Transimpedance AC coupled on T network

Olivier Monnom

Other Parts Discussed in Thread: OPA847, LMH6702, OPA820, LMH6609, OPA656

Hello,

I've designed a transimpedance amplifier for a photodiode (Cj=18pf). The circuit has two gains selectable by conducting or not the BJT Q1 (see schematic below). Unfortunately, I was not able to choose the low noise OPA847 since it's not stable at low gain. I had to choose the CFA LMH6702MF. With this one, everything works fine but the noise is higher of course.

My question: is there a component unity gain stable, less noisy than the LMH6702MF that can do the same work?

The BW of the circuit is about 40[MHz] at high gain (equiv. feedback resistor = 2500[ohm]).

Regards,

Olivier

over 11 years ago

0 Hooman Hashemi over 11 years ago

TI__Mastermind 22825 points

Hello Olivier,

I suspect that your noise is dominated by the inverting input noise current (18.5pA/RtHz for the LMH6702 vs. 2.5pA/RtHz for the OPA847) flowing through your 2.5kohm eq. feedback resistance.

Here is a list of devices with low input noise current (<2pA/RtHz), at least 800MHz of GBW which are unity gain stable:

OPA820 (1.6pA/RtHz)

LMH6609 (1.6pA/RtHz)

OPA656 is lower speed (500MHz), but has the lowest noise current (FET input) with 6.1nV/RtHz voltage noise.

Regards,

Hooman

0 Olivier Monnom over 11 years ago in reply to Hooman Hashemi

Prodigy 70 points

Thank you Hooman, I'll try these.

Regards,

Olivier

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Transimpedance AC coupled on T network