Electronic Design

Bob's Mailbox

Hello Bob: I'm designing wideband photodiode amplifiers and using Jerald Graeme's excellent book (Photodiode Amplifiers—Op Amp Solutions) as a reference. In the case of a composite transimpedance amplifier (TIA) discussed in Chapter 6, do you know whether the phase compensation requirement that dictates a value of CF for stability/gain peaking (i.e., the formula on page 58) changes? (Refer to my column "What's All This Transimpedance Amplifier Stuff, Anyhow?") Be sure to write down these questions: What BW (min and max) do you need? What noise do you need in that BW? What is your Z source (R, C)? What is the minimum and maximum signal size? Data? These questions can help you define your circuit and amplifier needs. These are usually defined by real circuits and not by a formula. If one circuit doesn't solve your problem, you may need another circuit. No book makes it easy! /rap) I've designed and built TIAs before, but not high frequency. I'm getting ready to design a photodetector for an analog application to measure pulses (amplitudes) as small as 10 µA and as fast as 50-ns pulse width (10-ns rise/fall times). (If you have signals as fast as that, you are definitely interested in op amps with low V noise, and the I noise probably won't be so important, unless you get silly. You have to get low V noise/(ZS) at the high frequency of interest. And you haven't mentioned your CS, so I can't guess what your ZS is. It had better be a small CS... /rap ) Right now I have a spreadsheet (People who use spreadsheets expect some kind of quasi-linear problem. These TIA problems force you to change your whole circuit, so it's not very linear. /rap) where the fixed parameters like the op-amp parameters and diode parameters are entered. Then RF, damping, and signal current (pulse amplitude) can be played with. What gets calculated is the CF (compensation), bandwidth, noise (diode shot, amp current, Johnson, amp voltage, and SNR). The tricky part is the amplifier voltage noise, which experiences noise gain that dominates wideband designs. That is why the datasheet for TI's OPA656 FET amp recommends using the OPA846 (replaced OPA686) and OPA847 (replaced OPA687) bipolar amps, which have lower voltage noise and input capacitance. Besides the TIA topology, I entered in the composite topology and bootstrap but didn't see a real advantage to those. The composite reduces noise bandwidth (and signal bandwidth) with a second op amp in the loop with the TIA, so it probably has some advantages over a separate filter after the TIA. (I have never been enthusiastic about that approach—not a winner. /rap) The bootstrap just made the pole from the compensation capacitance (which is a sum of a few capacitances) dominate versus the second-order pole from the limited open-loop gain and feedback zero. This second-order pole is the bandwidth given by the standard TIA bandwidth formula with 45° of phase margin. But if the RF, CF pole is less than that, it dominates, and bandwidth corresponds to 1/(2*π*RF*CF).
John Dailing
Pease: Let me know if you have a problem. Sometimes, some foolish person sets you a task that cannot be accomplished. Then you need a friend to explain "this has become impossible..."

Hi Bob: When I read your March 1 column ("What's All This ‘Others Stay Lighted' Stuff, Anyhow?"), I had to e-mail you about phasing generators to the grid. With all the wind generators being installed at remote locations, phasing would be a large problem, except wind generators use induction generators. An induction generator is just a three-phase motor (or single-phase motor) that is run above synchronous rpm. Since an induction generator has to run 5% to 8% above synchronous rpm, there is no phasing needed. Just get it close to synchronous rpm and connect to the grid. If the wind generator rpm is low, it will act as a motor and bring it up to speed or load it down to the correct rpm. No phasing or governor is needed. At high wind speeds, the blades stall so the generator does not over-rev. At low wind speeds, the generator becomes a motor and keeps the speed up.
Steven Schmitt
Pease: You're saying that if an induction generator's synchronous speed is 120 rpm (for example) and you bring the generator up to 119 or 122 and throw the switch to connect it 20 times, there will never be a huge surge of current? Not enough to blow breakers? Many big wind generators now use electronic switching for best efficiency at all rotor and wind speeds. So, any such system would have its own needs for synchronization. Your simplified version probably wouldn't apply. Fair enough? Your statement might apply to small, simple wind systems.

Dear Bob: Your recent article about polypropylene caps (\\[\\[What-s-All-This-Capacitor-Leakage-Stuff-Anyhow-151|"What's All This Capacitor Leakage Stuff, Anyhow?\\]\\]") raises a question I've had for a while about Y5V dielectric ceramic caps. I got burned a few years ago on a design where I wanted high capacitance in a low volume and used these near their maximum operating voltage. At that voltage, they only have 10% of their rated capacitance. So what's the use of these things?
Mike Partridge
Pease: At low voltages and at room temp, i.e., cheap consumer stuff. They are pretty lousy, but the world has a lot of applications for a lousy cap. So these do sell some.

Comments invited! [email protected] —or: Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

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