![]() So I adjusted the settings to keep the entire plot within (0 < ICE < 8mA) and the result is Image 5. But my intended application (as a bias spreader in an audio amplifier) will run the optoisolator at relatively low currents. The damn thing just gets better and better as you crank up the current. The curve tracer has a cool feature that lets you plot Beta versus collector current I enabled that feature and instead got a plot of Current Transfer Ratio versus output current, in Image 4. Current Transfer Ratio (= ICE / ILED) varied from about 20X to 38X. I fixed the LED current at 0.4mA and tested a bunch of different FOD852 parts, overlaying the I-V curves on a single plot, to get an idea of their variability. Again we see a whiff of quasi-saturation, at low VCE and high ICE. Collector current is plotted on the Y axis. Image 2 treats the optoisolator as a weird NPN transistor: "base current" (actually: LED current) is stepped from 0.3mA to 0.55mA, while collector voltage is swept. I realized I had a tube of Darlington optoisolators, Fairchild FOD852s, so I put those on the tester too.Īs shown in image 1, these LEDs seem to "turn on" at slightly lower voltage: 1.1V or so, rather than the 1.2V of the 4N35s. is also relatively constant, at about 2.5 or so. zip archive, if you want to examine it more closely. The raw data (excel spreadsheet) is included in a. I connected it this way and plugged it into my curve tracer, set to measure NPN transistors. If we ground the cathode of the input (LED) side, and if we also ground the emitter of the output (phototransistor) side, then the optoisolator behaves as an NPN transistor with unusually low beta. So I decided to measure the 4N35 at a variety of VCE voltages, and find out whether it misbehaves at some or all VCEs. ![]() If this voltage stays constant then the current through those resistors also stays constant, which means the output stage bias current is constant.ĭepending upon whether the output stage is a double or a triple, and depending upon whether the transistors are MOSFETs or BJTs, the bias spreader voltage (= optoisolator VCE) could be as low as 2.4 volts, and it could be as high as 12 volts. I'm thinking about using the 4N35 to "servo" the voltage across the output transistors' emitter resistors / source resistors. ![]()
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