it just matter of choice between P-N-P & N-P-N transistors. In early days (that time instrumenation standard +24Vdc was not implemented), P-N-P transisters were more cheaper than N-P-N transisters. So, it was decided to use P-N-P transistor and as we all know that for P-N-P transistors, a -ve bias voltage was required instead of +ve voltage. So, that’s why in vibration Monitoring system -ve voltage use and now a days it’s compatible for all systems.
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