audibly silence the unit. Raising
the linear control input to + 10 V while maintaining the exponential input at
zero will increase the control current to I rnA and provide unity gain. The
expression for gain, therefore, is G = O.IEI, where G is the gain and EI is
the voltage applied to the linear input.
With the values shown, the exponential input has a sensitivity of 8
dEN. Thus, + 10 at this input would raise the gain 80 dE above the zero
input value and give unity gain. RE can be adjusted to provide exactly 8
dEN, which incidently is quite close to the increase in amplitude necessary
for a doubling of perceived loudness. The nonstandard impedances of the
control inputs may be easily corrected with additional" op-amp buffers. Note
that a TL-82 dual FET amplifier was used for A I and A2. The high slew rate
is necessary at high audio frequencies and the low bias current is needed for
predictable operation with the extremely low reference current used in the
exponential converter transistors.