Connecting photodiode to TIA - Page 1
Forum section: electronics / beginners

author: elki
author_rank: Regular Contributor
timestamp: August 20, 2024, 05:03:14 pm
content: Hi all! What is the usual practice when connecting a photosensitive detector to a trans impedance amplifier? The BNC and SMA connections come with non negligible impedance, so I am wondering if there is some other standard that would allow to reduce the input impedance for the amplifier.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2024, 05:46:00 pm
content: What is the bandwidth of your application?At DC through audio frequencies the impedance of common small coaxial connectors is merely a shunt capacitance. Similarly, short coaxial cables are capacitive at low frequencies.Is your photodiode reverse-biased (with relatively low capacitance) or in zero-bias (photovoltaic) mode with higher capacitance and lower shunt resistance?At higher bandwidth, it is normal to run the photodiode with reverse bias into a 50 ohm termination rather than into a TIA.

author: tggzzz
author_rank: Super Contributor
timestamp: August 20, 2024, 06:26:27 pm
content: TIAs with capacitance at the input can have stability problems.The input cable's capacitance may be significant.

author: elki
author_rank: Regular Contributor
timestamp: August 20, 2024, 07:24:49 pm
content: The bandwidth is in the range of 100s of MHz. The photodiode is reverse biased. A voltage amplifier is another possible approach, however, I would like for the amplified signal to have a linear response with respect to the input current.Quote from: TimFox on August 20, 2024, 05:46:00 pm

author: elki
author_rank: Regular Contributor
timestamp: August 20, 2024, 07:26:19 pm
content: Indeed, there are always pros and cons of every approach. I am currently leaning towards minimizing the distance between the photodiode and the amplifier using jumper-type of connections.Quote from: tggzzz on August 20, 2024, 06:26:27 pm

author: tggzzz
author_rank: Super Contributor
timestamp: August 20, 2024, 07:43:32 pm
content: Quote from: elki on August 20, 2024, 07:26:19 pmTIAs are discussed in The Art of Electronics 3, and in Phil Hobbs' bookhttps://electrooptical.net/Building_ElectroOptical_Systems/"Building electro-optical systems : making it all work" is directly relevant since "[it is] intended in the first instance for use by oppressed graduate students".Phil Hobbs is referred to several times in TAoE.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2024, 08:06:17 pm
content: Quote from: elki on August 20, 2024, 07:24:49 pmIf you run a reverse-biased photodiode into a 50 ohm load, the response will be linear so long as the voltage developed across the load is small compared with the reverse bias.The simplified model of the photodiode is a current generator with a parasitic diode (to the bias supply); the parasitic diode can be modeled roughly as a high resistance in parallel with a voltage-dependent capacitance, so long as the net reverse voltage is reasonably high.The net capacitance of the diode (small) with the 50 ohms (assuming a matched transmission line and coax connectors) will limit the high frequency response.This approach does not have the stability problem of the TIA with shunt capacitance, but that can be avoided with careful detailed design.

author: elki
author_rank: Regular Contributor
timestamp: August 20, 2024, 08:27:46 pm
content: Indeed, as long as the voltage drop is much smaller than the bias voltage, there should be no issue. However, as you point out, there is a tradeoff between gain and bandwidth when using the voltage amplifier. In the TIA case, one can stabilize the performance by adding a small feedback capacitor.Going back to my initial question, I guess that at the end of the day, one should just reduce as much as possible the path from the photodetector output to the amplifier stage when using the TIA approach. Common jumper wires would work just fine, I imagine.Quote from: TimFox on August 20, 2024, 08:06:17 pm