post amplifiers in optical receivers

[John Xu]

Hi,
There are two types of circuits applied for manitude control in optical receives, AGC(automatically gain-controlled amplifier) and LA(limiting amplifiers). Configued as closed-loop structure, the AGC has a loop time constants much larger than the main amplifier. For this, it is easy to understand.But for the LA, is is asserted that it is open-loop. It is also easy to understand if it has no dc offset controll loop. However, there is a dc feedback loop which is always consists of a large RC constants, why it can be seen as open loop?

Thanks in advance
John

 

[asia]

In my opinion, large RC contants is not influence the frequency response of LA
but the LA that I have studied has no DC feedback loop, can you post a circuit?then we can analyze it

 

[John Xu]

Hi, Asia,
Thanks for the reply.

DC offset feedback loop is ued to cancell the dc offset because of the large dDC gain of the LA. PLs. refer the attached paper, the introduction part, paragraph two. My question is from this. The LA indeed have a feedback loop. From which view, it can be seen as open loop?

"The large RC contants is not influence the frequency response of LA ", can you explain more clearly??

Thanks in advance
John