Understanding transistor datasheets.

mmarab · Oct 22, 2010

Hi, i'm new to the forum, so hello

My electronics knowledge is getting better, i have been reading a lot, but i have lots of gaps and lack some basic knowledge, which im trying to pick up.

Anyway, i understand the concept of a transistor, how it can be used for switching and i have tried it on a breadboard and wow i got it working etc. So thats great, but i dont really understand the datasheet information and what are the important bits. I have the following transistor:

Device_Name: BC635
VCEmaxV: 45 - maximum voltage between the collector-emitter junction
VCBmaxV: 45 - the maximum voltage between the collector-base
VEBmaxV: 5 - the maximum voltage between the emitter-base junction
IcMax: 1 - the maximum current in milliampers you can put through the transistor
A PtotW: 0.83 - power the transistor can dissipate in milliwats
hFE@IcmA: 63@150 - current gain

But how do i actually use this information, what does it all mean? I have been searching for an example on the internet where they actually have a real example with power supply etc, but i havent found one which uses this information and explains it.

If anyone could help, that would be much appreciated

Thanks.

ronv · Oct 22, 2010

Transistors

Here is a picture with some examples.
I forgot power dissipation which can be calculated using collector current X voltage from collector to emitter. In this case about .3 watts. (2x.150). Often in switching designs saturation is used instead of hfe. It is usually defined as Ic/Ib = 10. This gives the lowest collect emitter voltage and reduces the power in the transistor. In this case the power could be reduced to about .05 watts. Many designs don't reverse bias the base emitter junction to turn the transistor off. The reverse bias (-5 volts here) will speed up the turn off over just making the off voltage 0.

ronv · Oct 22, 2010

Tx

Forgot the picture.

crutschow · Oct 22, 2010

Here's a short description of the parameter use:

Device_Name: BC635

VCEmaxV: 45 - maximum voltage between the collector-emitter junction
Voltage never to be exceeded. Normally you operate some below that.

VCBmaxV: 45 - the maximum voltage between the collector-base
Same as for VCEmaxV

VEBmaxV: 5 - the maximum voltage between the emitter-base junction
This is the absolute maximum reverse bias voltage that you should stay below.

IcMax: 1 - the maximum current in milliamperes you can put through the transistor
The circuit must be designed to never exceed this current.

A PtotW: 0.83 - power the transistor can dissipate in milliwatts
Absolute maximum dissipated power, usually at 25C ambient temperature, equal to the collector current times the collector voltage at that current. Must be reduced as ambient temperature rises due to room or circuit temperature increases.

hFE@IcmA: 63@150 - current gain
Current gain is used to determine required base current for a given collector current when used as an amplifier (not a switch). Normally you design with the minimum value to insure that any transistor will work in the circuit.

Jaguarjoe · Oct 22, 2010

crutschow said:
Here's a short description of the parameter use:

Device_Name: BC635

IcMax: 1 - the maximum current in milliamperes you can put through the transistor
The circuit must be designed to never exceed this current.

A PtotW: 0.83 - power the transistor can dissipate in milliwatts
Absolute maximum dissipated power, usually at 25C ambient temperature, equal to the collector current times the collector voltage at that current. Must be reduced as ambient temperature rises due to room or circuit temperature increases.

For this device, Ic units are amps, not ma. Power is watts, not mw.

mmarab · Oct 25, 2010

Thanks for all the information guys, much appreciated.

That has help further my understanding.

ronv thanks for the image, that helped. Just one question, the resistor before the base, is that used to keep below the 5v?

crutschow · Oct 25, 2010

mmarab said:
Just one question, the resistor before the base, is that used to keep below the 5v?

The base emitter junction looks like a diode, thus a resistor is required to limit the current when a forward voltage is applied (the normal transistor "on" condition).

The 5V limit is for the reverse direction which is not significantly affected by the resistor. You must never apply a reverse base-emitter voltage larger than 5V or you can break down the junction and damage the transistor.

Jaguarjoe · Oct 25, 2010

The spec sheet shows Veb which is not the same as Vbe.
Veb is the max positive voltage that can be applied across the E-B junction with current flowing from E- B.
Vbe is the voltage across the B-E junction with current flowing from B-E. This is the P-N junction voltage drop which is fixed at approximately 0.7V for a silicon transistor and 1.4 volts for a Darlington transistor

olu1yomi · Jul 21, 2013

how to know the transistor output current

How do i know d transistors output current without
D datasheet or with d datasheet.i want to use it as an
Amplifier,to amplifier current.i intendysingBC638

audioguru · Jul 21, 2013

olu1yomi said:
How do i know d transistors output current without
D datasheet or with d datasheet.i want to use it as an
Amplifier,to amplifier current.i intendysingBC638

Of course you need the datasheet of the transistor to design a circuit using it.
I get datasheets from www.datasheetarchive.com .

Your circuit design determines the output current. The datasheet states the maximum allowed output current and your design should limit its output current to less.

olu1yomi · Jul 22, 2013

audioguru said:
Of course you need the datasheet of the transistor to design a circuit using it.
I get datasheets from www.datasheetarchive.com .

Your circuit design determines the output current. The datasheet states the maximum allowed output current and your design should limit its output current to less.

thanks your reply really helped.pls can u give me the type of design that will do as an amplifier with bc 368 transistor..thanks

Mr RB · Jul 23, 2013

Google for "NPN common emitter amplifier" for low power amp design, or google "3 transistor NPN headphone amplifier" to see examples of small power amps that will (barely) drive a speaker or headphone, using small NPN transistors like yours.

If you want to build an amp that is loud you will need bigger transistors and a much more complex design.

Frank Nash · Jul 23, 2013

https://alltransistors.com/

audioguru · Jul 23, 2013

olu1yomi said:
thanks your reply really helped.pls can u give me the type of design that will do as an amplifier with bc 368 transistor..thanks

A BC368 is an OLD low voltage, low power transistor. It melts when it dissipates only 625mW because you cannot cool it.
Also a power amplifier is usually class-AB with TWO complimentary transistors with metal tabs that can be cooled that heat only when they are working hard instead of only one transistor (class-A) that heats a lot ALL THE TIME.

Maybe you want to make a low power single transistor voltage signal amplifier instead of a power amplifier?
Maybe you should use a power amplifier IC (class-AB) that has a metal tab that can be bolted to a heatsink.

Mosaic · Jul 24, 2013

Also, if distortion control is important in the amplification of the signal, the small signal parameters of the transistor will guide the DC bias point. If your signal has a dc offset you may want to isolate the ac component with a capacitor coupled input.

WTP Pepper · Jul 24, 2013

hFE@IcmA: 63@150 - current gain [COLOR=royalblue said:
Current gain is used to determine required base current for a given collector current when used as an amplifier (not a switch).[/COLOR] Normally you design with the minimum value to insure that any transistor will work in the circuit.

I disagree. if you use a transistor as a switch to say switch a relay on, then the Hfe is an important calculation to ensure you supply enough base current to allow enough collector current to flow through the relay coil to activate the relay.

e.g. 1uA base current through a transistor with a Hfe of 100 will not switch a relay that requires 10mA through it to trigger. However 1mA base current would generally guarantee it would. It's down to the base resistor and the drive behind it. If you cannot supply the base current in this case then an N channel MOSFET is a good alternative as it requires zero gate drive current, just the voltage.

Mosaic · Jul 24, 2013

WTP Pepper said:
an N channel MOSFET is a good alternative as it requires zero gate drive current, just the voltage.

Not quite correct. The MOSFETS have a charge associated with the parasitic gate capacitance that must be satisfied to raise the gates voltage to the threshold. Depending on the size of the FET this can require several amps for perhaps a few tens of nanoseconds.

crutschow · Jul 24, 2013

WTP Pepper said:
I disagree. if you use a transistor as a switch to say switch a relay on, then the Hfe is an important calculation to ensure you supply enough base current to allow enough collector current to flow through the relay coil to activate the relay.

e.g. 1uA base current through a transistor with a Hfe of 100 will not switch a relay that requires 10mA through it to trigger. However 1mA base current would generally guarantee it would. It's down to the base resistor and the drive behind it. ...........................

I believe you misunderstood my (not a switch) comment. Using the listed transistor Hfe when using it as a switch is not usual (or good) engineering practice. If you look at the data sheet for BJTs, for minimum ON voltage when used as is switch a forced (dc) hFE (or Beta) of 10 (or in some cases 20) is used. In a good design, the minimum data sheet active region hFE is never used to determine the required base switching current.

audioguru · Jul 24, 2013

The hFE of a transistor is always listed when it is linear with plenty of collector to emitter voltage so it is an amplifier, not a switch.
When a transistor is a switch then hFE numbers are not used, the saturation voltage is listed when the base current is 1/10th or 1/20th of the collector current, even if the minimum hFE is very high.

WTP Pepper · Jul 25, 2013

Mosaic said:
Not quite correct. The MOSFETS have a charge associated with the parasitic gate capacitance that must be satisfied to raise the gates voltage to the threshold. Depending on the size of the FET this can require several amps for perhaps a few tens of nanoseconds.

To switch a simple relay, this is irrelevant. As a switch inside say a SMPS then you have a point.

Understanding transistor datasheets.

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