. Electronic apparatus for biological research . HT+ HT+ Figure HT+ Figure the two sides of the amplifier are perfectly similar in all respects. In a practical circuit with real-life components this cannot be the case. What happens is that an in-phase input produces at the first stage anodes potential changes which are in the same direction but not quite equal—that is, an in- phase output with a small anti-phase component. This anti-phase component will suffer amphfication by the second stage, so that the practical rejection ratio is always inferior to the theoretical. The situatio
. Electronic apparatus for biological research . HT+ HT+ Figure HT+ Figure the two sides of the amplifier are perfectly similar in all respects. In a practical circuit with real-life components this cannot be the case. What happens is that an in-phase input produces at the first stage anodes potential changes which are in the same direction but not quite equal—that is, an in- phase output with a small anti-phase component. This anti-phase component will suffer amphfication by the second stage, so that the practical rejection ratio is always inferior to the theoretical. The situation can be mitigated by introducing special gain-balancing arrangements. The principal offenders in causing inequaUty between the two sides are the valves, whose g^ values are subject to rather wide variations both in manufacture and in aging. The object of gain balancing networks is to correct for differences in the performance of the two valves comprising each stage; hence theoretically each stage requires a balancing adjustment. In practice, amplifiers must not have too many knobs; it is clear that since lack of gain balance in the first stage—when handling an in-phase input—leads to amplification of the concomitant anti-phase component by all the succeeding stages, the first stage is the one which requires balancing most, and in practice the provision of first-stage gain-balancing only is usual. In the coupled differential voltage amplifier no HT negative supply with respect to earth is necessary, and the stage has the form of Figure MAKING R^ A PENTODE A good ratio of anti-phase to in-phase gain with a differential stage depends on having i?^ large. Unfortunately, if-^^ is an ordinary resistance and is to be large, the negative supply must be very negative indeed if the valve currents 184
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