Speeding optoisolators

A b-e resistor, ballpark 47k to 1M, really speeds them up, at the cost of ctr. Given reasonable led drives, in the 5 mA ballpark, it's worth it.

How about some sort of feedback loop, driving the base such as to keep the collector current constant? Light increases c-b leakage (I think that's what happens), the loop backs off external base current, and Vbe hardly changes at all. That might be fast.

John

I'll give that a whirl. Don't know if my Spice models are any good though.

...Jim Thompson

Could be quite a bit faster. As John pointed out already, photodiode mode into a TIA is about as good as it gets.

My common-base circuit still allows about 35mV of swing across the phototransistor, so even here a true TIA would help some.

Cheers, James Arthur

Like many "simple" circuits, it's not entirely simple in that several parameters (drive, CTR, storage time, transimpedance gain) interact.

Here's the design procedure: 1) Set R41 for desired LED current. 2) Set R42-R43 divider for ~1v. 3) Choose R44 for reliable operation of Q42 at the lowest anticipated photocurrent. Allow for LED aging and CTR variation.

The speed--over which you have no more control--is then determined by how quickly R44 can pull the opto's phototransistor out of saturation.

Increasing photocurrent increases the phototransistor's storage time, slowing the circuit, but considerable overdrive is necessary to cover CTR variations and LED aging.

Here's my go at a lower-voltage version that solidly passes a 650kHz squarewave:

FIGURE 5. ========== +2.7v -+- R51 | 3.6k 1/2 | + >----/\\/\\/----. MOCD217 +---------. | OPTO | | .--. .---|---------/--. | | | | --- |/ | | --' '-- | \\ / ---> | | \\ R55 4v p-p | -+- |>. | / 2.2k | | | | \\ '---|----------|-' | --. .-- - >-------------' | | | | | | '--' R52 | | tf=150nS, tr=175nS 2.7k |

Thanks

don

Well, run a little quiescent current through the led, just enough to bias up the base some.

John

View in a fixed-width font such as Courier.

. . . . . FIGURE 5. R22 . ========== 1k . ---/\\/\\--- . | | . | | . | |\\ | . 1/2 +-----|-\\ | |\\ . + >-------------. MOCD217 | | >-+---|+\\ .--. . | OPTO | 3V--|+/ | >-> | | . .--. .---|---------/-- |/ 3.5V-|-/ --' '-- . | | | --- |/ | |/ . --' '-- | \\ / ---> | | . 1.5mA | -+- |>. | dual comp . | | | | . '---|----------|-' . - >-------------' | . | . === . GND . . . . . . .

That allows you to manipulate the base drive through a reasonably low impedance and use positive feedback. The old NEC Optocoupler Handbook contained at least one Schmitt trigger circuit using a single external transistor to do this; it was as fast as you get using these opto's.

Here is another idea: negative feedback through the other optocoupler. As soon as the first optocoupler starts to conduct, it cuts of the LED current via the other optocoupler. So both optocouplers are always in the barely conducting condition; there are only the slight variations of the currents and voltages.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

That's a good idea, and he has a dual anyway. It may tricky with the time delay ( feedback transport delay) inherent to oc's though, but it can be worked.

Dual comp? The first stage should be an opamp.

But do that, and add a little LED pre-bias current, and it looks pretty good.

John

The current waveform from the opto is a ramp, so you could get a pretty nice flat-topped impulse output this way. The tradeoff is you'd have to keep the amplitude pretty low to avoid Miller slowing the opto's transistor. If you've got a fast comparator handy it's not bad though.

James

On Oct 7, 4:03 pm, John Larkin wrote:

That does help if your LED is normally off and you really need that first pulse response. There is still some charge-up time though, as the LED "bias current" isn't the full amount, so the photobase still has to charge up a little to pass full current when the signal pulse arrives.

Using an LT1818 I've got a TIA simulted similar to the one Fred's posted. With the LED pre-biasing, the TIA output is nearly up to full swing by the 2nd pulse @ 2MHz, versus 8 pulses required when run without the improvement.

At 1MHz, the 3rd pulse is usable even without pre-biasing the LED.

At this level of speed and complexity though, the simplest solution would be to invert the driving signal, thus ensuring the phototransistor's biased and ever-ready for quick action.

James ~~~~~~~~~~~~~~~~~~~

Version 4 SHEET 1 1184 724 WIRE 16 80 -80 80 WIRE 288 80 96 80 WIRE -240 208 -288 208 WIRE -144 208 -160 208 WIRE -80 208 -80 80 WIRE -64 208 -80 208 WIRE 224 208 128 208 WIRE 288 208 288 80 WIRE 400 208 288 208 WIRE 1104 208 400 208 WIRE -288 224 -288 208 WIRE 688 224 560 224 WIRE 400 240 400 208 WIRE 688 240 688 224 WIRE 1104 240 1104 208 WIRE 176 272 128 272 WIRE -64 304 -96 304 WIRE 176 304 176 272 WIRE 560 336 560 224 WIRE 688 336 688 320 WIRE -288 352 -288 304 WIRE -96 352 -96 304 WIRE 224 352 224 208 WIRE 224 352 176 352 WIRE 528 352 224 352 WIRE 992 368 592 368 WIRE 1104 368 1104 320 WIRE 400 384 400 320 WIRE 528 384 400 384 WIRE 784 400 688 400 WIRE 400 416 400 384 WIRE 688 416 688 400 WIRE 784 432 784 400 WIRE 400 528 400 496 WIRE 560 528 560 400 WIRE 688 528 688 496 WIRE 688 528 560 528 WIRE 480 576 432 576 WIRE 576 576 544 576 WIRE 176 656 176 352 WIRE 432 656 432 576 WIRE 432 656 176 656 WIRE 464 656 432 656 WIRE 576 656 576 576 WIRE 576 656 544 656 WIRE 992 656 992 368 WIRE 992 656 576 656 FLAG -288 352 0 FLAG -96 352 0 FLAG 1104 368 0 FLAG 784 432 0 FLAG 688 336 0 FLAG 176 304 0 FLAG 400 528 0 SYMBOL Optos\\\\MOC207 32 272 R0 SYMATTR InstName U1 SYMBOL voltage -288 208 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 -414 101 Left 0 SYMATTR Value PULSE(0 4.6 10uS 50nS 50nS .2uS 500nS) SYMATTR InstName V1 SYMBOL res -144 192 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 4.7k SYMBOL voltage 1104 224 R0 SYMATTR InstName V2 SYMATTR Value 3.3v SYMBOL diode -144 224 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL res 112 64 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL res 560 640 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL cap 544 560 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 10pF SYMBOL res 384 224 R0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL voltage 688 224 R0 SYMATTR InstName V3 SYMATTR Value 5v SYMBOL voltage 688 400 R0 SYMATTR InstName V4 SYMATTR Value 5v SYMBOL Opamps\\\\LT1818 560 368 R0 SYMATTR InstName U2 SYMBOL res 384 400 R0 SYMATTR InstName R2 SYMATTR Value 10k TEXT -144 520 Left 0 !.tran 0 20uS 0 100nS TEXT -720 104 Left 0 ;This version pre-biases the LED and the summing node,\\nfor faster initial response.