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I am trying to saturate a transistor , a BC547 A , but I am having a hard time figuring out something.What hFE should I use to compute the base and collector current to make it turn on fully?

Here is the datasheet

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  • \$\begingroup\$ Interest only - you can get super low Vsat by using forced beta of say 0.1 ie Ib = 10 x Ic. Sounds insane BUT can be useful if eg using transistor to turn on a voltage divider and you want to minimise voltage error caused by transistor switch. I did that in a commercial design long ago. Nowadays I'd use a MOSFET with suitably low Rdson. \$\endgroup\$ – Russell McMahon Jul 11 '15 at 14:37
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Use an Hfe of 10 and you'll always saturate the transistor as long as the collector current isn't high enough to drive the transistor's raw Hfe to below 10.

Study figures 3 and 4 on the data sheet.

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  • \$\begingroup\$ Thank you , I came across a tutorial that confirms your answer. \$\endgroup\$ – Daniel Tork Jul 12 '15 at 5:24
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If the collector current is between 10mA and 100mA use Ic/Ib = 20, because the limits are specified on the data sheet for that ratio. You can expect Vce(sat) between a maximum of 600mV and 250mV depending on collector current. If the collector current is much larger than 100mA or extremely tiny (nA) you may want to select a different device or increase the base current respectively.

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Estimate Ic(sat). Then take hfe to be about 20, and calculate your base drive. Be careful not to overheat the transistor; look at VCE(sat) * Ic(sat) to get an idea of power dissipation.

Look at VCE to estimate how deep into saturation your transistor is. TIP: Look at the VCE sat in the electrical characteristics. That tells you what Ib takes the transistor into saturation for a given Ic. If your Ic is less you can get away with lower Ib and hence slightly lower overall power.

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  • \$\begingroup\$ Why must hFE be so small , below the minimum ratings? \$\endgroup\$ – Daniel Tork Jul 11 '15 at 13:29
  • \$\begingroup\$ @DanielTork hfe is a small signal parameter. Which means its value is for a "small" input signal excursion around the bias point, and changes with the bias point. The hfe(min) of 100 is only for linear operation - VCE is 5V and Ic is only 2mA. \$\endgroup\$ – kabZX Jul 11 '15 at 13:38
  • \$\begingroup\$ Those hfe specifications are given at a specific Vce, often 5V. They don't apply at much lower Vce (like 0.2V, which is normally used as the definition of saturation). At that point, it's difficult to turn the transistor on much harder : which is just another way of saying hFE is very low at Vce=0.2V. \$\endgroup\$ – Brian Drummond Jul 11 '15 at 13:45
  • \$\begingroup\$ So it's advisable to use the hFE as 20.I also understand that this works for the other types of transistor , too. \$\endgroup\$ – Daniel Tork Jul 11 '15 at 15:40
  • \$\begingroup\$ I read somewhere on this site that the DC current gain can be any below the linear ratio to saturate. \$\endgroup\$ – Daniel Tork Jul 11 '15 at 17:36

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