Patent GB-1142894-A (GB)
Title: Direct coupled differential transistor amplifier with improved common mode performance
IPC: H03F3/45
Filed: 1966-09-15
Abstract: 1,142,894. Transistor amplifying circuits. BECKMAN INSTRUMENTS Inc. 15 Sept., 1966 [18 Oct., 1965], No. 41270/66. Heading H3T. A direct-coupled differential transistor amplifier has a plurality of stages including at least two differential voltage amplifying stages, and a common mode feedback path having a voltage gain substantially greater than one connected from a common mode output point in the last of the plurality of stages, at which point only the amplified common mode signal appears, back to the first stage. The invention is shown applied to an amplifier comprising a first differential stage, formed by transistors 104, 106 direct-coupled via emitter-followers 136, 138 respectively to a second differential stage comprising transistors 144, 146. The second stage drives a third differential stage 166, 168 via emitter-followers 158, 160 respectively, which in turn is direct-coupled to a fourth differential stage 182, 184. A complementary emitterfollower output stage comprising transistors 204, 206 is driven from the collector of transistor 184, while transistor 182 drives an emitter-follower 216. The emitter circuit of transistors 104, 106 comprises a resistance network 116, 118, 120, 122, 264 and shunt resistors 110 which may be selected to adjust the gain. The gain of transistors 166, 168 may also be varied, their emitter circuits comprising resistors 170, 172 together with selectable shunt resistors 174. Overall negative feedback is applied to the arrangement by means of resistors 124, 126 interconnecting the emitters of transistors 216 and 204, 206 respectively and 104, 106 respectively. Resistors 224, 226 establish a common mode point at their join 222 from which a common mode feedback signal is derived and amplified in an amplifier comprising emitter-coupled pair 228, 230. This is direct-coupled to a differential pair 246, 248 in which transistor 248 drives a complementary transistor 254 in series with transistor 246: an amplified feedback signal is taken from the collector of this transistor and is applied via resistors 120, 122 to the emitters of transistors 104, 106 in parallel. In addition, further rejection of common mode signal is obtained by making the supply potential of transistors 104, 106 follow the mean emitter potential, which is substantially equal to the common mode signal on their bases. A potential proportional to the mean emitter potential is derived from resistor 264, amplified in a Darlington pair comprising transistors 258, 260 and applied from the emitter of transistor 260 to the bottom of a chain of diodes 148, 266, 268, 270, 272 from the top of which the collector circuits of transistors 104, 106 are fed. Variable resistor 149 in series in this chain is utilized for balancing transistors 144, 146. Transistor 274 comprises a buffer for the current source which is made up of transistors 276, 278, resistors 280, 282 determining the collector voltage of transistor 278 and the current output being determined by the voltage across Zener diode 186 and resistors 283, 285, 287. The arrangement also includes means for producing an improved offset voltage temperature coefficient, which forms the subject-matter of Specification 1,142,893.