Hi, Is it possible to saturate the NPN BJT transistor in this biasing ? Vcc ___ | | | _| C B | ---- Resistor ----| | - | E |---------------- Output | Resistor | | | --- - Ground Since the transistor is OFF when '0' to base and hence output is '0'. Then when '1' to base I would like to have 4.5V at the output, is that possible to bias the transistor in saturation region (since Vce no more <0.2V)?
Yes, if you drive it with a voltage at least one diode drop above Vcc. Otherwise, it wont saturate, only clip. Kevin Aylward http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.
Sure. Saturation implies that the collector to emitter voltage is less than the base to emitter voltage (the collector to base junction becomes forward biased). If this circuit is driven with a voltage more positive than the collector supply, the transistor may well saturate.
Yup, "inverted state saturation". This can be very cool; if one were to pull the left side of the resistor up above Vcc, the transistor drop will go very low, like normal saturation. Pull up more, and it can go to zero, and then *below* zero with the right transistor. I did a 16-bit DAC once using discrete transistors and an R-2R wirewound resistor network, with the base currents of the first few stages tweaked for exactly zero saturation voltage. John
OK, one more question. If my transistor datasheet stated that the minimum hFE (DC current transfer ratio) is 100 and the maximum is 400, can I still force this transistor into saturation where hFE normally is 10 in saturated transistor? Thank you.
Yes. That beta spec applies only at some collector to emitter minimum voltage and some maximum collector current. If the minimum collector voltage is not met, the hfe falls dramatically, since it is reverse bias across the collector to base junction that sweeps the charge carriers injected into the base region by the emitter (perhaps I should say, emitted by the emitter) efficiently to the collector (the reverse bias collects those charges out of the reverse biased layer of the base region). The base is called what it is because originally it was the physical object that emitters and collectors were diffused into from opposite sides. Once the base voltage rises above the collector voltage, the base to collector junction becomes forward biased, and base current diverts directly to the collector (in addition to any feeble emitter charge collection taking place). At some value of base collector forward bias, the base to collector current must exceed the base to emitter forward bias current, since there is no load resistor limiting this current in the collector path. At that point, the hfe must be less than 1.
Yup. If you jam in 1/10 as much base current as the load current, that would be called a "forced beta" of 10. But more base current doesn't always lead to lower saturation voltage. For either the normal or inverted setup, there's some base current that makes for the lowest Vce. A forced beta of 10 should be pretty good, though. John
