of Microprocessors

206 MUSICAL ApPLICATIONS OF MICROPROCESSORS
volved but a few observations can be made with little effort. D5 and D6 are
protective diodes that normally do not conduct; thus, they can be ignored.
Resistors R2 and R3 in conjunction with the supply voltages act like current
sources and bias the diode bridge. Since the diodes are all matched, equal
currents flow in each one and in turn give equal dynamic impedances of
about 100 ohms each. The dynamic impedance of the bridge acts with R 1 to
form a voltage divider for the signal voltage. With zero signal input, the
impedance of the overall bridge is also 100 ohms so the attenuation is about
330, one-third that used in uncompensated circuits.
For positive input voltages, the current through D1 decreases and
increases through D2. Because of the constant bias current, the current
through 1)4 must therefore decrease, while that through D3 increases. Since
the dynamic impedance of the diodes is inversely proportional to their currents,
the bridge impedance will change also. For small signals, the impedance
decreases are closely matched by the increases and the bridge impedance
remains constant. As the signal voltage rises, the diodes with increased
impedance dominate (since they are in series with a decreased impedance
diode) and the bridge impedance rises. The input voltage tolhe 3080 is
therefore boosted to counteract its tendency to flatten the waveform peaks.
The circuit runs away when the signal current exceeds the bridge bias current
so D5 and D6, which are in the diode array Ie anyway, prevent any possible
damage to the 3080.
The improvement offered by this circuit is impressive. With a 50-mV
peak voltage into the 3080, the harmonic distortion is only 0.25%. The
increased signal amplitude also improves the signal-to-noise ratio by nearly
14 dB to a total of 80 dB. Resistors R2 and R3 should be carefully matched
as well as the 15-V power supplies to prevent even order harmonic distortion.
Vbio, V+
I3' 14
01 02
II 12
V-
Fig. 6-18. Gilbert multiplier