Post by thread:[EE] : How to read the amplifier sheet.

I am quite new at reading the amplifier datasheet. Evaluating an amplifier for my needs is still a bit fuzzy. I am interested in using an amplifier to increase the current that flows from the collector to emitter. I do know that the base current does control the current that flows between the collector and emitter. The problem is that the datasheet is vague. http://www.fairchildsemi.com/ds/BC/BC338.pdf http://www.fairchildsemi.com/ds/2N/2N3904.pdf from these 2 datasheet I can make the conclusion that the bc338 "allows" a great current flow between the collector and emitter. While the other only "tops" off at 100mA with a gain of 100. Unfortunately in real life I measure the gains quite differently. The gains is not 100 for bc338 but in a magnitude of 20? This is puzzling. Please help me clarify this question of mine. John __________________________________ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com

At 07:44 PM 10/12/2005 -0700, you wrote: >Unfortunately in real life I measure the gains quite >differently. The gains is not 100 for bc338 but in a >magnitude of 20? The data sheet gives min/max figures for hFE at certain Vce and collector current and temperature. hFE is quite dependent on collector current-- low at very low collector current, rising to a peak at some particular current, and lower again for high current. The two transistors you reference have quite different maximum current ratings as well, chances are the higher rated one has its peak gain at a higher collector current. If you're measuring only 20 for hFE at a reasonable collector current, you might be doing something wrong. Remember that the hFE is measured at a Vce of 1V. If you allow the transistor to saturate (Vce < Vbe) then you enter the realm of "forced beta", where the current is limited by the load, not by the transistor. A forced beta (Ic/Ib) of 10-20 is normal for saturated switching designs. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff

--- Spehro Pefhany <.....speffKILLspam@spam@interlog.com> wrote: {Quote hidden}> At 07:44 PM 10/12/2005 -0700, you wrote: > > >Unfortunately in real life I measure the gains > quite > >differently. The gains is not 100 for bc338 but in > a > >magnitude of 20? > > The data sheet gives min/max figures for hFE at > certain Vce and collector > current and temperature. > > hFE is quite dependent on collector current-- low at > very low collector > current, rising to a peak at some particular > current, and lower again > for high current. The two transistors you reference > have quite different > maximum current ratings as well, chances are the > higher rated one has its > peak gain at a higher collector current. > > If you're measuring only 20 for hFE at a reasonable > collector current, > you might be doing something wrong. Remember that > the hFE is measured > at a Vce of 1V. If you allow the transistor to > saturate (Vce < Vbe) > then you enter the realm of "forced beta", where the > current is > limited by the load, not by the transistor. from this datasheet http://www.fairchildsemi.com/ds/2N/2N3904.pdf what is the saturation voltage for Vce and Vbe. It looks like the saturation for Vce is 0.2V?? While Vbe is 0.85V. I am confused on this part(saturation). Regards, John {Quote hidden}> > A forced beta (Ic/Ib) of 10-20 is normal for > saturated switching designs. > > Best regards, > > Spehro Pefhany --"it's the network..." > "The Journey is the reward" > speffKILLspaminterlog.com Info for > manufacturers: http://www.trexon.com > Embedded software/hardware/analog Info for > designers: http://www.speff.com > ->> Inexpensive test equipment & parts > http://search.ebay.com/_W0QQsassZspeff > > > --

> from this datasheet > > http://www.fairchildsemi.com/ds/2N/2N3904.pdf > > what is the saturation voltage for Vce and Vbe. It > looks like the saturation for Vce is 0.2V?? While Vbe > is 0.85V. I am confused on this part(saturation). Mid posting is the very worst of all choices !!!! Try and put you text at bottom (I prefer) or top (some misguided people prefer :-) ) and trim old text to a minimum. A transistor's Vce can be lower than its Vbe at any given time - the two are not directly related. Vce is typical for a saturated transistor - it will vary widely with device and current. 'Saturation' is the state when increasing the base current does not decrease the Vce. Vbe of 0.85 V is high but not untypical. Essentially all silicon transistors would be being very hard driven at this Vbe. RM

This site may help you http://www.kpsec.freeuk.com/components/tran.htm To try and answer your question. The Vce voltage of 0.2 volts is the result of a low resistance path from the collector to emitter. This how the transistor is designed in order to pass maximum current. Were the Vbe voltage is the forward voltage drop across the base to emitter junction that acts just like a voltage drop across a diode. _______________________________________ Roy Tauranga New Zealand _______________________________________ > > what is the saturation voltage for Vce and Vbe. It > looks like the saturation for Vce is 0.2V?? While Vbe > is 0.85V. I am confused on this part(saturation). > -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.344 / Virus Database: 267.11.14/131 - Release Date: 12/10/2005

At 09:42 PM 10/12/2005 -0700, you wrote: > from this datasheet > >http://www.fairchildsemi.com/ds/2N/2N3904.pdf > > what is the saturation voltage for Vce and Vbe. It >looks like the saturation for Vce is 0.2V?? While Vbe >is 0.85V. I am confused on this part(saturation). Okay, what the specifications you are referring to means is that if you *force* a current through the base of 1.0mA, and adjust the load such that the collector current is 10mA, then take voltage measurements, you'll be guaranteed to find* that both: 1. Collector to emitter voltage is 0.2V or less 2. Base to emitter voltage is between 650mV and 850mV * Note the test conditions- 25°C, and the test is done with 300usec pulses of < 2% duty cycle. The most important consequence of those oddball conditions is that the junction temperature is going to be pretty close to 25°C. From those numbers you can calculate/infer how much base current you must supply to be guaranteed to switch a load in the 10mA - 50mA range, and beyond if want to risk extrapolating. I think maybe your problem is that you don't yet understand which are dependent and which are the independent variables. In the case of hFE, the collector current is an independent variable. In the case of Vce(sat), the collector current is a dependent variable. P.S. The numbers on that data sheet are your guarantee of what the transistor will do at those particular conditions. It's a bit unlikely you'll actually be using it at those exact conditions. From knowledge of how a transistor works you can infer to a fair degree (always allow some safety margin) how it will behave under other conditions. The 'typical' curves can help with this (but remember that *you* must guarantee operation at worst-case figures and conditions if you want a reliable design). To this end, see this more detailed data sheet from On semi: http://www.onsemi.com/pub/Collateral/2N3903-D.PDF You may find the figures 15-18 interesting. The *typical* Vce at 10mA with Ic/Ib = 10 is only 100mV @ Tj= 25°C , half the minimum guaranteed, but you can see that it's typically just starting to lift off the 100mV 'floor'. If you have Ic/Ib =10 and Ic = 3-5mA, you can be pretty sure that Vce will be well under 200mV. You can also see that hFE typically peaks at around 6-10mA IC, and has a strong temperature dependence (fig. 15). Do NOT use the typical curves directly for design-- use the guaranteed numbers. The typical curves are for insight into behavior between and beyond the guaranteed points. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam.....interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff