of Microprocessors

242 MUSICAL ApPLICATIONS OF MICROPROCESSORS
+15V
TO SWITCH
9-----0 CONTROL
INPUT
-15V
Fig. 7-14. FET switch driver
between whatever positive and negative supply voltages are connected. The
output is negative for a logic high input, which would drive N-channel FET
switches off. In operation, Ql performs as a level shifter by feeding the
approximately 3-mA logic input current through to the base of Q2. The
220-pF capacitor speeds up the turnoff ofQ2 (turnon of N-channel FETs) by
removing its stored base charge. As shown, the circuit will provide switching
times of around 200 nsee. Without Cl this deteriorates to around 1 f.Lsee.
Much faster operation (down to 20 nsec or less) can be obtained by changing
Cl to 100 pF, Q2 to a 2N3646, and reducing its collector load resistor to the
1-2.2K n range.
Recently, completely integrated analog switches have appeared on the
market. These "BIFET" devices accept normal logic levels and analog supply
voltages of ± 15 V and provide several NFET switching functions per package.
Each switch in a typi,cal array of four can handle ± lO-V analog signals
and switching time is about 0.5 f.Lsee. These are perfect for most applications
not requiring really high speed or exceptionally low on resistances.
Current-to-Voltage Conversion
Returning to DACs, it is often the case that the output amplifier limits
many performance parameters, particularly settling time. The purpose of the
amplifier, of course, is to isolate the DAC resistor network from load variations
so that accuracy is maintained. By far the simplest output buffer is
simply a voltage follower op-amp connected to a voltage output DAC network.
Although simple, it is relatively slow when general-purpose op-amps
are connected as voltage followers because of the heavy frequency compensation
required. Specialized voltage-follower amplifiers such as the LM310 are
so fast, however, that overall speed may be dominated by the DAC output
capacitance.
Nearly all commercial DACs are inherently current-output devices.
Many of these can actually generate small voltages across small load resistors
with no loss of linearity. A non-inverting amplifier with gain can amplify the