of Microprocessors

202 MUSICAL ApPLICATIONS OF MICROPROCESSORS
V+
DIFFERENTiAL(+
INPUT
VOLTAGE _
IC V-
SYMBOL
V+
Ii
+
lout = 11-12
OUTPUT
IC
]2
V-
SIMPLIFIED SCHEMATIC
Fig. 6-14. Operational transconductance amplifier
posed entirely of transistors. A current mirror is essentially a commonemitter
transistor stage with a current gain of unity to close tolerances. A
current mirror is a very commonly used device in linear ICs but to the
author's knowledge has never been packaged and sold as a discrete device.
Like transistors, current mirrors may be either npn (arrows entering) or pnp
(arrows leaving). Since the current gain is unity, the collector current is
exactly equal to the base current.
The controlled-gain cell of the 3080 is a two-transistor differential
amplifier. Tracing the simplified 3080 schematic will reveal that the differential
gain cell emitter current is equal to Ie by virtue of CMl. Ql's
collector current, 11, gives a positive contribution to the output current via
CM3, while Q2's collector current 12, is inverted by CM4 and gives a
negative output contribution. The net output current is therefore 11-12,
which means that the current mirrors have performed a differential to singleended
current conversion. Since the output is connected only to transistor