Gain-Bandwidth-Product (GBWP) and Transimpedance Amplifiers (TIA) - Page 1
Forum section: electronics / projects

author: Falcon_MP
author_rank: Newbie
timestamp: March 11, 2025, 02:53:28 pm
content: Hello together!I have a question regarding the gain bandwidth product of op amps.As I understand the gain bandwidth product, it is the product of a the gain from an op amp multiplied with the bandwidth that I want to amplify. So for example if I want to amplify frequencies from 0 to 1 GHz and the gain bandwidth product of an op amp is 1 GHz, then my amplification would be 1. Did I understand that correctly?The thing is that I have to build a transimpedance amplifier with a very high bandwidth (10 MHz to 1 GHz). The amplifier will be used to convert the current signal from a photodiode into a voltage signal. I have searched for op amps with enough bandwidth and found some chips with gain bandwidth products of up to 8 GHz. Is it actually possible to use the whole bandwidth of 8 GHz with these chips? Are there some limitations in how much bandwidth I can actually get out of these amplifiers?Thanks in advance!Best Regards

author: jwet
author_rank: Frequent Contributor
timestamp: March 11, 2025, 03:08:16 pm
content: What you've said is mostly correct first order. You could take a 1 GHz GBWP amp and make a 1 GHz follower- its not a good idea, you have no excess gain and you lose all the beauty of negative feedback.You're missing one point- you don't have to take all your gain in one stage, you use a transimpedance amp to get your current out of the mud and then add additional gain with post amps. Making a stable (non oscillator) 1 Ghz amp is challenging and it will take some finesse.Transimpedance amps are current operated and have unusual requirements for the Amp. Good book- "Photodiode Amplifiers" by Jerald Graeme is my bible for this stuff. There are some unusual interactions between the sensor capacitaince and the amp that adversely affect noise gain and cause problems like gain peaking where the noise gain rises at high frequencies- this makes the problems very challenging. Get an understanding of these things.

author: TimFox
author_rank: Super Contributor
timestamp: March 11, 2025, 04:09:05 pm
content: Quote from: jwet on March 11, 2025, 03:08:16 pmI highly recommend that book by Graeme.Photodiode capacitance is key to proper design of the feedback.A TIA is inherently a feedback impedance (R and C) from the output back to the junction of diode and inverting input, with frequency-dependent gain and phase from that node forward to the output.

author: Falcon_MP
author_rank: Newbie
timestamp: March 11, 2025, 04:18:38 pm
content: Thank you!Yes, I had the same idea to use subsequent op amps to improve the gain. So in general one would design a TIA with a gain of around 1 and then use subsequent op amps, is that correct?Thanks for the book recommendation. I will definitely have a look.

author: jwet
author_rank: Frequent Contributor
timestamp: March 11, 2025, 04:33:42 pm
content: TIA's have transresistive gains measure in ohm. Resistances (gains) of 100 ohms to 1 meg are not uncommon though at very high speed, you'll likely be at the lower end of this range.There is a related circuit called a Charge Amp that is kind of leaky integrator often used in fast photodiode and PMT apps. The gain set by a small capacitance with a large R across it - to keep it from pumping up to the rail. Sometimes circuits kind of bridge the two domains being transresistance at low F's and turning into charge amps. After a charge amp, you generally do signal shaping (filtering) to optimize noise. Nuclear instrumentation use this architecture. "Radiation Detection and Measurement" by Knoll will give you a good overview. Charge amp specifics are a bit of a black art.One other related app is laser diode range receivers. There are asics for this. Look at TI and Maxim (Analog).My best advice is still to read Graeme- its a complicated topic really though the circuits are deceptively simple looking.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 12, 2025, 03:44:37 pm
content: GBW, is defined as the product of the open loop voltage gain and the frequency at which it is measured.Or good, bad, whatever.... The details of how to caclulate the necessary GBW are on page 2 of the attached>>You want to set the TIA for the highest gain that won't lead to saturation. Know your limits.Also see thishttps://tools.analog.com/en/photodiode/this wizard will help sort out some parameters for you.At high speeds you'll need to be very careful with layout, even a pF of stray can wreck your plans. None of that perf board folly here.Charge amps are red herring.

author: ConKbot
author_rank: Super Contributor
timestamp: March 12, 2025, 04:24:33 pm
content: For a TIA circuit, noise gain is what you can use to work out what your opamp GBWP roughly needs. Your compensation cap (in parallel with your gain resistor) and the photodiode capacitance make a capacitive divider. Use that as your feedback divider to see what the gain is, and knowing your GBWP, you have a first order estimate of your usable bandwidth.