MPF102 FET Preamplifier Arduino

A Darlington would perform even better, but the circuit would be slightly more complicated. (See the schematic at bottom-right. ) Fortunately, the added complexity is inside the part! The reason for the Darlington`s superiority is that it can provide the lowest output impedance. I don`t know how low you need the output impedance to be. For the Darl
MPF102 FET Preamplifier Arduino - schematic

ington, I strongly recommend the 2N6426 from Mouser. Similar devices are the 2N6427 and MPSA13. Each Darlington transistor  is actually an IC with two super beta transistors inside. Either one of these internal devices is a superman.  Together, they rule the universe. If you don`t want to try the 2N6426, you could build your own Darlington with two 2N3904 transistors from Radio Shack. Yet two 2N3904s only have a combined gain of 40K. The 2N6426 has a gain of 300K. Either version of the circuit should work, but the 2N6426 has the edge. (All that gain provides extra elbow room in the form of sensitivity. Actually, nobody uses the whole 300K. Think about it: In theory, 1 milliamp on the base gets you 300 amps on the collector. In real life, that much current would crater your PC board!) Here`s the circuit The Arduino digital output pulses an infrared LED that`s in series with a 220-ohm, half-watt resistor. (One LED for each motor coil. ) The resistor reduces the Arduino current to a safe value for the LED. Be sure to use a half-watt resistor here. The LED could be active-high or active-low. (The drawing below shows both versions of the circuit. ) The 12-volt stepper supply powers the phototransistor. The phototransistor`s emitter is in series with a 5. 6K, quarter-watt resistor. Another resistor, 56K, quarter watt, couples the emitter signal to the Darlington array input. Whenever the LED lights, the phototransistor sends a...

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