of Microprocessors

238 MUSICAL ApPLICATIONS OF MICROPROCESSORS
CONTROL o--------'lNv--~
UPPER TRANSISTOR
IS ON IF MORE
POSITIVE THAN Vref
LOWER TRANSISTOR IS ON
IF CONTROL VOLTAGE
IS NEGATIVE
(A)
ON
2R
CURRENT
OUTPUT
NODE
(8)
Fig.7-12. Bipolar transistor switches for DACs. (A) Voltage switch. (B) Current
switch.
Figure 7-12 shows two possible configurations of transistor switches in
DACs. The first circuit is a double-throw voltage switch that switches its
output between Vrif and ground with very little error. With the simple
resistive-voltage-to-control-current converter shown, the control logic swing
for this switch must be from a negative voltage to a positive voltage greater
than Vrif. An inherently bipolar (+ and - output voltages) DAC may be
made if the bottom transistor is connected to - Vre/instead ofground and the
control swing is increased. Note that the Vre./S can change over a wide range
as long as the drive is sufficiently greater than either and that they do not
cross (Vre/ on npn more negative than Vre/ on pnp). Total voltages (Vre/+
- Vrej-) up to the collector breakdown can be accommodated with this circuit.
The second circuit is often used in monolithic DACs. Weighted resistors
are used to establish binary-weighted currents. The current either flows
through the common-base-connected switching transistor into the output
line or through the steering diode into the switch driver. If the current gain
of the transistors is high, very little signal current is lost through the base.
Note that this is strictly a current output DAC and that the current must be
sunk at ground or negative potentials. The temperature dependence of out-