Multiple transistors sharing a common substrate are available in integrated-circuit packages called transistor arrays. Compared to medium- and large-scale integrated circuits, arrays contain relatively few devices, but they have the advantage that all or most device terminals are accessible at external pins. This accessibility allows the user to connect the components in a variety of ways and provides great flexibility in applications while retaining the integrated circuit’s advantages of compactness and rugged structure. Also, like integrated-circuit components, the common fabrication processes used to create the components on one substrate make it possible to
obtain devices with closely matched characteristics. Often 11 transistor array will be manufactured with certain internal device connections already in place. For example, Darlington pairs are made with collector and emitter-to-base interconnections already formed. The example of a transistor array containing a Darlington pair and two ‘indepC~lllcnt NPN transistors. A schematic diagram and manufacturer’s specification sheet for the CA301H/3018A is shown in Figure 11-23. Note that the specifications show that the I-In (de (3) values are matched to within :!::10% and the de values of
VH£are matched to within :!::2mV (for the CA3018A). Also note that both emitter terminals in (he Darlington pair (O, and 04) are accessible at external pins. This arrangement makes it possible to bias each transistor separately. The individual collector terminals arc also accessible. These can be tied together externally. or a resistor can he inserted in series with the collector of 04. as is sometimes done to reduce feedback from 04 to 03 when the Darlington pair is used in a commonemitter configuration.
RCA Application Note ICAN-5296 shows how the CA3018 can be used to construct a video (high-frequency) amplifier. A schematic diagram is shown in Figure 11-24. Note that 0\ and 02 form a cascode amplifier. with 02 in the commonemitter configuration and 0\ as the common-base stage. OJ and 04 are used in this example as a pair of cascaded emitter followers. The voltage gain of the amplifier is reported to be 37 dB over a midband range from 6 kHz to 11 MHz.
MULTISTAGE FET AMPLIFIERS
The snmc principles we applied to the analysis of is stage B.l’I’ amplifiers used to analyze multistage FET amplifiers. In Illany ways the analysis is simpler. because the very large gate-to-source resistance of a .lFET or MOSFET allows LIS to ignore some of the loading cl’lccts that had to he considered in B.lT designs . The next example illustrates this point.
Direct-Coupled FET Amplifiers
Direct-coupled field-effect transistors are widely used in linear integrated circuits. The next example illustrates a 3-stage. direct-coupled amplifier that uses complementary JFETs (N channel and P channel). similar in concept to the direct-coupled. complementary BJT amplifier discussed earlier. Note in particular how the P channel stage (02) is biased.
The JFETs in Figure 11-26 all have loss = 8 mA and iVpl = 2 V. Find the de values of the drain-to-ground voltage VD• the source-to-ground voltage Vs and the drainto source voltage V/ls• of each transistor. Solution. Note that QI and 02 are both common-source stages and 03 is a source follower.
The gate-to-ground voltage of Ot is determined by the voltage divider across the input:
O! is a P-channef lFET biased with its source side connected to the positive supply voltage and its drain side grounded. Writing Kirchlwfl”s voltage law from through R.I!o across the gate-to-source junction. and from the gate of to ground.