Canberra 814 / Ortec 4890

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Garrett Young
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Joined: Sat May 27, 2006 1:51 am
Real name: Garrett Young

Canberra 814 / Ortec 4890

Post by Garrett Young » Sat Mar 04, 2017 8:49 pm

Both of these NIMs are combination preamp/amp/disc and use BJTs for the charge sensitive preamp. The specs indicate that these have poor mV/coulomb gain factors (3.3mV/C) compared with JFET based preamps (200mV/C). I'm wondering if anyone is certain about the vulnerability of the BJT based preamp to rapid changes in bias voltage. I know that JFET based preamps can be damaged by sudden application or removal of bias voltage but my general inclination is that a BJT would be more robust. I want to use a 814 or 4890 with a fixed HV supply that only has on/off control but of course don't want to damage the NIM. Thoughts?
- Garrett

User avatar
Garrett Young
Posts: 123
Joined: Sat May 27, 2006 1:51 am
Real name: Garrett Young

Re: Canberra 814 / Ortec 4890

Post by Garrett Young » Mon Mar 06, 2017 1:21 am

Update:

The Ortec 4890 has steering diodes on the input that should protect the BJT from sudden application/removal of voltage. Probably not best practice to abruptly change the bias voltage but it's nice to know some protection is present. I might add low leakage diodes (FJH1100) to my JFET preamp once I discover the root cause of the repeated failure.
- Garrett

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