. Electronic apparatus for biological research . all Speaker Input o Coupling transformer : Gilson, W0780/6V Output transformer : Gilson, W0781/6V Figure pair of output transistors in class B push-pull. The circuit is largely self- explanatory; the starred 56 kQ resistor introduces negative feedback round the output and penultimate stages. The maximum audio output power is 215 mW, and, with a 5 in. loudspeaker is sufficient to fill a large room with sound. The writer has used this amplifier, with an additional R-C coupled transistor at the front and the feedback removed, to demon- strate
. Electronic apparatus for biological research . all Speaker Input o Coupling transformer : Gilson, W0780/6V Output transformer : Gilson, W0781/6V Figure pair of output transistors in class B push-pull. The circuit is largely self- explanatory; the starred 56 kQ resistor introduces negative feedback round the output and penultimate stages. The maximum audio output power is 215 mW, and, with a 5 in. loudspeaker is sufficient to fill a large room with sound. The writer has used this amplifier, with an additional R-C coupled transistor at the front and the feedback removed, to demon- strate muscle action potentials via a concentric needle electrode. A transistorized myographic amplifier has been described by George*^. For detailed design procedure for audio amplifiers, Jones and Hilbourne may be consulted^. Units with output powers ranging from 20 mW to 20 W are described. Direct-coupled amplifiers As with valves, we distinguish between 'straightforward' amplifiers and carrier amphfiers. The phototransistorized and transistorized galvano- meter amplifier remains to be announced. 'Straightforward'' amplifiers—The sensitivity of meters may be improved by the addition of a single transistor in a simple circuit such as Figure , 696
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