But I also have worked on this general problem many times over the years and have several suggestions that go be-yond Jerry's book. More on this later. Most bipolar op amps have much higher current noise than FETs. It's a rare case when an op amp with bipolar input transistors is better, except when R S is very low or resistive or in cases where the input is capacitive but the bandwidth is narrow. C You usually want to avoid an op amp with large input capacitance.
Unfortunately, most data sheets don't properly specify the op amp's input capacitances, neither differential-mode nor common-mode. But it's fair to assume that most "low-noise" op amps have a larger input capacitance than ordinary op amps. You may want to ask the manufacturer, or you might just decide to measure it yourself. So if you want to get low noise, you must optimize very carefully. Specifically, begin by computing the im-pedance Z S of your sensor at the maximum frequency of interest:.
For a good amplifier, the voltage noise and the current noise times Z S should both be as small as you can get. If one of these noises is much larger than the other, then you're probably far off optimum. E If you have any choice of what sensor you employ, try to find a lower-capacitance sensor. Furthermore, make a low-capacitance layout between the sensor and the op amp.
If you want to get fast response, low noise, or wide bandwidth, Jerry's book offers some pretty good advice.
the photodiode in these applications. The classical design topologies shown in Figure 1 are discrete solutions that use an operational amplifier (op amp) with a. Photodiode Amplifiers: OP AMP Solutions [Jerald G. Graeme] on ykoketomel.ml * FREE* shipping on qualifying offers. Publisher's Note: Products purchased from.
More on that later. Because some are better than others, I bet you can use Paul Grohe's selector guide to find some low-noise op amps. See www. Also, Jerry neglected to mention that you can design your own op amp with better, lower voltage noise and better bandwidth. But you can "roll your own" surprisingly easily and accomplish even better performance for a specified application!
I'm not proposing that you design a complete op amp, but it's simple to just add a new low-noise front end ahead of a suitable op amp.
The basic idea is to add a couple of good low-noise FETs in front of an existing op amp. Most op amps don't operate the front-end transistors as rich as the output.
Yet in a case like this, there's no reason at all not to run more current through the front end than in the rest of the op amp. What's the voltage noise of this amplifier? When you're designing an op amp, remember this: adding gain is one of the cheapest things you can add. You on-ly need to be careful about how to give that gain away—to roll it off. In this case, it's easy. The R1-C1 network in Figure 5 just rolls off the gain for a fairly smooth frequency response. We can roll off the amplifier's gain simply in two swoops.
The low-frequency gain is rolled off by R X and C X. Then after the gain rolls off flatly, we roll it off some more by R Y and C Y. This isn't exactly rocket science. We just want to make it a practical design. But this is a whole system design. You can't very well design and optimize the op amp alone.
It's the op amp, the feedback system, the noise filters, and the post-amplifiers that have to be considered and optimized all together.
Then, at a higher frequency, let it roll off some more in some vaguely controlled way. This would make a lousy general-purpose op amp, but it might be ideal for a case where the noise gain is rising, such as in a transimpedance amplifier. Look at the old LM When you choose the correct damping networks, it can provide a gain of out to some high frequency like 1 MHz.
Also note that I added a second pair of 2Ns to improve the voltage noise. Yes, this will approximately double the input capacitance. But if your C S is already large, this may easily improve the signal-to-noise ratio.
If it's good to have two, will three be better? I'll let you figure that out!
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But, yes, four or five may provide definite improvements I won't recommend that you design your own op amp if you can buy one that does the job. But if the best one you can buy isn't good enough, then there's some hope here. Designing your own composite op amp is not that hard, and not that expensive, even if you are going to build one or 10 or The post-amplifier can be inexpensive. Either way, it's not that difficult, but the design compromises are slightly different.
In future columns on this topic, I will comment on other aspects of design and optimization for transimpedance am-plifiers. Meanwhile, try to avoid Tee networks in the feedback network. They often cause poor signal-to-noise ratios. Next time, I'll explain that completely. Yes, a Tee network might help you avoid buying MO resistors, but that's only okay when you have proven that the noise is okay.
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