Transistor Base to Emitter Voltage

In summary, the difference between Base-Emitter Saturation Voltage and Base-Emitter on Voltage is that Vbesat occurs when the BJT is driven into saturation, while Vbeon is the nominal forward voltage drop across the b-e junction when the BJT is in the active region. Vbesat is larger than Vbeon due to the increase in base current needed to saturate the device. Additionally, Vbeon is specified at a certain collector current and temperature, usually 25°C.
  • #1
alexhong81
6
0
just wondering, what is the difference between Base-Emitter Saturation Voltage and Base-Emitter on Voltage ?
VBE (sat) = 0.5V
VBE (on) = 0.7V
 
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  • #2
The sat value should be higher than the on value. The first value of 0.5V sounds more like "Vcesat" instead of "Vbesat". Can you double check.?
Anyway, when a bjt is driven fully into saturation, Vce drops to a low value, 0.1 to maybe 0.5V, or even higher/lower in some cases. The on voltage for moderate current values at room temp are around 0.65V. If driven harder, the bjt saturates, & Vbe increases a little.

Claude
 
  • #3
I am sorry, you are right, I got the wrong value here.
I am looking at this NPN transistor ZXTD6717E6, Diodes Inc.

VCE(sat) = 0.245V max (havent seen such low VCEsat before)
VBE(sat) = 0.93V typical
VBE(on) = 0.865V typical

If i am not wrong, When VCE > 0.245V and VBE > 0.93V the transistor will "on" and current ID will flow.
Then what is VBE(on) 0.865V? What is the difference between VBE(sat) and VBE(on)?

Thanks~~
 
  • #4
Vbeon must be specified at some value of colector current & some temp, usually 25C. If the bjt is in its active region, i.e. Vce>0.7V or so, then Vbeon is the nominal forward voltage drop across the b-e junction.

To saturate the device, a base current greater than Ic/hFE is needed. Most general purpose bjt devices spec saturation at Ic/Ib equal to 10. In this case, the base is overdriven so that Vce drops to a low value, known as Vcesat. This is a condition where the device acts like a closed switch.

Because the base current is higher than its value when the device is in the active region, Vbe is higher as well, & this is Vbesat. Since Vbesat occurs when base current is increased, then Vbesat>Vbeon. But the voltage across a forward biased p-n junction increases only as the logarithm of the current, a large increase in Ib results in a relatively small increase in Vbe. Hence Vbesat is larger than Vbeon by a small margin.

Did this help?

Claude
 
  • #5
Thanks Claude.
See whether my interpretation of your explanation is correct:

BJT in active region, VCE>VCE(sat): VBE(on) = nominal base emitter voltage.
BJT in saturation region, VCE<VCE(sat), Ib>Ic/HFE
BJT in active region, Ib higher, VBE(sat) higher.

1. What happens when VBE is lower than VBE(on)?
2. What happens when VBE is higher than VBE(on)?
3. What happens when VBE is lower than VBE(sat)?
4. What happens when VBE is higher than VBE(sat)?

Thanks again Claude
 
  • #6
alexhong81: BJT in active region, VCE>VCE(sat)
BJT is in active region when: Nearly VCE > VBEon

1. What happens when VBE is lower than VBE(on)?
All Transistor currents are nearly zero: IB=IC=IE=0

2. What happens when VBE is higher than VBE(on)?
For simple Bias circuits VBE can not be higher than VBEon. Driving BE diode above VBEon may lead to Avalanche break down. But transistor CAN work with over voltages, its work is highly dependent to the type of transistor and the circuit and we will have to consider the precise transistor physical models like Ebers-Moll or Gummon-Poll models.

3. What happens when VBE is lower than VBE(sat)?
There is no VBEsat for first order circuit analysis of transistor bias circuits. We have VCEsat!

4. What happens when VBE is higher than VBE(sat)?
The question is wrong!

Microelectronic circuit design by Richard Jaeger is highly recommended.
 
  • #7
Thanks...guess i will have to do some reading up first...
 

FAQ: Transistor Base to Emitter Voltage

What is the role of the base to emitter voltage in a transistor?

The base to emitter voltage is a crucial parameter in a transistor as it controls the flow of current between the base and emitter terminals. It acts as a switch, allowing or blocking the flow of electrons from the emitter to the collector, which ultimately determines the output current of the transistor.

How does the base to emitter voltage affect the amplification of a transistor?

The base to emitter voltage directly affects the amplification or gain of a transistor. By increasing this voltage, the transistor can be driven into saturation, resulting in a larger output current. However, it is essential to keep the voltage within a certain range to prevent damage to the transistor.

What happens if the base to emitter voltage is too high?

If the base to emitter voltage is too high, the transistor can be damaged due to excessive current flow. This can lead to overheating and ultimately failure of the device. It is essential to use appropriate circuit design and voltage regulation to prevent this from happening.

Does the base to emitter voltage vary for different types of transistors?

Yes, the base to emitter voltage can vary for different types of transistors. For example, in a bipolar junction transistor (BJT), the base to emitter voltage is typically around 0.7 volts, while in a field-effect transistor (FET), it can range from 1-5 volts. It is essential to consult the datasheet for the specific transistor to determine its base to emitter voltage.

How does temperature affect the base to emitter voltage?

Temperature can affect the base to emitter voltage by changing the characteristics of the transistor. In general, an increase in temperature can cause the base to emitter voltage to decrease, resulting in a decrease in amplification. It is important to consider temperature variations in transistor circuits and use appropriate cooling measures if necessary.

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