B.J.T transistor

[aamir1]

hi guyz !!

Please tell me what is the requirement for a B.J.T to saturate is it Ib ?? or Ic ??

suppose a load needs 1A Ic and i design the circuit for 1A Ic current in which transistor saturates but if the current decreases some how to 500mA and the Ib is kept same will the transistor be still in saturation mode??

 

[alec_t]

If Ic = 1A, design for Ib = 100mA. If Ic drops to 500mA then the bjt will still be saturated.

 

[aamir1]

how you told me that i should design it for 100mA ??? what is the reason behind it and does it mean that the saturation of B.J.T depends on Ib only??

 

[alec_t]

how you told me that i should design it for 100mA ???

A rule of thumb to ensure transistor saturation is that the base current should be at least 1/10 of the collector current. That is eqivalent to assuming the transistor minimum current gain is 10.

does it mean that the saturation of B.J.T depends on Ib only??

Not sure. My transistor theory is somewhat rusty . IIRC if Vc is exceedingly low (say only a few mV) the bjt may not be saturated.

 

[aamir1]

what do u mean by IIRC ??

 

[aamir1]

and also what if the beta of transistor is lower than 10 suppose 5 for any transistor

 

[tosborn]

IIRC = if I remember correctly

 

[alec_t]

what if the beta of transistor is lower than 10 suppose 5 for any transistor

Then the transistor needs Ib > Ic/5 to ensure saturation.

 

[Mike odom]

Then the transistor needs Ib > Ic/5 to ensure saturation.

Yes, this is correct.
In theory, the transistor should saturate at Ib = Ic/beta. The general rule of thumb is you take the beta / 10 to ensure saturation. This is used when you are using the transistor as a switch, not an amplifier, so when in saturation, the Votage across the Collector to Emitter (Vce) is minimal, so the power dissipated by the transistor is less (less heat).

Therefore, if beta = 5, then Ib = Ic/.5, or Ib = 2Ic to ensure saturation, which seems crazy, but so does a transistor with a beta of 5. They are usually > 100.

 

[aamir1]

the problem is Beta is a function of temperature so the datasheet gives a range of beta so which beta should be used and what is it named by in the datasheet ??

 

[aamir1]

Please see the attached datasheet the beta highlighted is given is as 5 while the curve shows above 10 beta at 10A ic im really confused please help


View attachment 69081

 

[ronv]

Don't confuse beta with saturation.
Beta (hfe in the datasheet) is the current gain of the transistor but not in saturation.
Saturation is the point where applying more base current does not significantly reduce the voltage from collector to emitter. (Vbe SAT).
So while you may have a transistor that has a beta of 100 meaning it will supply 100 times the base current in collector current the collector to emitter voltage may be large creating heat in the transistor. That is why a "rule of thumb" of 10 X base current will saturate the transistor. Not a very good explanation on my part. Maybe someone has a simpler one.

See this datasheet for Hfe and Vbe Sat:


https://pdf1.alldatasheet.com/datasheet-pdf/view/75121/MICRO-ELECTRONICS/2N2222.html

 

[unclejed613]

you're over-thinking the problem. there aren't a lot of transistor types that have betas below 10 anymore. large power transistors often have betas in the 20-50 range, and smaller ones 50-200 (or much higher). germanium transistors used to have betas in the 2-15 range, but you will not run into these except in rare cases like a very old car radio or keyboard amp (early 1960s vintage).

 

[Mike odom]

the problem is Beta is a function of temperature so the datasheet gives a range of beta so which beta should be used and what is it named by in the datasheet ??

let's start with a data sheet or part number of the transistor you are trying to use...

 

[Mike odom]

That is why a "rule of thumb" of 10 X base current will saturate the transistor. Not a very good explanation on my part. Maybe someone has a simpler one.

That is not the rule of thumb. The rule of thumb is to divide beta by 10, not the collector current. Of course, this rule works for a 2n2222, because it has a beta of 100, which would give you a divide by 10 of the collector current. But, the rule of thumb, as I stated eariler, is to divide the gain by 10, not the collector current, to make sure you have 10x the base current you need to saturate the transistor.

The beta does have a small function of temperature. It also has a function of collector current, and collector-emitter voltage. When used as an amplifier, you try to pick a collector current that falls into the linear range. This is why the data sheet or spec list gives an average gain. The actual gain is taken off a chart as it is not fully linear over all ranges of base current and collector-emitter voltages.

 

[Mike odom]

this is from the data sheet posted in #12


View attachment 69084

 

[Mike odom]

Please see the attached datasheet the beta highlighted is given is as 5 while the curve shows above 10 beta at 10A ic im really confused please help

ok, so I didn't see this post at first.... look at the bottom of your chart... find the collector current you are shooting for... then go up until the curve crosses this line... at this point go left and find the gain of the transistor for this collector current... this is the gain. Divide this by 10, and design for that current into the base. Note: I = E/R, but E = input voltage - .7 base voltage and then divide this by the base resistor to get the base current.

The chart does show a gain of 15. But the spec sheet says a transfer of 5. Note that this is a MINIMUM value. What it is saying is that the typical is 15, but the minimum guarenteed is 5.
Note also that at 4A, the gain is given from 20 to 70. When they produce semiconductors, it is an 'etched' process, so not all parts work EXACTLY the same, even on the same die. Sometimes to tighten up the specs they laser trim. Sometimes they grade the parts and separate them by performance. The curves show typical performance, and probably over 70% of the parts produced operate close to that curve. They give minimums and maximums so that the parts work to spec. When designing, always use the minimum gain value. Another trick would be to buy thousands and grade them, if you need exact performance in an amplifier. For a switch, just use the minimum.

 

[Mike odom]

So, with a minimum gain of 5, at 10A, the question would be, why would you ever pick this part as a switch. Especially when MOSFETs do a much better job at higher currents like this.

 

[aamir1]

the equation Ic=B*Ib does it work in saturation mode ?? as part of my knowledge it only works in active mode am i correct?

 

[aamir1]

So, with a minimum gain of 5, at 10A, the question would be, why would you ever pick this part as a switch. Especially when MOSFETs do a much better job at higher currents like this.

The problem is i am designing a H bridge of 8A since the motor max current is 8A it is a D.C motor and the voltage is 24V i dont want to use the IR2110 mosfet controller that`s y im using the B.J.T i will control them by micro controller. So if i design the bridge for 8A if the motor takes 2A at no load how can i be sure that the transistors will be saturated at 2A since they are designed for 8A. Am i missing something in transistor theory. i ve also attached the circuit i designed please have a look at it......

View attachment 69091